Autumn is one of the most beautiful times of the year. The diversity and richness of nature during this period simply amazes the mind, the simple and complex leaves are so different from each other. The leaf arrangement of each plant is special (it can be alternate or whorled), and it is by this that one can determine which species it belongs to. Let's take a closer look at the features and functions of each type of leaf.

Definition in botany

Along with flowers, roots, stems and shoots, leaves are the most important vegetative organs in plants, which are also responsible for the function of photosynthesis. In addition, they perform many other jobs, such as participating in the processes of respiration, evaporation and guttation of plants. There are the following simple and complex ones, each of them has its own characteristics and is found in a certain type of plant.

Very often, leaf blades are mistaken for leaves, but in fact it is an organ that consists of a blade (veins run through it) and a stalk, which originates at the base and connects the leaf blade with stipules. It always occupies a lateral position on the stem, with all the leaves arranged on it in a certain sequence in such a way as to ensure optimal access to the sun's rays. Its dimensions can vary from 2 cm to 20 m (for tropical palm trees).

External structure and forms

One of the features of these organs is their flat shape, which ensures maximum contact of the plant surface with the air and sunlight. The shapes are simple and differ from each other in appearance. Simple ones have only one leaf blade, which is connected to the base using a petiole. Complex ones consist of several leaf blades located on one petiole. Remember what the thickest vein looks like in the middle, to which two or three stipules are attached on each side. Such a complex one is called opposite, because the leaf blades are located symmetrically to each other.

The main components are the plates and veins that run along their surface, as well as the petiole, stipules (although not all plants have them) and the base with which the element is connected to the stem of a tree or other plant.

Unlike the shape of a simple leaf, several varieties can be found in complex ones, which have their own distinctive properties and features.

Internal structure

The upper surface of the leaf blades is always covered with skin, which consists of a layer of colorless cells of the integumentary tissue - the epidermis. The main functions of the skin are protection from external mechanical damage and heat transfer. Due to the fact that its cells are transparent, sunlight passes through it unhindered.

The lower surface also consists of these transparent cells, tightly adjacent to each other. However, among them there are small paired green cells, between which there is a gap. This part is called the stomata. By opening and reconnecting, green cells open and close the entrance to the stomata. During these movements, moisture evaporation and gas exchange occur. It is known that on the surface of one leaf blade there are from 90 to 300 stomata per 1 mm 2.

Interesting fact: green cells are almost always located on the side of the leaf where maximum air exchange occurs. So, for example, in plants floating on water, egg capsules or water lilies, the stomata are on the outer side facing the air.

Varieties

Scientists distinguish two main types of leaves: a simple leaf and a complex leaf. The structure of each of them has its own characteristics. Depending on the appearance, the number of plates and the shape of their edges, compound leaves can also be divided into several types. So, here are the most common types, if selected by external characteristics:

• fan-shaped (shape resembles a semicircle);
• spear-shaped (sharp, sometimes there are spines on the surface);
• lanceolate (rather wide, with narrowed edges);
• oval (ovoid shape, which sharpens slightly closer to the base);
• palmate and lobed (they can sometimes be confused, since they both have several lobes);
• palmate (the plates diverge from the petiole, the appearance resembles fingers);
• needle-shaped (thin and quite sharp).

This list can be continued for a long time, but the complex shape of the leaf has several more types, depending on the shape of the edges, as well as the location of the leaf blades themselves.

Types of complex plants

By the edges of the plates you can often determine which species a particular plant belongs to. The following forms are most common in nature:

• whole-edged - have smooth edges with no teeth at all;
• serrated - as the name implies, such leaves have teeth along the edges;
• fine-toothed - they resemble a saw, which has very sharp and small incisors;
• wavy - these have wavy cuts that do not have a strict order or standard shape.

Features of each type

It is worth talking in more detail about the distinguishing features of simple and compound leaves, as this can help determine what kind of plant it is and what species it belongs to. So, one of the most noticeable features of each type is the number of plates. If three elements are present, then we have trifoliate sheets. If five are palmate, and if there are more, then they are called pinnately divided. On each plate you can observe a special venation system, thanks to which nutrients enter the internal tissues. In simple and complex varieties they differ in shape and structure. Here are the most common types of vein arrangement:

• arcuate (when the veining resembles the shape of a menorah - one of the symbols of Judaism);
• transverse;
• longitudinal;
• palmate;
• parallel;
• mesh;
• feathery.

Another distinguishing feature is the way the leaves are arranged on the stem. Simple and complex - all, without exception, are attached to plant stems in two ways:

• using a cutting, in which case the plant is classified as petiolate;
• without a cutting, when the base grows and covers the stem, then we have a sessile plant.

Plant leaves: simple and complex

If we classify plants according to leaf characteristics, we can note the following facts. Simple ones are usually common in all herbaceous plants, including shrubs and trees. Complex ones are found in both shrubs and trees, however, unlike simple ones, during leaf fall they do not fall off all at once, but in parts: first the blades themselves, and then the cuttings.

Let's look at the names of simple and complex leaves in plants using examples. Most trees growing in Russia have simple leaves. Aspen, birch and poplar have different shapes: lanceolate, rounded with jagged edges and spear-shaped, respectively. With the onset of autumn cold, the leaves of each of them fall off entirely. They are also found in fruit trees such as apple, pear and cherry; crops such as oats and corn also have simple leaves.

Complex forms are present on leguminous plants, such as the pinnately compound leaves of peas. The following trees have palmate leaves: maple, chestnut, lupine, etc. Remember meadow clover, its shape is called trifoliate with ciliated edges.

What functions do leaves perform?

The simple and complex forms of these organs are largely determined by climatic conditions. In hot countries, trees have large leaves, which serve as a kind of protective fence from the sun's rays.

However, the main irreplaceable function is participation in photosynthesis. As you know, it is through this process that trees can convert carbon dioxide into oxygen by absorbing solar energy.

The second most important process is cellular respiration. With the help of mitochondria, leaves take in oxygen, and through the stomata they exhale carbon dioxide, which is then used during photosynthesis. Since photosynthesis occurs only in light, carbon dioxide is stored at night in the form of organic acids.

Transpiration is the evaporation of water from the surface of leaves. Thanks to this, the overall temperature and humidity of the plant is regulated. The intensity of evaporation depends on the size and thickness of the plates and on the wind speed at a certain point in time.

Adaptation and modification

Many leaves - simple and complex - have the ability to adapt to environmental conditions. In the process of evolution, they acquired the ability to change. Here are the most amazing of them:

• the ability to produce wax that lies on the surface and prevents excessive evaporation of water droplets;
• they form reservoirs for water during rains, this happens due to the fusion of the edges in such a way that a bag-like container is formed (such forms can be found in many tropical vines);
• the ability to change the surface of the plates, rugged leaves prevent the effects of strong winds, thereby protecting plants from damage.

Many facts related to the life activity of these irreplaceable plant organs still remain poorly understood. These beautiful decorations of nature itself, in addition to the above functions, perform another aesthetic task - they delight people with their splendor and variety of bright colors!

The leaf is a very important organ of the plant. This is the part of the shoot whose main functions are transpiration and photosynthesis. The structural features of the leaf lie in its high morphological plasticity, great adaptive capabilities and variety of shapes. The base can expand in the form of stipules - leaf-shaped oblique formations on each side. In some cases, they are so large that they play a certain role in photosynthesis. The stipules can be adherent to the petiole or free; they can be shifted to the inner side, and then they are called axillary.

External leaf structure

The leaf blades are not the same in size: they can be from a few millimeters to ten to fifteen meters, and in palm trees - even as much as twenty meters. The structure of the leaf determines the lifespan of the vegetative organ; it is usually short - no more than a few months, although in some it ranges from one and a half to fifteen years. Shape and size are hereditary traits.

Leaf parts

The leaf is a lateral vegetative organ that grows from the stem, has a growth zone at the base and is bilaterally symmetrical. It usually consists of a petiole (except for sessile leaves) and a leaf blade. In a number of families, the leaf structure also suggests the presence of stipules. The external organs of plants can be simple - with one plate, or complex - with several plates.

The leaf pad (base) is the part that connects the leaf to the stem. The educational tissue located here gives rise to the petiole and leaf blade.

The petiole is a narrowed part, with its base connecting the stem and the leaf blade. It orients the leaf relative to the light, acts as a place where the intercalary educational tissue is located, due to which the growth of the vegetative organ occurs. In addition, the petiole weakens impacts on the leaf during rain, wind, and hail.

The leaf blade is usually a flat, expanded part that performs the functions of gas exchange, photosynthesis, transpiration, and in some species also the function of vegetative propagation.

Speaking about the anatomical structure of the leaf, it is necessary to say about the stipules. These are leaf-shaped paired formations at the base of the vegetative organ. When the leaf unfolds, they may fall off or remain. Designed to protect axillary lateral buds and intercalary educational tissue.

Complex and simple leaves

The structure of a leaf is considered simple if it has one leaf blade, and complex if it has several or many blades with joints. Due to the latter, the plates of complex leaves do not fall together, but one at a time. But some plants may fall entirely.

Whole leaves can be lobed, divided or dissected in shape. In a lobed leaf, the cutouts along the edge of the plate are up to 1/4 of its width. A separate organ is characterized by a larger depression; its blades are called lobes. The dissected sheet along the edges of the plate has cutouts reaching almost to the midrib.

If the blade is elongated, with triangular segments and lobes, the leaf is called planum-shaped (for example, in a dandelion). If the lateral lobes decrease toward the base and are unequal in size, and the final lobe is rounded and large, the result is a lyre-shaped external organ of the plant (for example, in radish).

The structure of a leaf with several plates is significantly different. There are palmate, trifoliate, and pinnate organs. If a complex leaf includes three blades, it is called trifoliate, or trifoliate (for example, maple). A leaf is considered palmate when its petioles are attached to the main petiole at one point, and the blades diverge radially (for example, lupine). If the lateral plates on the main petiole are present on both sides along the length, the leaf is called pinnately compound.

Solid plate shapes

In different plants, the shapes of leaf blades are not the same in the degree of dissection, outline, type of base and apex. They can have round, oval, triangular, elliptical and other shapes. The plate can be elongated, and its free end can be blunt, pointed, sharp or pointed. The base is extended and narrowed towards the stem, it can be heart-shaped or round.

Attaching to the stem

Considering the structure of a plant leaf, a few words should be said about how it is attached to the shoot. Attachment is carried out using long or short petioles. There are also sessile leaves. In some plants, their bases grow together with the shoot (descending leaf), and it happens that the shoot completely pierces the plate (pierced leaf).

Internal structure. Skin

The epidermis (upper skin) is a covering tissue located on the reverse side of a plant organ, often covered with cuticle, hairs, and wax. The internal structure of the leaf is such that on the outside it has a skin that protects it from drying out, mechanical damage, penetration of pathogens into internal tissues and other adverse effects.

The skin cells are living, they are different in shape and size: some are transparent, large, colorless, tightly adjacent to each other; others are smaller, with chloroplasts that give them a green color; such cells can change shape and are arranged in pairs.

Stoma

Skin cells can move away from each other, in which case a gap appears between them, which is called stomatal. When the cells are saturated with water, the stomata opens, and when the fluid outflows, it closes.

The anatomical structure of the leaf is such that air enters the internal cells through the stomatal slits and gaseous substances exit through them. When plants are not sufficiently supplied with water (this happens in hot and dry weather), the stomata close. In this way, representatives of the flora protect themselves from desiccation, since when the stomatal slits are closed, water vapor does not escape out and is stored in the intercellular spaces. Thus, during dry periods, plants retain water.

Main fabric

The internal structure of the leaf is not complete without columnar tissue, the cells of which are located on the upper side facing the light, fit tightly to each other, and have a cylindrical shape. All cells have a thin membrane, a nucleus, chloroplasts, cytoplasm, and a vacuole.

Another main tissue is spongy. Its cells are round in shape, loosely arranged, and between them there are large intercellular spaces filled with air.

What the structure of the plant leaf will be, how many layers of spongy and columnar tissues are formed, depends on the lighting. Leaves grown in light have much more developed columnar tissue than leaves grown in dark conditions.

The general decorative appearance of trees and shrubs is greatly influenced by the shape (ornament), size,

color and arrangement of leaves on branches.

The qualities of a leaf can not only enhance the effect of the basic architectural properties of the entire woody plant, but also be a leading factor in the composition of green spaces.

Along with the shape of the crown, the size, ornament and color of the leaf are of primary importance when selecting woody plants for single specimens and foreground groups.

The coloring of the leaves not only enhances the effect of the mass of the crown, its size, density, but is also an important decorative means when constructing an “aerial” perspective and contrasting color schemes in park compositions when creating areas of plantings of a certain color (“red”, “golden”, “blue”) ").

Finally, seasonal variability in leaf color is the most important means for enhancing the seasonal decorative effect in garden plantings.

Let us consider separately each of the indicated decorative qualities of the leaf.

FORM (ORNAMENT) OF LEAVES

Most species of woody plants have their own typical leaf shape, different from other species.

False leaves-phyllodes of Acacia longifolia

The following parts of the leaf are distinguished: leaf blade, petiole, leaf sheath and stipules. Some of these parts are sometimes missing.

The leaf blade of tree species can be flat (mainly in deciduous species) or needle-shaped (mainly in coniferous species).

The leaf petiole varies in length and is sometimes absent. Leaves with petioles are called petiolate, leaves without petioles or with very short petioles are called sessile. So, for example, in goat honeysuckle (Lonicera caprifolium L.) upper leaves are sessile (in pairs), fused at their bases around the stem.

Sometimes the leaf blade is absent, and its petiole grows in the form of a more or less wide green blade and performs its functions. These leaf-shaped petioles are called phyllodes and are found, for example, in some true acacias.

Stipules are formed at the base of the petiole. Most tree species do not have stipules. Their presence is typical for legumes, some Rosaceae and a few representatives of other families. Sometimes the stipules fall off soon after they form. During the entire growing season, stipules are retained in goat willow, as well as in some hawthorns, for example, spiny hawthorn, and are an interesting decorative detail of the leaf. In white acacia, the stipules are transformed into spines.

The leaf sheath is the expanded part of the base of the petiole that encloses the stem. In arboreal plants it is poorly developed, but is characteristic of the umbelliferous and grass families.

The place where a leaf is attached to the stem is called a node, and the parts of the stem between the nodes are called internodes.

In the leaf blade, a system of so-called “veins” or “nerves” is distinguishable - these are conductive vascular-fibrous bundles that connect through the petiole with the conductive vascular-fibrous system of the branches and trunk of the plant. They carry water and solutions of mineral salts into the leaves, and the organic substances produced in them (assimilates) are removed from the leaves.

The distribution and branching system of veins (“venation”) in a leaf is different for different groups of plants and can serve as one of the characteristic classification features.

The following main systems of venation, or nervation, of leaves are distinguished:

arcuerrous - a series of veins of equal thickness are directed in an arcuate manner from the base of the leaf to its apex; such venation is common in monocots (lily of the valley, tulip), less often in dicotyledons (plantain);

parallel nerve - the veins in the leaf blade run almost parallel, converging only at the top of the leaf; such venation is characteristic of cereals;

pinninervous - among the veins the main one stands out, which is a continuation of the petiole; From the pinnate, lateral veins of the first order branch off, which further branch into a network of small veins connecting with each other; distinguish: a) completely pinnate venation, in which the lateral veins end directly or forked in a forked manner at the edge of the leaf or in its teeth (in birch, elm, oak); b) imperfect pinnate venation, when the lateral veins, without reaching the edge of the leaf, branch out, connecting with each other in loops, or give thin branches that reach the edge of the leaf (in pear, willow, poplar, bird cherry, cherry, apple tree);

palmate nerve (palm nerve) - several identical, radially diverging veins emerge from the petiole; each of them branches into a network of small veins (in grapes, maple, linden, aspen, mulberry, fig).

The venation (or nervation) of a leaf is sometimes so clearly expressed, for example, in viburnum rugosa, that it has a certain decorative value.

Leaves are classified according to their structure: a) simple and b) complex.

A simple leaf has one petiole and one leaf blade attached to it.

A compound leaf has several individual leaf blades on its petiole, called “leaflets,” each of which has its own individual petiole attached to a common petiole.

Birch, beech, elm, hornbeam, pear, oak, maple, and linden have simple leaves.

Horse chestnut, false acacia (white acacia), walnut, pueraria, rose, rowan, and ash have complex leaves.

simple leaves

Based on the outline of the leaf blade, simple leaves are distinguished: rounded, in which the length and width of the leaf blade are approximately the same (aspen leaf); oval - slightly elongated in length, rounded at the base and apex (skumpia); ovate - the leaf blade is rounded at the base, gradually narrowing upward (beech, hornbeam); obovate - the upper part of the leaf is rounded, wide, gradually tapering towards the base (elm, black alder); oblong - the length exceeds the width by approximately 3 times and the ends of the leaves are rounded; lanceolate - narrow leaves, gradually tapering upward, the length of the leaf is 3-5 times greater than the width (in willow); needle-shaped (in pine).

Depending on the degree of integrity of the leaf blade, simple leaves are: a) whole and b) dismembered.

Leaves in which the leaf blade is whole (birch, beech, pear, linden) are called whole; leaves in which the leaf blade has more or less deep grooves are called dissected.

There are 3 types of dissected leaves:

lobed, in which the notches do not exceed ¹/3 of the distance from the edge of the leaf to its midrib; For example, oak has such leaves;

separate, having deeper grooves, but not reaching the midrib of the leaf (norway maple, sycamore maple, sycamore, amber wood);

dissected, in which the notches reach the midrib of the leaf; Such leaves have the so-called dissected leaf forms of many types of deciduous trees.

Compound Leaves

Among the compound leaves, in which individual leaflets are attached by their own petioles and to the common petiole of the leaf, they are distinguished: trifoliate, palmate, and pinnate.

U A trifoliate leaf consists of three leaflets - for example, ptelea, pueraria, broom; in palmate-compound petioles the petioles are attached to the main petiole at one point and the leaflets diverge radially, for example horse chestnut; in pinnately compound leaves are located along the length of the petiole, on both sides; At the same time, a distinction is made between pair-pinnate leaves, arranged in pairs on a common petiole (for example, silver moss, yellow acacia, and imparipinnate leaves, in which the tip of the leaf ends with one unpaired leaflet - black locust, mountain ash, common ash, walnut, Amur velvet, ash-leaved maple .)

If the individual leaves of a pinnately compound leaf are also complex, then such leaves are called: a) double pinnate false - when tank petioles are attached to the main petiole, and leaflets are attached to them by their sharpenings, for example, leaves of Albizia Lankaran (silk acacia), Melia Himalayan , silver acacia and b) triple pinnate - when lateral petioles sit on the main petiole, and to the latter, in turn, are attached more lateral petioles, on which leaflets sit, for example, leaves of Manchurian aralia, bunduk.

Double and triple pinnate leaves, in turn, are paired and unpaired.

Depending on the nature of the edge of the leaf blade of simple and complex leaves, leaves are distinguished: entire-edged, without any grooves (beech); orbicular, in which the notches are sharp and the protrusions are rounded (aspen); jagged, having wedge-shaped notches, and the projections are triangular, sharp (elm); serrated with oblique, sharp projections (Caucasian linden).

Tree species often exhibit deviations from the typical leaf shape characteristic of a given species. Sometimes these are temporary deviations associated with the age of the entire plant or part of it. Thus, in some conifers (for example, cryptomeria, as well as cypress and thuja), the graceful thread-like or thin and soft needle-like needles of young one- or two-year-old plants are replaced with age by the coarse needle-like or flat scaly needles typical of this species.

The young, or so-called “juvenile” form of the needles is very beautiful; in ornamental gardening, spectacular ornamental trees and bushes with these needles are grown from cuttings taken from very young plants with juvenile needles.

A wide variety of leaf shapes on the same tree, due to the age of the shoots, is also observed in eucalyptus trees.

In some tree species, on the same branches, in addition to leaves of a typical shape for a given species, leaves of a different shape also appear.

So, for example, in white mulberry (Morus alba L.), In addition to the typical leaves with a single leaf blade, lobed leaves appear. Especially often and in a very bizarre shape, lobed leaves appear on young shoots of the crown and shoots of white mulberry.

At the white poplar (Populus alba L.),“In addition to the typical leaves with a single blade, lobed leaves of various shapes are found on the same tree, especially on coppice shoots and on young branches of the species.

The presence of lobed leaves of various shapes on the same tree, along with pelvic leaves, is also observed in the fig tree (Ficus carica L.) and ivy.

The presence of leaves of different shapes on the same plant is called heterophyly.

In addition to the variety of leaf shapes observed in some tree species on the same specimen, almost all tree species have discontinuous forms in which all leaves differ from the leaf shape typical of a given species.

Specimens with deviations in leaf shape, independent of the age of the plant and the environment and uniform for the entire plant, are distinguished as independent varieties (forms) of this species.

Varieties in leaf shape, as well as the previously noted varieties in crown shape, occurred as a result of an abrupt change in the typical forms of a given species during seed propagation or vegetatively from the growth bud of shoots. Changes in leaf shape are so great that it is often difficult without the presence of flowers or fruits to establish whether a given plant belongs to one species or another.

These deviations in leaf shape from the typical one for a given species are inherited (through seed propagation), but usually in gardening practice they are fixed by grafting and rooting cuttings. Abrupt deviations in leaf shape from the typical ones are very important in ornamental gardening. They provide rich material for architectural compositions, most often used in single plantings and in small groups in the background, making it easy to observe the originality of the foliage of these forms.

Changes in leaf shape are most often expressed in the narrowing of the leaf blade (the so-called narrow-leaved and lanceolate forms), the formation of blades, strong dissection of the leaf blade (dissected forms), the similarity of the leaf shape of a given species with the shape of a leaf of another species from another family and a change in the size of the leaf blade.

At the same time, changes in leaf size of a shaped nature should not be confused with changes associated with the age of the plant, the location of the leaf in the crown and the influence of the environment.

The most decorative leaves are of the following shapes: lanceolate, or linear, - lanceola - tus, linearis; filamentous - fitifortnis, tenui folius; broadleaf - tatifolius; narrow-leaved - angustifolius; lobed - lobatus; dissected - laciniatus, dissectus; pinnate- pinnatus, plumosas; curly, curly - crispus; ugly - monstrosus, as well as leaves similar to leaves of other species, for example beech, oak-leaved form (Fagus silvatica L.. var. quercifolia); black alder, rowan leaf form (Alnus glutinosa Gaertn.Var. sorbifolia).

Depending on the size of the leaf, the following forms are distinguished: large-leaved - macrophylla and small-leaved - microphylta.

The most interesting varieties of tree species by shape (ornament) and leaf size are given.

The leaf is an extremely important organ of the plant. The leaf is part of the shoot. Its main functions are photosynthesis and transpiration. The leaf is characterized by high morphological plasticity, variety of shapes and great adaptive capabilities. The base of the leaf can expand in the form of oblique leaf-like formations - stipules on each side of the leaf. In some cases they are so large that they play a role in photosynthesis. The stipules are free or adherent to the petiole; they can move to the inner side of the leaf and are then called axillary. The bases of the leaves can be turned into a sheath that surrounds the stem and prevents it from bending.

External leaf structure

Leaf blades vary in size: from a few millimeters to 10-15 meters and even 20 (for palm trees). The lifespan of leaves does not exceed several months, in some - from 1.5 to 15 years. Leaf size and shape are hereditary traits.

Leaf parts

A leaf is a lateral vegetative organ growing from a stem, having bilateral symmetry and a growth zone at the base. A leaf usually consists of a leaf blade, a petiole (with the exception of sessile leaves); A number of families are characterized by stipules. Leaves can be simple, having one leaf blade, and complex - with several leaf blades (leaflets).

Leaf blade- an expanded, usually flat part of a leaf that performs the functions of photosynthesis, gas exchange, transpiration and, in some species, vegetative propagation.

Leaf base (leaf cushion)- part of the leaf connecting it to the stem. Here is the educational tissue that gives growth to the leaf blade and petiole.

Stipules- paired leaf-shaped formations at the base of the leaf. They may fall off when the leaf unfolds or remain. They protect the axillary lateral buds and intercalary educational tissue of the leaf.

petiole- the narrowed part of the leaf, connecting the leaf blade with the stem at its base. It performs the most important functions: it orients the leaf in relation to the light, it is the location of the intercalary educational tissue, due to which the leaf grows. In addition, it has a mechanical significance for weakening impacts on the leaf blade from rain, hail, wind, etc.

Simple and compound leaves

A leaf may have one (simple), several or many leaf blades. If the latter are equipped with joints, then such a sheet is called complex. Thanks to the joints on the common leaf petiole, the leaflets of compound leaves fall off one by one. However, in some plants, complex leaves may fall off entirely.

The shape of the leaves is whole; they are distinguished as lobed, divided and dissected.

Bladed I call a sheet in which the cutouts along the edges of the plate reach one quarter of its width, and with a larger recess, if the cutouts reach more than a quarter of the width of the plate, the sheet is called separate. The blades of a split sheet are called lobes.

Dissected called a leaf in which the cutouts along the edges of the blade reach almost to the midrib, forming segments of the blade. Separate and dissected leaves can be palmate and pinnate, double palmate and double pinnate, etc. Accordingly, a palmately divided leaf and a pinnately dissected leaf are distinguished; unpaired pinnately dissected leaf of potato. It consists of a terminal lobe, several pairs of lateral lobes, between which are located even smaller lobes.

If the plate is elongated and its lobes or segments are triangular, the leaf is called plow-shaped(dandelion); if the lateral lobes are unequal in size and decrease towards the base, and the final lobe is large and rounded, a lyre-shaped leaf (radish) is obtained.

As for complex leaves, among them there are trifoliate, palmate and pinnately compound leaves. If a compound leaf consists of three leaflets, it is called trifoliate, or trifoliate (maple). If the petioles of the leaflets are attached to the main petiole as if at one point, and the leaflets themselves diverge radially, the leaf is called palmate (lupine). If on the main petiole the lateral leaflets are located on both sides along the length of the petiole, the leaf is called pinnately compound.

If such a leaf ends at the top with an unpaired single leaf, it turns out to be an imparipinnate leaf. If there is no terminal leaf, the leaf is called pinnate.

If each leaflet of a pinnately compound leaf is, in turn, compound, then the result is a doubly pinnately compound leaf.

Shapes of solid leaf blades

A compound leaf is one whose petiole has several leaf blades. They are attached to the main petiole with their own petioles, often fall off independently, one by one, and are called leaves.

The shapes of leaf blades of different plants differ in outline, degree of dissection, and the shape of the base and apex. The shapes can be oval, round, elliptical, triangular and others. The leaf blade is elongated. Its free end can be sharp, blunt, pointed, pointed. Its base is narrowed and drawn towards the stem, and can be round or heart-shaped.

Attaching leaves to stem

The leaves are attached to the shoot by long or short petioles or are sessile.

In some plants, the base of a sessile leaf grows over a long distance with the shoot (descending leaf) or the shoot pierces the leaf blade right through (pierced leaf).

Shape of leaf blade edge

Leaf blades are distinguished by the degree of dissection: shallow cuts - jagged or finger-like edges of the leaf, deep cuts - lobed, separated and dissected edges.

If the edges of the leaf blade do not have any notches, the leaf is called entire. If the notches along the edge of the sheet are shallow, the sheet is called whole.

Bladed leaf - a leaf whose blade is divided into lobes up to 1/3 of the width of the half-leaf.

Separated leaf - a leaf with a blade divided to ½ the width of a half-sheet.

Dissected leaf - a leaf whose blade is dissected to the main vein or to the base of the leaf.

The edge of the leaf blade is serrated (sharp corners).

The edge of the leaf blade is crenate (rounded projections).

The edge of the leaf blade is notched (rounded notches).

Venation

On each leaf it is easy to notice numerous veins, especially distinct and raised on the underside of the leaf.

Veins- these are conductive bundles connecting the leaf to the stem. Their functions are conductive (supplying leaves with water and mineral salts and removing assimilation products from them) and mechanical (the veins support the leaf parenchyma and protect the leaves from rupture). Among the variety of venation, a leaf blade is distinguished with one main vein, from which lateral branches diverge in a pinnate or pinnate type; with several main veins, differing in thickness and direction of distribution along the plate (arc-neural, parallel types). Between the described types of venation, there are many intermediate or other forms.

The initial part of all the veins of the leaf blade is located in the leaf petiole, from where in many plants the main, main vein emerges, then branching out into the thickness of the blade. As you move away from the main vein, the lateral veins become thinner. The thinnest ones are mostly located on the periphery, and also far from the periphery - in the middle of areas surrounded by small veins.

There are several types of venation. In monocotyledonous plants, venation is arcuate, in which a series of veins enter the blade from the stem or sheath, arcuately directed toward the top of the blade. Most cereals have parallel veins. Arc venation also exists in some dicotyledonous plants, for example, plantain. However, they also have a connection between the veins.

In dicotyledonous plants, the veins form a highly branched network and, accordingly, the venation is distinguished as reticular-neural, which indicates a better supply of vascular bundles.

Shape of base, apex, leaf petiole

According to the shape of the top of the blade, the leaves are blunt, sharp, pointed and pointed.

Based on the shape of the base of the plate, leaves are distinguished into wedge-shaped, heart-shaped, spear-shaped, arrow-shaped, etc.

Internal structure of the leaf

Leaf skin structure

The outer skin (epidermis) is the covering tissue on the reverse side of the leaf, often covered with hairs, cuticle, and wax. On the outside, the leaf has a skin (covering tissue), which protects it from the adverse effects of the external environment: from drying out, from mechanical damage, from penetration of pathogenic microorganisms into the internal tissues. Skin cells are living, they vary in size and shape. Some of them are larger, colorless, transparent and fit tightly to each other, which increases the protective qualities of the integumentary tissue. The transparency of the cells allows sunlight to penetrate into the leaf.

Other cells are smaller and contain chloroplasts, which give them their green color. These cells are arranged in pairs and have the ability to change their shape. In this case, the cells either move away from each other and a gap appears between them, or they move closer to each other and the gap disappears. These cells were called guard cells, and the gap that appeared between them was called stomatal. The stomata opens when the guard cells are saturated with water. When water drains from the guard cells, the stomata closes.

Stomatal structure

Through the stomatal slits, air enters the internal cells of the leaf; through them, gaseous substances, including water vapor, escape from the leaf to the outside. If the plant is insufficiently supplied with water (which can happen in dry and hot weather), the stomata close. By this, plants protect themselves from desiccation, since water vapor does not escape outside when the stomatal slits are closed and is stored in the intercellular spaces of the leaf. In this way, plants retain water during dry periods.

Main sheet fabric

Columnar fabric- the main tissue, the cells of which are cylindrical in shape, tightly adjacent to each other and located on the upper side of the leaf (facing the light). Serves for photosynthesis. Each cell of this tissue has a thin membrane, cytoplasm, nucleus, chloroplasts, and vacuole. The presence of chloroplasts gives the green color to the tissue and the entire leaf. The cells that are adjacent to the upper skin of the leaf, elongated and arranged vertically, are called columnar tissue.

Spongy tissue- the main tissue, the cells of which have a rounded shape, are loosely located and large intercellular spaces are formed between them, also filled with air. Water vapor coming from the cells accumulates in the intercellular spaces of the main tissue. Serves for photosynthesis, gas exchange and transpiration (evaporation).

The number of cell layers of columnar and spongy tissues depends on lighting. In leaves grown in light, columnar tissue is more developed than in leaves grown in dark conditions.

Conductive fabric- the main tissue of the leaf, penetrated by veins. Veins are conductive bundles, since they are formed by conductive tissues - bast and wood. The bast carries out the transfer of sugar solutions from the leaves to all organs of the plant. The movement of sugar occurs through the sieve tubes of the bast, which are formed by living cells. These cells are elongated, and in the place where they touch each other with their short sides in the membranes, there are small holes. Through holes in the membranes, the sugar solution passes from one cell to another. Sieve tubes are adapted to transport organic matter over long distances. Living cells of smaller sizes adhere tightly along the entire length to the side wall of the sieve tube. They accompany the cells of the tube and are called companion cells.

Structure of leaf veins

In addition to bast, the conductive bundle also includes wood. Water with minerals dissolved in it moves through the vessels of the leaf, as well as in the root. The plant absorbs water and minerals from the soil through its roots. Then from the roots, through the vessels of the wood, these substances enter the above-ground organs, including the cells of the leaf.

The numerous veins contain fibers. These are long cells with pointed ends and thickened lignified membranes. Large leaf veins are often surrounded by mechanical tissue, which consists entirely of thick-walled cells - fibers.

Thus, along the veins there is a transfer of sugar solution (organic matter) from the leaf to other plant organs, and from the root - water and minerals to the leaves. Solutions move from the leaf through sieve tubes, and to the leaf through wood vessels.

The lower skin is the covering tissue on the underside of the leaf, usually bearing stomata.

Leaf activity

Green leaves are organs of air nutrition. The green leaf performs an important function in the life of plants - organic substances are formed here. The structure of the leaf corresponds well to this function: it has a flat leaf blade, and the pulp of the leaf contains a huge number of chloroplasts with green chlorophyll.

Substances necessary for the formation of starch in chloroplasts

Target: Let's find out what substances are necessary for the formation of starch?

What we do: Let's place two small indoor plants in a dark place. After two or three days, we will place the first plant on a piece of glass, and next to it we will place a glass with a solution of caustic alkali (it will absorb all the carbon dioxide from the air), and we will cover it all with a glass cap. To prevent air from entering the plant from the environment, lubricate the edges of the cap with Vaseline.

We will also place the second plant under a hood, but only next to the plant we will place a glass of soda (or a piece of marble) moistened with a solution of hydrochloric acid. As a result of the interaction of soda (or marble) with acid, carbon dioxide is released. A lot of carbon dioxide is formed in the air under the hood of the second plant.

We place both plants in the same conditions (in the light).

The next day, take a leaf from each plant and first treat it with hot alcohol, rinse and apply iodine solution.

What we see: in the first case, the color of the leaf did not change. The leaf of the plant that was under the cap, where there was carbon dioxide, became dark blue.

Conclusion: this proves that carbon dioxide is necessary for the plant to form organic matter (starch). This gas is part of atmospheric air. Air enters the leaf through the stomatal slits and fills the spaces between the cells. From the intercellular spaces, carbon dioxide penetrates into all cells.

Formation of organic substances in leaves

Target: find out in which cells of the green leaf organic substances (starch, sugar) are formed.

What we do: Place the indoor plant fringed geranium in a dark closet for three days (so that there is an outflow of nutrients from the leaves). After three days, remove the plant from the closet. Attach a black paper envelope with the word “light” cut out to one of the leaves and place the plant in the light or under an electric light bulb. After 8-10 hours, cut the leaf. Let's remove the paper. Place the leaf in boiling water and then in hot alcohol for a few minutes (chlorophyll dissolves well in it). When the alcohol turns green and the leaf becomes discolored, rinse it with water and place it in a weak iodine solution.

What we see: blue letters will appear on a discolored sheet (starch turns blue from iodine). Letters appear on the part of the sheet on which the light fell. This means that starch has formed in the illuminated part of the leaf. It is necessary to pay attention to the fact that the white strip along the edge of the sheet is not colored. This explains the fact that there is no chlorophyll in the plastids of the cells of the white stripe of the geranium leaf. Therefore, starch is not detected.

Conclusion: Thus, organic substances (starch, sugar) are formed only in cells with chloroplasts, and light is required for their formation.

Special studies by scientists have shown that sugar is formed in chloroplasts in light. Then, as a result of transformations from sugar in chloroplasts, starch is formed. Starch is an organic substance that does not dissolve in water.

There are light and dark phases of photosynthesis.

During the light phase of photosynthesis, light is absorbed by pigments, excited (active) molecules with excess energy are formed, and photochemical reactions take place in which excited pigment molecules take part. Light reactions occur on the membranes of the chloroplast, where chlorophyll is located. Chlorophyll is a highly active substance that absorbs light, stores primary energy and further converts it into chemical energy. Yellow pigments, carotenoids, also take part in photosynthesis.

The process of photosynthesis can be represented as a summary equation:

6CO 2 + 6H 2 O = C 6 H 12 O 6 + 6O 2

Thus, the essence of light reactions is that light energy is converted into chemical energy.

Dark reactions of photosynthesis occur in the matrix (stroma) of the chloroplast with the participation of enzymes and products of light reactions and lead to the synthesis of organic substances from carbon dioxide and water. Dark reactions do not require the direct participation of light.

The result of dark reactions is the formation of organic compounds.

The process of photosynthesis occurs in chloroplasts in two stages. In the grana (thylakoids) reactions caused by light occur - light, and in the stroma - reactions not associated with light - dark, or carbon fixation reactions.

Light reactions

1. Light, falling on the chlorophyll molecules that are located in the membranes of grana thylakoids, leads them to an excited state. As a result of this, electrons ē leave their orbits and are transferred by carriers outside the thylakoid membrane, where they accumulate, creating a negatively charged electric field.

2. The place of the released electrons in chlorophyll molecules is taken by water electrons ē, since water undergoes photodecomposition (photolysis) under the influence of light:

H 2 O↔OH‾+H + ; OH‾−ē→OH.

Hydroxyls OH‾, becoming OH radicals, combine: 4OH→2H 2 O+O 2, forming water and free oxygen, which is released into the atmosphere.

3. H+ protons do not penetrate the thylakoid membrane and accumulate inside, using a positively charged electric field, which leads to an increase in the potential difference on both sides of the membrane.

4. When a critical potential difference (200 mV) is reached, H + protons rush out through the proton channel in the ATP synthetase enzyme, built into the thylakoid membrane. At the exit from the proton channel, a high level of energy is created, which is used for the synthesis of ATP (ADP+P→ATP). The resulting ATP molecules move into the stroma, where they participate in carbon fixation reactions.

5. Protons H + that come to the surface of the thylakoid membrane combine with electrons ē, forming atomic hydrogen H, which goes to the reduction of NADP + carriers: 2ē+2H + =NADP + →NADP∙H 2 (carrier with attached hydrogen; reduced carrier ).

Thus, the chlorophyll electron activated by light energy is used to attach hydrogen to the carrier. NADP∙H2 passes into the stroma of the chloroplast, where it participates in carbon fixation reactions.

Carbon fixation reactions (dark reactions)

It is carried out in the stroma of the chloroplast, where ATP, NADP∙H 2 from granal thylakoids and CO 2 from the air arrive. In addition, there are always five-carbon compounds there - pentoses C 5, which are formed in the Calvin cycle (CO 2 fixation cycle). This cycle can be simplified as follows:

1. CO 2 is added to pentose C5, resulting in the appearance of an unstable hexagonal compound C6, which splits into two three-carbon groups 2C3 - trioses.

2. Each of the 2C 3 trioses accepts one phosphate group from two ATPs, which enriches the molecules with energy.

3. Each of the trioses 2C 3 attaches one hydrogen atom from two NADP∙H2.

4. After which some trioses combine to form carbohydrates 2C 3 → C 6 → C 6 H 12 O 6 (glucose).

5. Other trioses combine to form pentoses 5C 3 → 3C 5 and are again included in the CO 2 fixation cycle.

Total reaction of photosynthesis:

6CO 2 +6H 2 O chlorophyll light energy →C 6 H 12 O 6 +6O 2

In addition to carbon dioxide, water takes part in the formation of starch. The plant receives it from the soil. The roots absorb water, which rises through the vessels of the vascular bundles into the stem and further into the leaves. And already in the cells of a green leaf, in chloroplasts, organic matter is formed from carbon dioxide and water in the presence of light.

What happens to organic substances formed in chloroplasts?

Starch formed in chloroplasts, under the influence of special substances, is converted into soluble sugar, which enters the tissues of all organs of the plant. In some tissue cells, sugar can be converted back into starch. Reserve starch accumulates in colorless plastids.

From sugars formed during photosynthesis, as well as mineral salts absorbed by roots from the soil, the plant creates the substances it needs: proteins, fats and many other proteins, fats and many others.

Part of the organic substances synthesized in the leaves is spent on the growth and nutrition of the plant. The other part is put into reserve. In annual plants, reserve substances are deposited in seeds and fruits. In biennials in the first year of life, they accumulate in the vegetative organs. In perennial herbs, substances are stored in underground organs, and in trees and shrubs - in the core, the main tissue of the bark and wood. In addition, at a certain year of life, they also begin to accumulate organic substances in fruits and seeds.

Types of plant nutrition (mineral, air)

In living plant cells, metabolism and energy constantly occur. Some substances are absorbed and used by the plant, others are released into the environment. Complex substances are formed from simple substances. Complex organic substances are broken down into simple ones. Plants accumulate energy, and during photosynthesis, release it during respiration, using this energy to carry out various life processes.

Gas exchange

Thanks to the work of the stomata, leaves also perform such an important function as gas exchange between the plant and the atmosphere. Through the stomata of the leaf, carbon dioxide and oxygen enter with atmospheric air. Oxygen is used during respiration, carbon dioxide is necessary for the plant to form organic substances. Oxygen, which is formed during photosynthesis, is released into the air through the stomata. Carbon dioxide that appears in the plant during respiration is also removed. Photosynthesis occurs only in light, and respiration occurs in light and in darkness, i.e. constantly. Respiration occurs continuously in all living cells of plant organs. Like animals, plants die when breathing stops.

In nature, there is an exchange of substances between a living organism and the environment. The absorption of certain substances by the plant from the external environment is accompanied by the release of others. Elodea, being an aquatic plant, uses carbon dioxide dissolved in water for nutrition.

Target: Let's find out what substance Elodea releases into the external environment during photosynthesis?

What we do: We cut the stems of the branches under water (boiled water) at the base and cover them with a glass funnel. Place a test tube filled to the brim with water on the funnel tube. This can be done in two ways. Place one container in a dark place, and expose the other to bright sunlight or artificial light.

Add carbon dioxide to the third and fourth containers (add a small amount of baking soda or you can breathe into a tube) and also place one in the dark and the other in sunlight.

What we see: after some time, in the fourth option (a vessel standing in bright sunlight), bubbles begin to appear. This gas displaces water from the test tube, its level in the test tube is displaced.

What we do: When the water is completely replaced by gas, you must carefully remove the test tube from the funnel. Close the hole tightly with the thumb of your left hand, and quickly insert a smoldering splinter into the test tube with your right hand.

What we see: the splinter lights up with a bright flame. Looking at the plants that were placed in the dark, we will see that gas bubbles are not released from the elodea, and the test tube remains filled with water. The same thing with test tubes in the first and second versions.

Conclusion: it follows that the gas released by elodea is oxygen. Thus, the plant releases oxygen only when all the conditions for photosynthesis are present - water, carbon dioxide, light.

Evaporation of water by leaves (transpiration)

The process of water evaporation by leaves in plants is regulated by the opening and closing of stomata. By closing the stomata, the plant protects itself from water loss. The opening and closing of stomata is influenced by external and internal environmental factors, primarily temperature and intensity of sunlight.

Plant leaves contain a lot of water. It comes through the conduction system from the roots. Inside the leaf, water moves along the cell walls and through the intercellular spaces to the stomata, through which it leaves in the form of steam (evaporates). This process is easy to check if you make a simple device, as shown in the figure.

The evaporation of water by a plant is called transpiration. Water evaporates from the surface of a plant leaf, especially intensively from the surface of the leaf. A distinction is made between cuticular transpiration (evaporation by the entire surface of the plant) and stomatal transpiration (evaporation through stomata). The biological significance of transpiration is that it is a means of transporting water and various substances throughout the plant (suction action), promotes the entry of carbon dioxide into the leaf, carbon nutrition of plants, and protects the leaves from overheating.

The rate of water evaporation by leaves depends on:

• biological characteristics of plants;
• growth conditions (plants in arid areas evaporate little water, in humid areas - much more; shady plants evaporate less water than light ones; plants evaporate a lot of water in hot weather, much less in cloudy weather);
• lighting (diffused light reduces transpiration by 30-40%);
• water content in leaf cells;
• osmotic pressure of cell sap;
• soil, air and plant body temperatures;
• air humidity and wind speed.

The greatest amount of water evaporates in some tree species through leaf scars (the scar left by fallen leaves on the stem), which are the most vulnerable spots on the tree.

The relationship between the processes of respiration and photosynthesis

The entire process of respiration takes place in the cells of the plant organism. It consists of two stages during which organic matter is broken down into carbon dioxide and water. At the first stage, with the participation of special proteins (enzymes), glucose molecules break down into simpler organic compounds and a little energy is released. This stage of the respiratory process occurs in the cytoplasm of cells.

At the second stage, simple organic substances formed in the first stage, under the influence of oxygen, decompose into carbon dioxide and water. This releases a lot of energy. The second stage of the respiratory process occurs only with the participation of oxygen and in special cell bodies.

Absorbed substances, in the process of transformations in cells and tissues, become substances from which the plant builds its body. All transformations of substances occurring in the body are always accompanied by energy consumption. A green plant, as an autotrophic organism, absorbs light energy from the Sun and accumulates it in organic compounds. During the process of respiration during the breakdown of organic substances, this energy is released and used by the plant for vital processes that occur in cells.

Both processes - photosynthesis and respiration - proceed through a succession of numerous chemical reactions in which some substances are converted into others.

Thus, during the process of photosynthesis, sugars are formed from carbon dioxide and water received by the plant from the environment, which are then converted into starch, fiber or proteins, fats and vitamins - substances the plant needs for nutrition and energy storage. In the process of respiration, on the contrary, the breakdown of organic substances created during photosynthesis into inorganic compounds - carbon dioxide and water. In this case, the plant receives the released energy. These transformations of substances in the body are called metabolism. Metabolism is one of the most important signs of life: with the cessation of metabolism, the life of the plant ceases.

The influence of environmental factors on leaf structure

The leaves of plants in humid areas are usually large with a large number of stomata. A lot of moisture evaporates from the surface of these leaves.

The leaves of plants in arid places are small in size and have adaptations that reduce evaporation. These are dense pubescence, a waxy coating, a relatively small number of stomata, etc. Some plants have soft and succulent leaves. They store water.

The leaves of shade-tolerant plants have only two or three layers of rounded cells, loosely adjacent to each other. Large chloroplasts are located in them so that they do not shade each other. Shade leaves tend to be thinner and darker green in color because they contain more chlorophyll.

In plants in open areas, the leaf pulp has several layers of columnar cells tightly adjacent to each other. They contain less chlorophyll, so light leaves are lighter in color. Both leaves can sometimes be found in the crown of the same tree.

Protection against dehydration

The outer wall of each leaf skin cell is not only thickened, but also protected by a cuticle, which does not allow water to pass through well. The protective properties of the skin are significantly increased by the formation of hairs that reflect the sun's rays. Due to this, the heating of the sheet is reduced. All this limits the possibility of water evaporation from the leaf surface. When there is a lack of water, the stomatal fissure closes and steam does not escape outside, accumulating in the intercellular spaces, which leads to the cessation of evaporation from the leaf surface. Plants in hot and dry habitats have a small plate. The smaller the leaf surface, the less danger of excessive water loss.

Leaf modifications

In the process of adapting to environmental conditions, the leaves of some plants have changed because they began to play a role that is not characteristic of typical leaves. In barberry, some of the leaves have changed into spines.

Leaf senescence and leaf fall

Leaf fall is preceded by leaf senescence. This means that in all cells the intensity of life processes - photosynthesis, respiration - decreases. The content of substances already present in the cells that are important for the plant decreases and the supply of new ones, including water, is reduced. The breakdown of substances prevails over their formation. Unnecessary and even harmful products accumulate in cells; they are called the end products of metabolism. These substances are removed from the plant when its leaves are shed. The most valuable compounds flow through the conducting tissues from the leaves to other organs of the plant, where they are deposited in the cells of storage tissues or are immediately used by the body for nutrition.

In most trees and shrubs, during the aging period, the leaves change color and become yellow or purple. This happens because chlorophyll is destroyed. But besides it, plastids (chloroplasts) contain yellow and orange substances. In the summer they were, as it were, disguised by chlorophyll and the plastids were green. In addition, other yellow or red-crimson coloring substances accumulate in the vacuoles. Together with plastid pigments, they determine the color of autumn leaves. Some plants have leaves that remain green until they die.

Even before the leaf falls from the shoot, a layer of cork forms at its base at the border with the stem. A separating layer is formed from it outside. Over time, the cells of this layer separate from each other, as the intercellular substance that connects them, and sometimes the cell membranes, becomes slimy and destroyed. The leaf is separated from the stem. However, it still remains on the shoot for some time thanks to the conducting bundles between the leaf and the stem. But there comes a moment when this connection is disrupted. The scar at the site of the detached leaf is covered with a protective cloth, cork.

As soon as the leaves reach their maximum size, aging processes begin, leading ultimately to the death of the leaf - its yellowing or redness associated with the destruction of chlorophyll, the accumulation of carotenoids and anthocyanins. As the leaf ages, the intensity of photosynthesis and respiration also decreases, chloroplasts degrade, some salts accumulate (calcium oxalate crystals), and plastic substances (carbohydrates, amino acids) flow out of the leaf.

During the aging process of a leaf, near its base in dicotyledonous woody plants, a so-called separating layer is formed, which consists of easily exfoliated parenchyma. Along this layer, the leaf is separated from the stem, and on the surface of the future leaf scar A protective layer of cork fabric is formed in advance.

On the leaf scar, cross sections of the leaf trace are visible in the form of dots. The sculpture of the leaf scar is different and is a characteristic feature for the taxonomy of lepidophytes.

In monocots and herbaceous dicotyledons, a separating layer, as a rule, does not form; the leaf dies and is destroyed gradually, remaining on the stem.

In deciduous plants, the shedding of leaves in the winter has an adaptive significance: by shedding their leaves, the plants sharply reduce the evaporating surface and protect themselves from possible damage under the weight of snow. In evergreen plants, massive leaf fall is usually timed to coincide with the beginning of the growth of new shoots from the buds and therefore occurs not in autumn, but in spring.

Autumn leaf fall in the forest has important biological significance. Fallen leaves are a good organic and mineral fertilizer. Every year in their deciduous forests, fallen leaves serve as material for mineralization produced by soil bacteria and fungi. In addition, fallen leaves stratify seeds that fell before leaf fall, protect roots from freezing, prevent the development of moss cover, etc. Some types of trees shed not only leaves, but also one-year-old shoots.

1 - raspberry; 2 - horse chestnut; 3 - acacia; 4 - strawberries.

The shape of the leaves of different plants is not similar to each other. But even the most diverse leaves can always be combined into two large groups. One group is formed simple leaves, the other - complex.

How to distinguish a simple sheet from a complex one? There is only one leaf blade on the petiole of each simple leaf. And compound leaves have several leaf blades located on one petiole, which are called leaflets.

Among simple leaves, there are whole, lobed, divided and dissected leaves.

Many trees have whole leaves: birch, linden, poplar, apple, pear, cherry, bird cherry, aspen and others. The leaf is considered whole, if its plate is entire or has shallow notches.

Bladed they call a leaf in which, like an oak, the cut-out blades along the edges of the blade reach one quarter of its width.

If the cuts in the leaf blade do not reach the midrib or base of the leaf slightly, the leaves are called separate. If the leaf is cut to the midrib or to the base, it is called dissected.

Lobed leaves- these are the leaves of maple, oak, hawthorn, currant, gooseberry and some other plants.

Take several leaves of different plants, for example: raspberry, rowan, ash, poplar, maple, oak. Compare the leaves of rowan, raspberry, and ash with the leaves of poplar, linden, maple and oak. How are they different from each other? The leaves of ash, rowan and raspberry have several leaf blades - leaflets - on one petiole. These are compound leaves. The leaves of poplar, maple and oak are simple. In simple leaves, the leaf blade falls off along with the petiole during leaf fall, while in complex leaves, the individual leaves that make up the leaf may fall off earlier than the petiole.

A compound leaf consisting of three leaf blades, like a clover, is calledtrisyllabic or trifoliate.

If a leaf is formed by several leaf blades attached at one point, as, for example, in lupine, it is calledfinger-complex.If the leaflets of a compound leaf are attached along the entire length of the petiole, then such a leaf ispinnately complex.

Among pinnately compound leaves, a distinction is made between imparipinnate and paripirnate.

Nepar pinnate leaves are those that end in a leaf blade that does not have its own pair. An example of imparipinnate leaves would be the leaves of rowan, ash, and raspberry. Piripnately compound leaves are less common, but you still know some plants with such leaves. These are, for example, field peas, mouse peas and sweet peas.

Both simple and compound leaves of dicotyledonous and monocotyledonous plants are arranged on the stems in a certain order. The portions of the stem that bear leaves are calledstem nodes,and the sections of the stem between the nodes - internodes.

The arrangement of leaves on the stem is calledleaf arrangement.

Most plants have another leaf arrangement, for example: rye, wheat, birch, apple, sunflower, ficus, rose. Their leaves are arranged spirally around the stem one at a time, as if alternating with each other, which is why this arrangement is called alternate.

The leaves of lilac, jasmine, maple, fuchsia, and dead nettle are located on the stem not one at a time, but two at a time: one leaf opposite the other. This leaf arrangement is called opposite.

Sometimes there are plants with whorled leaf arrangement. Their leaves grow on the stem in bunches, whorls, arranged in three or more leaves per node, and form a kind of ring (whorl) around the stem. Among indoor plants, oleander has a whorled leaf arrangement, in an aquarium - elodea, among wild plants - northern bedstraw

Leaf arrangement:

1 - next; 2 - opposite; 3 - whorled; a - internode; b - node.

Based on their structure, leaves are divided into simple and complex.

Leaves that have one leaf blade that falls off entirely in the fall are called simple.

In lobed leaves, the blade has cuts that divide it into lobes (maple, oak).

Leaves in which the leaf blade consists of several leaflets attached to the main petiole with the help of their petioles and falling off in parts in the fall are called compound.

Compound leaves are: