Various leaf edge shapes. Types of leaves according to the shape of the leaf blade, the shape of the edge and base - examples of plants with different leaf shapes

Leaves

It is easier to identify trees and shrubs by their leaves than by other organs. The flowers and fruits of many tree species are inconspicuous and located high. Their flowering time often coincides with school holidays, making display and collection of natural material difficult. Tree leaves are usually large with obvious morphological characteristics. The shape of the leaves is more or less characteristic of each plant species.

Classes can be carried out partially during lessons when studying the external structure and shape of leaves, during extracurricular activities, before and after excursions to the forest, parks, squares, as well as on excursions in summer pioneer camps.

Purpose of the lesson

Observe, describe and compare the morphological characteristics of leaves in various trees and shrubs.

To instill in schoolchildren some skills and abilities in using determinants (familiarity with signs, terms). Develop observation skills.

Equipment

For every student: sets of dry leaves numbered in folders; description plan; plant lists; tasks; simple pencils with eraser; tweezers, magnifying glasses.

For the whole class: instructional visual aids - drawings of the shapes of simple and complex leaves, base, apex, edge, venation, division of the leaf blade; herbariums and collections of leaves and shoots.

Hardwood

In the fall, without damaging the plants, you can easily collect fallen leaves and dry them in presses or under an iron. This will be good material that can be used throughout the school year, especially in the winter when covering the Leaf topic. It is better to hand out leaves to students without sewing them on so that they can see the underside of the leaf. Sets of leaves work well in folders with pockets.

You should become familiar with the basic concepts of leaf morphology in the previous lesson. When describing leaves, it is necessary to analyze as many morphological characteristics as possible to develop observation, taking into account that often leaves in shape and other characteristics, even on one annual shoot, but in different places can differ significantly from each other. The variability of the shape of aspen leaves is shown in Fig. 2.

Plan for describing trees and shrubs by leaves

1 - sheet simple or complex; 2 - sheet petiolate or sessile; 3 - leaf blade shape: a) simple leaves - round, oval, oblong, lanceolate, linear, ovate, obovate; b) complex leaves - pinnately complex (paired and unpaired), palmately complex; 4 - shape of the base of the leaf blade: wedge-shaped, rounded, heart-shaped; 5 - leaf tip shape: dull, sharp; 6 - venation: feathery, palmate; 7 - dissection of the leaf blade: whole, lobed, separate, dissected; 8 - leaf blade edge shape: entire, jagged, serrate, crenate, notched; 9 - color, shine, pubescence and other signs (Tables VI, VII).

To describe the leaves, you need to keep special notebooks in which to write down only the answers to the questions on the plan, putting down their numbers. In this case, it is necessary to give drawings of leaves from life. The answers can be arranged in table form; then the same characteristics in different plants fall into the same column and they can easily be compared with each other. It is better to give tasks for independent work in written form.

Let us give examples of descriptions of leaves in the order of the questions in the plan (see tables III, IV, V).

Small-leaved linden. 1 - simple; 2 - petiole; 3 - ovoid; 4 - heart-shaped; 5 - pointed with an oblique apex; 6 - fingered; 7 - whole; 8 - crenate-toothed, entire in the lower half; 9 - dark green above, naked, softly hairy below.

Summer oak, common, or pedunculate. 1 - simple; 2 - petiole 3 - 7 mm; 3 - oblong-obovate; 4 - narrowed into a petiole; 5 - blunt or notched; 6 - feathery; 7 - bladed, 4 - 7 blunt blades; 8 - entire; 9 - dark green above, shiny, bluish-green below, naked on both sides.

Birch warty or silver birch. 1 - simple; 2 - petiole half as long as the leaf blade, 15 - 30 mm; 3 - triangular-ovate or rhomboidal, leaf blade length 30 - 70 mm, width 25 - 50 mm; 4 - straight cut or at an angle of 120°, sometimes slightly heart-shaped; 5 - acute; 6 - feathery; 7 - whole; 8 - entire-edge at the bottom, double-serrate at the top; 9 - bare on both sides.

Mountain ash. 1 - complex, odd-pinnate, 11 - 21 leaflets; 2 - petiole 80 - 170 mm, hairless or hairy; 3 - oblong; 4 - unequal at the base; 5 - acute; 6 - feathery; 7 - whole; 8 - solid in the lower part, serrated above; 9 - dark green above, naked, gray below.

Yellow acacia, or caragana. 1 - complex, pinnate, 4 - 8 pairs of leaflets; 2 - petiole, total petiole 50 - 80 mm, there are leathery, spiny stipules; 3 - oval; 4 - wedge-shaped; 5 - sharp with bristles; 6 - feathery; 7 - whole; 8 - entire; 9 - naked, hairy in youth.

Comparison tasks

1. Let’s compare the leaves of viburnum and Siberian hawthorn. What are the similarities and differences? (Table VIII).

Similarities: leaves are simple, petiolate. The leaf blades are ovoid (in hawthorn it is obovate); with pinnate venation, lobed. Difference: the base of the leaf blade in hawthorn is wedge-shaped, in viburnum it is rounded. Hawthorn has more blades, while viburnum usually has three. Hawthorn leaves are covered with short hairs on both sides, while viburnum leaves are bare, wrinkled on top and fluffy underneath. Hawthorn has larger stipules, while viburnum has thread-like stipules.

2. Let’s compare the complex leaves of common ash, ash-leaved maple, or American, and red elderberry (see Table VIII).

Similarities: Leaves are compound, odd-pinnate, with pinnate veining. Difference: ash has the largest leaf, 7-15 leaves; sheet length can be up to 40 cm, total petiole up to 15 - 25 cm. The lateral leaflets are almost sessile. Ash maple has 3 - 5, less often 7, leaves. Common petiole 10 - 22 cm and the lateral leaves have petioles. The red elderberry has 5 - 7 leaves, the total petiole is 5 - 11 cm with two stipules. The leaves are almost sessile. Leaves with a faint odor.

Common ash has an obovate-shaped upper leaf. The lateral leaflets are lanceolate and wedge-shaped at the base. The ash-leaved maple has an ovate-lanceolate upper leaflet, unequal-sided; the base is wedge-shaped. In the first pair, the leaves are lanceolate, wedge-shaped at the base, they are especially similar to the leaves of ash. In the second pair, the leaves are broadly ovate-lanceolate. The leaves of the red elderberry are almost sessile, oblong-oval with an oblique, pointed apex, and unequally round at the base.

Let's compare the dissection and shape of the edges of the leaf blades: in common ash and red elder the leaf blade is entire, but in ash-leaved maple the middle leaf and lower lateral ones are often lobed. The shape of the edge of the leaf blades: in ash, serrate or crenate-serrate; elderberry has serrated; ash-leaved maple has lateral leaves whole or with sparse teeth; the upper ones are coarsely toothed.

3. Let’s compare the leaves of common elm, hazel, gray alder, and hornbeam (see Table VIII).

What do these leaves have in common and what are the differences?

Similarities: leaves are simple, petiolate, with pinnate venation, with a whole leaf blade (in hazel sometimes almost lobed), a sharp apex, not entire. Difference: the shortest hairy petiole of elm 4 - 5 mm; at hazel 10 mm with glandular bristles; hornbeam 10 - 15 mm, long-haired, often glandular; for gray alder 10 - 25 mm, naked. Leaf blade of gray alder 40 - 90 mm(oblong-ovate), smooth elm has an oval or obovate shape. Irregularity at the base of the leaf blade is most common and severe in elm, and is also found in hornbeam. Gray alder has a rounded or wedge-shaped leaf base. The leaf edge of smooth elm is serrated, while that of hazel, gray alder and hornbeam is serrated (alder has large, prominent teeth). The upper side of all leaves is dark green, but in gray alder and hornbeam it is bare, in hazel it is finely hairy, rough with depressed nerves, and in elm it is rough. The underside of the leaves of gray alder is covered with gray felt over the entire surface; in the hornbeam - naked; in hazel it is hairy, sometimes with glandular bristles, in elm it is soft-hairy.

Exercise: draw the leaves of birch, maple, elm, hawthorn, and viburnum from memory. Who will draw it faster and more correctly? Label the leaves with the name of the plants.

Review questions

1. What tree and shrub species have simple leaves?

Answer: poplar, linden, aspen, hazel, Tatarian maple, hawthorn, viburnum, etc.

2. What tree and shrub species have compound leaves?

Answer: odd pinnate: ash maple, common ash, rowan, common rose hip, red elder, white acacia, etc.; paripirnate: yellow acacia, honey locust.

3. Name the simple leaves with palmate venation.

Answer: balsam poplar, small-leaved linden, maple, holly, etc.

4. Name simple leaves with pinnate veins.

Answer: oak, birch, Tatarian maple, hornbeam, alder, hazel (hazel), etc.

5. Name the simple leaves with a whole leaf blade.

Answer: balsam poplar, small-leaved linden, elm, warty birch, Tatarian honeysuckle, hornbeam, aspen, alder (hazel), etc.

6. Name the leaves simple lobed.

Answer: summer oak, Norway maple, ginnala maple, Siberian hawthorn, common viburnum, etc.

7. Name the simple, entire leaves.

Answer: oak, Tatarian honeysuckle, brittle buckthorn, etc.

8. Name the leaves with a serrated edge of the leaf blade.

Answer: balsam poplar, small-leaved linden (whole-edged in the lower half, crenate-toothed above), aspen, etc.

9. Name the leaves with a notched-toothed edge of the leaf blade.

Answer: Norway maple, etc.

10. Name the leaves with a serrated edge of the leaf blade.

Answer: warty birch, mountain ash, red elderberry, ginnala maple, Tatarian maple, etc.

11. What are the shapes of the leaf blades of summer oak, warty birch, Norway maple, and small-leaved linden?

Answer: obovate, triangular-ovate, round, ovoid.

12. Which leaves have a heart-shaped base of the leaf blade?

Answer: small-leaved linden, Norway maple, etc.

13. Which leaves are unequal at the base?

Answer: smooth elm leaves, etc.

14. What are the similarities and differences between the leaves of the warty birch and the downy birch?

Answer: similarity - leaves are simple, petiolate, with pinnate venation, the same length; the difference is that the shape of the leaf blade in warty birch is often triangular-ovate or rhombic, the base is wedge-shaped or truncated, less often rounded, and in downy birch the shape of the leaf blade is ovoid or oval, the base is round, heart-shaped, less often narrowed. The shape of the leaf tip of the warty birch is long-pointed, while that of the downy birch is short-pointed. The shape of the edge of the warty birch is sharp-serrated, while that of the downy birch is large-serrated. The warty birch has bare leaves, while the downy birch has young leaves that are densely pubescent and the pubescence on the petiole lasts a long time.

Conifers

Coniferous leaves are most often in the form of needles, less often in the form of scales (cypress, thuja). They can be located on shoots spirally(singly, in pairs, in bunches, in two rows), for example: spruce, pine, Siberian cedar, fir; opposite(crosswise), for example: cypress, thuja occidentalis; whorled(three to four-membered whorls), for example: juniper. In some coniferous species, the shoots on which the leaves are located are divided into elongated and shortened ones, for example, pine, Siberian cedar, larch); other species (spruce, fir) have only elongated shoots. Shortened shoots grow slightly during one summer, and elongated shoots grow by 35 cm and even more. Sometimes shortened shoots develop into elongated ones.

The needles are annual, soft, dying off in the fall of the first year (for example, in larch) and hard, long-term, dying off no earlier than the fall of the second year.

Plan for describing coniferous species by needles

1 - needle location: spiral (singly, in pairs, in bundles, two-row), opposite (crosswise), whorled (with three to four-membered whorls); 2 - shape, size and etc.; 3 - color, shine and other signs (see Table II).

Let us give examples of descriptions of coniferous leaves in order of the questions in the plan.

Scots pine. 1 - spiral, steamed needles, coming out of a leathery vagina of brown-gray color; 2 - semi-cylindrical or semi-circular, sharp at the end, finely serrated at the edges, rigid, strongly twisted; 3 - dark green on the upper convex side, and bluish or whitish on the lower grooved side; On the lower side there are closely spaced stomata.

Siberian cedar. 1 - spiral, 5 pieces in bunches, which are surrounded by a yellow-brown sheath that falls off early; the bundles are close to each other; the needles are densely located on the shoot; 2 - triangular, serrated at the edges, size 11 cm; 3 - bright or dark green, hard.

Siberian larch. 1 - spiral, on short shoots and old trees - in bunches from 25 to 50 pcs. in a bunch, and on elongated and young trees singly along the entire shoot; the size of the needles increases from the top to the base of the shoot, which is often surrounded by a crown of the longest needles; needle size 30 - 35 mm; 2 - narrow-linear, flat, slightly widened towards the apex, with blunt ends; 3 - bright green with a bluish bloom, soft, tender needles; Stomata are located in rows on both sides.

Norway spruce. 1 - spiral, singly around the shoot and directed in all directions; 2 - tetrahedral, short, hard, thin, prickly, length - 15 - 25 mm; 3 - dark green, shiny, dense, raised upward.

Siberian fir. 1 - spiral, singly, directed towards two opposite sides in a comb-like manner; 2 - flat, with a rib in the middle and two white stripes of stomatal rows; length up to 30 mm; 3 - the upper side is dark green, shiny, the lower side is paler; young fir shoots have a light, yellowish-green color; the needles are soft, narrow, dense; the end is blunt with a notch, so the needles are not prickly.

Review questions

How do Siberian cedar needles differ from Scots pine?

Answer: cedar needles are much longer, softer than those of pine, and are arranged in bunches of five needles (in pine - of two needles).

2. With which coniferous species does the Siberian larch have some similarities in the shape of its needles?

Answer: with spruce, but larch needles are much narrower and longer, and in addition they are soft and have a lighter tone.

3. How does fir differ from spruce in terms of needles?

Answer: fir needles have distinct upper and lower sides of different colors, they are flat, wide, while spruce needles are tetrahedral and the sides are difficult to distinguish; If fir needles are rubbed between your fingers, it gives off a balsamic scent reminiscent of lemon peel. The needles on the shoot of fir are comb-shaped on two opposite sides, while those of spruce are located in all directions.

4. From the needles of which tree is valuable essential oil produced for the perfume industry?

Answer: from fir needles.

There are a huge number of varieties in the world that differ in appearance, and the main feature of each plant is its leafy part. Leaves come in different sizes, shapes and colors, but these features are formed due to their unique cellular structure.

Therefore, today we will look at the external and internal structure of the leaf, as well as its main types and shapes.

What are leaves made of: external structure

The green plate in all cases is located on the side of the shoot, at the node of the stems. The vast majority of plants have flat-shaped foliage, which distinguishes this part of the plant from others. This type of sheet is not without reason, since its flat shape ensures maximum contact with air and light. This plant organ is limited by the leaf blade, petiole, stipule and base. In nature, there are also varieties of plants that lack stipules and petioles.

Did you know? Putang plates are considered the sharpest in the world. The plant is common in New Guinea and local tribes use it for shaving, claiming that they are no worse than a special razor.

Basic types and forms

Let's look at the different types and shapes of green plates and how they differ from each other.

Simple and complex

The leaves of most plants are simple because they contain only one leaf blade, but there are other types that have many leaf blades and are called compound leaves.

A simple variety has a leaf blade that can be whole or dissected. To determine the nature of the dissection, one should take into account how the protruding sections of the plate are distributed, depending on the main vein and petiole. We can talk about pinnateness if the parts that protrude beyond the base of the plate are symmetrical to the main vein. But if they protrude pointwise, from a certain place, then they are called fingered.

The names of complex varieties are similar to simple ones, but the word “complex” is added to them. These are palmate, pinnate, ternate and others.
To make it easier to understand simple and complex leaves, you can consider several plant examples.

Examples of simple ones are oak. Complex – , .

The following leaf plates are distinguished, which come in different shapes:

broadly ovoid;
rounded;
ovoid;
inversely broad-ovate;
elliptical;
obovate;
linear;
oblong;
obverse-narrow-ovate;
lanceolate;

The edges of the plant can be:

entire;
notched;
wavy;
spiny;
serrated;
double-toothed;
serrated;
crenate;

Along the top

The upper parts of the plate can be:

pointed;
pointed;
spinous;
dull;
notched;
cut off;
rounded.

Based on

The bases of the green plates can be of the following shapes:

round;
rounded wedge-shaped;
wedge-shaped;
reniform;
sagittal;
spear-shaped;
notched;
truncated;
drawn out.

When studying the appearance of the part of the plant in question, the veins, which are small bunches, are clearly visible. Thanks to the veins, the plate is fed with water and mineral salts, as well as the removal of organic substances accumulated in the plant.

The main types of venation are: arcuate, parallel, reticulate or pinnate, fingered.
As arcuate venation of leaves, we can give examples of the following plants: plantain, which have large venation, presented in the form of one central even vein, around which all other veins are arranged in an arcuate manner. For parallel veining, consider the examples of corn and wheat plants.

Examples of reticulate venation are the sheets. They have a main vein that is surrounded by many smaller ones, creating the appearance of a mesh.

As an example of finger venation, we can consider sycamore, caustic, presented in the form of large veins that diverge in a fan-shaped manner, have many smaller fan-shaped branches.

By leaf arrangement

Leaf arrangement is presented in the form of whorled, alternate, rosette and opposite.

As an example of a whorled leaf arrangement, we can consider forest leaf arrangement, regular leaf arrangement - vanilla leaves, rosette leaf arrangement - plantain leaves, opposite leaf arrangement - Rostkov's eyebright.

Internal structure of the leaf

If we talk about the internal structure, then it can be noted that we will talk about its cellular structure. In order to characterize the cellular structure of a leaf as accurately as possible, they resort to examining its cross section.

The upper part of the leaf blade is covered with skin, which is presented in the form of transparent cellular tissue. Skin cells are located very closely together, which provides maximum protection of internal cells from mechanical stress and drying. Due to the fact that the skin is transparent, this allows better penetration of sunlight into the inside of the leaf.

The lower part of the leaf is presented in the form of stomata - green cells with slits. They can diverge or converge, open or close a gap. Thanks to the stomata, moisture evaporates and gas exchange occurs.

A leaf is a vegetative organ of plants and is part of a shoot. The functions of the leaf are photosynthesis, water evaporation (transpiration) and gas exchange. In addition to these basic functions, as a result of idioadaptations to various living conditions, leaves, changing, can serve the following purposes.

Accumulation of nutrients (onions, cabbage), water (aloe);
protection from being eaten by animals (cactus and barberry spines);
vegetative propagation (begonia, violet);
catching and digesting insects (sundew, Venus flytrap);
movement and strengthening of weak stems (pea tendrils, vetch);
removal of metabolic products during leaf fall (in trees and shrubs).

General characteristics of the plant leaf

The leaves of most plants are green, most often flat, usually bilaterally symmetrical. Sizes range from a few millimeters (duckweed) to 10-15 m (palm trees).

The leaf is formed from the cells of the educational tissue of the growth cone of the stem. The leaf primordium is differentiated into:

Leaf blade;
the petiole by which the leaf is attached to the stem;
stipules.

Some plants do not have petioles; such leaves, unlike petiolate ones, are called sedentary. Not all plants have stipules either. They are paired appendages of various sizes at the base of the leaf petiole. Their shape is varied (films, scales, small leaves, spines), their function is protective.

Simple and compound leaves distinguished by the number of leaf blades. A simple leaf has one blade and falls off entirely. The complex one has several plates on its petiole. They are attached to the main petiole with their small petioles and are called leaflets. When a compound leaf dies, first the leaflets fall off, and then the main petiole.

The leaf blades are varied in shape: linear (cereals), oval (acacia), lanceolate (willow), ovate (pear), arrow-shaped (arrowhead), etc.

The leaf blades are pierced in different directions by veins, which are vascular-fibrous bundles and give the leaf strength. The leaves of dicotyledonous plants most often have reticulate or pinnate venation, while the leaves of monocotyledonous plants have parallel or arcuate venation.

The edges of the leaf blade can be solid; such a leaf is called whole-edged (lilac) or with notches. Depending on the shape of the notch, along the edge of the leaf blade, leaves are distinguished as serrated, serrated, crenate, etc. In serrated leaves, the teeth have more or less equal sides (beech, hazel), in serrated leaves, one side of the tooth is longer than the other (pear), crenate - have sharp notches and blunt protuberances (sage, budra). All these leaves are called whole, since their grooves are shallow and do not reach the width of the blade.

In the presence of deeper grooves, the leaves are lobed when the depth of the groove is equal to half the width of the blade (oak), separate - more than half (poppy). In dissected leaves, the notches reach the midrib or the base of the leaf (burdock).

Under optimal growth conditions, the lower and upper leaves of the shoots are not the same. There are lower, middle and upper leaves. This differentiation is determined in the kidney.

The lower, or first, leaves of the shoot are the bud scales, the outer dry scales of the bulbs, and the cotyledon leaves. The lower leaves usually fall off as the shoot develops. The leaves of basal rosettes also belong to the grass roots. Median, or stem, leaves are typical of plants of all species. The upper leaves usually have smaller sizes, are located near flowers or inflorescences, are painted in various colors, or are colorless (covering leaves of flowers, inflorescences, bracts).

Types of sheet arrangement

There are three main types of leaf arrangement:

Regular or spiral;
opposite;
whorled.

In the next arrangement, single leaves are attached to the stem nodes in a spiral (apple tree, ficus). In opposite case, two leaves in a node are located one opposite the other (lilac, maple). Whorled leaf arrangement - three or more leaves at a node envelop the stem in a ring (elodea, oleander).

Any leaf arrangement allows plants to capture the maximum amount of light, since the leaves form a leaf mosaic and do not shade each other.

Cellular structure of the leaf

The leaf, like all other plant organs, has a cellular structure. The upper and lower surfaces of the leaf blade are covered with skin. Living colorless skin cells contain cytoplasm and a nucleus and are located in one continuous layer. Their outer shells are thickened.

Stomata are the plant's respiratory organs

The skin contains stomata - slits formed by two guard, or stomatal, cells. Guard cells are crescent-shaped and contain cytoplasm, nucleus, chloroplasts and a central vacuole. The membranes of these cells are thickened unevenly: the inner one, facing the gap, is thicker than the opposite one.

A change in the turgor of guard cells changes their shape, due to which the stomatal fissure is open, narrowed or completely closed, depending on environmental conditions. So, during the day the stomata are open, but at night and in hot, dry weather they are closed. The role of stomata is to regulate the evaporation of water by the plant and gas exchange with the environment.

Stomata are usually located on the lower surface of the leaf, but they can also be on the upper surface; sometimes they are distributed more or less evenly on both sides (corn); In aquatic floating plants, stomata are located only on the upper side of the leaf. The number of stomata per unit leaf area depends on the plant type and growth conditions. On average there are 100-300 of them per 1 mm2 surface, but there can be much more.

Leaf pulp (mesophile)

Between the upper and lower skins of the leaf blade is the leaf pulp (mesophil). Beneath the top layer there are one or more layers of large rectangular cells that have numerous chloroplasts. This is a columnar, or palisade, parenchyma - the main assimilation tissue in which photosynthesis processes take place.

Under the palisade parenchyma there are several layers of irregularly shaped cells with large intercellular spaces. These layers of cells form spongy, or loose, parenchyma. Spongy parenchyma cells contain fewer chloroplasts. They perform the functions of transpiration, gas exchange and nutrient storage.

The pulp of the leaf is penetrated by a dense network of veins, vascular-fibrous bundles, which supply the leaf with water and substances dissolved in it, as well as remove assimilants from the leaf. In addition, the veins perform a mechanical role. As the veins move away from the base of the leaf and approach the top, they become thinner due to branching and the gradual loss of mechanical elements, then sieve tubes, and finally tracheids. The smallest branches at the very edge of the leaf usually consist only of tracheids.

Diagram of the structure of a plant leaf

The microscopic structure of the leaf blade varies significantly even within the same systematic group of plants, depending on different growing conditions, primarily on lighting and water supply conditions. Plants in shaded areas often lack palisade parenchyma. The cells of the assimilative tissue have larger palisades; the concentration of chlorophyll in them is higher than in light-loving plants.

Photosynthesis

In the chloroplasts of pulp cells (especially columnar parenchyma), the process of photosynthesis occurs in the light. Its essence lies in the fact that green plants absorb solar energy and create complex organic substances from carbon dioxide and water. This releases free oxygen into the atmosphere.

Organic substances created by green plants are food not only for the plants themselves, but also for animals and humans. Thus, life on earth depends on green plants.

All oxygen contained in the atmosphere is of photosynthetic origin; it accumulates due to the vital activity of green plants and its quantitative content is maintained constant due to photosynthesis (about 21%).

By using carbon dioxide from the atmosphere for the process of photosynthesis, green plants thereby purify the air.

Evaporation of water by leaves (transpiration)

In addition to photosynthesis and gas exchange, the process of transpiration occurs in the leaves - the evaporation of water by the leaves. The main role in evaporation is played by the stomata; the entire surface of the leaf partly takes part in this process. In this regard, a distinction is made between stomatal transpiration and cuticular transpiration - through the surface of the cuticle covering the epidermis of the leaf. Cuticular transpiration is significantly less than stomatal transpiration: in old leaves it is 5-10% of total transpiration, but in young leaves with a thin cuticle it can reach 40-70%.

Since transpiration occurs mainly through stomata, where carbon dioxide also penetrates for the process of photosynthesis, there is a relationship between the evaporation of water and the accumulation of dry matter in the plant. The amount of water that is evaporated by a plant to build 1 g of dry matter is called transpiration coefficient. Its value ranges from 30 to 1000 and depends on growth conditions, type and variety of plants.

To build its body, the plant uses an average of 0.2% of the water passed through, the rest is spent on thermoregulation and transport of minerals.

Transpiration creates a suction force in the leaf and root cells, thereby maintaining the constant movement of water throughout the plant. In this regard, the leaves are called the upper water pump, in contrast to the root system - the lower water pump, which pumps water into the plant.

Evaporation protects the leaves from overheating, which is of great importance for all plant life processes, especially photosynthesis.

Plants in dry areas and in dry weather evaporate more water than in humid conditions. In addition to the stomata, the evaporation of water is regulated by protective formations on the leaf skin. These formations are: cuticle, waxy coating, pubescence from various hairs, etc. In succulent plants, the leaf turns into spines (cacti), and its functions are performed by the stem. Plants in humid habitats have large leaf blades and no protective formations on the skin.

Transpiration is the mechanism by which water evaporates from plant leaves.

When evaporation is difficult in plants, guttation- release of water through stomata in a drop-liquid state. This phenomenon usually occurs in nature in the morning, when the air is approaching saturation with water vapor, or before rain. In laboratory conditions, guttation can be observed by covering young wheat seedlings with glass covers. After a short period of time, droplets of liquid appear at the tips of their leaves.

Excretion system - leaf fall (leaf fall)

A biological adaptation of plants to protect themselves from evaporation is leaf fall - the massive fall of leaves during the cold or hot season. In temperate zones, trees shed their leaves during the winter, when the roots cannot draw water from the frozen soil and frost dries out the plant. In the tropics, leaf fall occurs during the dry season.

Preparation for shedding leaves begins when the intensity of life processes weakens in late summer - early autumn. First of all, chlorophyll is destroyed; other pigments (carotene and xanthophyll) last longer and give the leaves an autumn color. Then, at the base of the leaf petiole, parenchyma cells begin to divide and form a separating layer. After this, the leaf is torn off, and a mark remains on the stem - a leaf scar. By the time the leaves fall, the leaves become old, unnecessary metabolic products accumulate in them, which are removed from the plant along with the fallen leaves.

All plants (usually trees and shrubs, less often herbs) are divided into deciduous and evergreen. In deciduous plants, leaves develop during one growing season. Every year, with the onset of unfavorable conditions, they fall off. The leaves of evergreen plants live from 1 to 15 years. The dying off of some old leaves and the appearance of new leaves occurs constantly, the tree appears to be evergreen (conifers, citrus fruits).

The shapes of leaves, flowers and plant roots are very diverse. Today we will talk about one of the main organs of all green plants. This is a leaf. It is located on the stem, occupying a lateral position on it. The shape of the leaves varies significantly, as do their sizes. For example, in duckweed, an aquatic plant, they are about three millimeters in diameter. The Amazonian Victoria leaf can reach up to a meter. In some tropical palm trees its length is 20-22 m.

General characteristics of plant leaves

The leafless tree is a broom of various sizes. It is often difficult to determine its species in winter, when the crown is bare. Trees with leaves that have fallen for the winter do not grow, although they remain alive. Only after they bloom do they begin to live to the fullest and acquire their characteristic shape. The leaf is not an axial organ, but it is closely related to the stem, which is the axis of the shoot.

Psilophytes, the most ancient land plants, did not have the dismemberment of the body that is familiar to us. In their structure, the root, leaf and stem were not distinguished. It happened a little later. In modern plants, the shape of leaves and their organization are very plastic. These organs differ from the stem and root in their characteristic features. The leaves of the shoot are its lateral organs. They are formed superficially (exogenously) as tubercles located in the growth cone. However, the leaves themselves do not. They grow from the base. They do not directly bear other leaf or axial organs. Their growth is limited to a certain period of time.

rules and exceptions

The leaf blade is the expanded part of the leaf. The petiole is its stem-like narrow part. It is with its help that the leaf blade is connected to the stem. The base is the part by which the cutting is attached to the stem. There are stipules at the base.

As a rule, the structure of the leaves is dorsoventral (dorsiventral). There is only one plane of symmetry, and it divides them into 2 halves, symmetrical to each other. However, there are many exceptions to these rules. For example, the leaves of fronds (ferns) grow at the top. As for pine needles, they increase in size over several years. Pine needles grow by intercalary growth at the base.

However, the most surprising exceptions to these rules can be considered the leaves of Velvichia mirabilis. This is a gymnosperm plant that is found in South Africa (Kalahari Desert). The pedestal-shaped trunk of Velvichia mirabilis (40 cm in height and 1 meter in diameter) forms only 2 leaves. Their length reaches three meters. The leaves are belt-shaped and leathery. These leaves die off at the ends and continually grow at the base. As a result, their life expectancy can exceed 100 years.

How to classify leaves?

The external diversity of leaves is so great that it is impossible to create a unified classification system based on one or more characteristics. There are several classifications, which we will now discuss.

Classification by petiole

There are three ways in which leaves are attached to the stem. Plants with and without petioles are distinguished. In the first case, the leaves of such a plant are called petiolate, and in the second - sessile. The base of some plants grows, covering the stem above the node. In this case, the leaf is called vaginal. The stem seems to be embedded in it. If a sessile leaf of a plant descends down the stem, it is called decurrent. A typical example is thistle. If the leaf of a plant covers the stem, it is called stem-encompassing.

Complex and simple leaves

Let's move on to the next classification. Leaf blades can also be very diverse in shape, size, structure and other parameters. There may be one or several. If there is only one blade, the leaves are called simple. The shape of tree leaves in this case can be oval, round, lanceolate, oblong, ovoid, linear, obovate. When there are several blades on one petiole, we are talking about complex species. The location of the leaf blades can also be different. The form can be as follows: intermittently pinnate, trippennate, doubly pinnate, odd pinnate, paripnate, palmate, ternate.

However, simple leaves are not so simple. Let's consider this using the example of the monstera plant, known to many. Its leaf consists of only one leaf blade, therefore it is considered simple. However, its shape is very bizarre. Leaves of this type are called dissected. There are other types. If the dissection of the blade does not exceed a quarter of its width, the shape of the tree leaves is lobed. If it is cut into a third, it is called separate. It also happens that the cut reaches the main one. In this case, the shape of the plant leaves is dissected.

Number of cuts, shape of leaf blades and edges

Let's move on to the next classification. Plants can also differ in the number of cuts on the leaf. If it is divided into 3 parts, it is called trifoliate, if into 5 - finger-shaped, if into a larger number of parts - pinnate-(dissected, divided, lobed).

Leaf blades are also classified by shape. There are many forms: ovoid, round, spear-shaped, lanceolate, linear, oblong, heart-shaped, arrow-shaped, etc. The edges can also be classified on the same basis. The most common form of leaf edge is entire (whole-edged leaves). However, there are several other types. There are jagged, crenate, spiny-toothed (spinous), serrate, and sinuous leaves according to the shape of the edge.

Heterophily

Are you familiar with this concept? If not, then we note that the leaves on one shoot can have different shapes, colors and sizes. It is this phenomenon that is called heterophily. It is characteristic, for example, of arrowhead, buttercup and many other species.

Veins of plants

When examining the leaf blade of a plant, you will notice that it has veins. These are conducting vessels. Their location on the sheet can also be different. Venation is the way leaves are arranged. There are several types: reticulate (pinnate and finger-like), dichotomous, arcuate, parallel. Monocots are characterized by an arcuate or reticulate pattern, and for dicotyledons, a reticulate pattern.

We propose to examine and compare oak and maple leaves and determine their shape.

Oak leaves

Oak is a plant characteristic of temperate climates. It can be found in various regions of the Northern Hemisphere. Tropical highlands are the southern limit of its growth. Its leaves are leathery. In evergreen species they stay on the tree for several years, while in other species they fall off annually or remain on the branches, gradually breaking down and drying out. The oak leaf shape is lobed. However, sometimes there are also whole ones. This form is observed in some evergreen species. White, for example, has quite large leaves (up to 25 cm). This type of tree has an oblong-oval leaf shape. In spring, the crown acquires a bright red color, and in summer it changes its color to bright green, while the lower part becomes white. The color of the leaves varies in autumn. It can range from deep purple to burgundy. The shapes of autumn leaves do not change.

Red oak (otherwise called northern) is a tall tree (up to 25 m) with a dense crown. Its leaves are large and have pointed lobes. This tree got its name because of its foliage, which has a reddish color in autumn and spring.

maple leaves

The maple is native to Eurasia. This is a deciduous tree with a dense, rounded, wide crown. It reaches a height of 30 meters. The tree can live up to 200 years under favorable conditions. Its leaves are large, their diameter reaches 18 cm. They have pronounced veins. The shape is as follows: it has 5 blades ending in pointed lobes. In this case, the three front blades do not differ from each other, and the two lower ones are somewhat smaller. There are rounded recesses between all of them. The leaf petioles are long. As for color, it also varies depending on the time of year. In summer, the leaves are dark green above and light green below. In autumn they acquire brown, red, burgundy and brown shades.

So, we looked at the basic shapes of leaves. In conclusion, we will talk about their role.

Meaning of leaves

The most important function is the formation of organic substances. The large, flat leaf plate catches sunlight. It is in the leaves that leaks occur. With their help, the plant also evaporates water. It can change the intensity of this process by closing and opening stomata. In addition, gas exchange occurs with the help of leaves. Carbon dioxide and oxygen enter through the stomata. Oxygen is needed for respiration, and carbon dioxide is needed by the plant for the synthesis of organic substances. During leaf fall, unnecessary substances are removed, the surface of above-ground organs decreases during an unfavorable period. The plant evaporates less water, the crown accumulates less snow, which means it will not break.

- arose in the process of evolution as a result of flattening of the branches of the main shoot. It performs functions photosynthesis, gas exchange And evaporation of water. The evaporation of water from the surface of the leaves ensures a constant flow of water with substances dissolved in it, and also protects the plant from burns and overheating.

Leaf parts

The sheet consists of leaf blade, petiole, base And stipules.

The basis the leaf is attached to the stem. In some plants (cereals, umbellifers), the base of the leaf grows and forms a tube - vagina, which covers the stem.

petiole provides the most favorable location of the plate in relation to the light. Many leaves do not have a petiole; they are called sessile.

Stipules- these are special, usually paired outgrowths at the base of the leaf. They look like films, scales, spines or small leaves. The stipules of many plants protect the leaves in the bud and are shed when they bloom ( birch, linden, apple tree). Leaf-shaped stipules in some plants perform the function of photosynthesis (peas, meadow chin).

The main part of the sheet is plate. It has sufficient density due to the presence of veins, which consist of mechanical and conductive tissues. The veins branch in various ways in the plate. There are parallel (at cereals, sedge), arc (y lily of the valley, plantain), pinnately reticular (at cherries, pears), palmate reticularis (at maple) venation.

Leaf Shape

There are simple and complex leaves.

simple leaves

Simple ones have one leaf blade, whole or dissected, and one articulation with the stem.

According to the degree of dissection of the leaf blade simple leaves are divided into lobed, separate And dissected(Fig. 33).

Lobed are called leaves that have cuts in the leaf blade, reaching approximately 1/2 the width of the half blade (maple, oak).
U separate leaf cuts extend further than 1/2 the width of the leaf half-blade (dandelion, radish).
Dissected leaves that have cuts that extend to the midrib or base of the leaf are called (potatoes, yarrow).

By arrangement of blades, lobes and segments leaves are called pinnately lobed And finger-lobed, pinnately divided And finger-separated, pinnately dissected And finger-dissected.

A single leaf blade can be rounded, elliptical- length is 2 times the width; oblong- length exceeds width by 3-4 times; linear- length exceeds width by more than 5 times; ovoid- the length is greater than the width, and the widest point is below the middle of the sheet; obovate- the widest place above the middle of the leaf; lanceolate- an analogue of the oblong one, but pointed at the apex (Fig. 34). The edge of the leaf blade may be whole-edged, sinuate, serrated, serrated, crenate, wavy.

Compound Leaves

Compound leaves consist of several leaflets, which are attached to a common petiole (rachis) using their own petioles. Compound leaves are divided into ternary complexes, pinnately complex And finger-complex(Fig. 35).

Pinnately compound leaves. In pinnately compound leaves, the leaflets are arranged in pairs on a common petiole, and if the petiole ends in one leaflet, the leaf is called odd-pinnate (rowan, ash), if with two leaves - pari-pinnate(peas, yellow acacia).

Palmately compound leaves. In palmately compound leaves, the leaflets are attached to the end of the petiole and radiate from one point (lupine, chestnut). There are leaves that are twice or thrice pinnately compound (in many umbrella).

Heterophylly (various leaves)

There are plants whose leaves have different shapes even on the same shoot. This phenomenon is called heterophylly, or diversity of leaves, it is especially characteristic of aquatic plants. Material from the site

Sheet sizes

Sheet sizes can be very diverse. Some tropical palms have especially large leaves. For example, at raffia they reach 20 m in length and up to 12 m in width. Of the plants in our flora, the leaves are quite large burdock, hogweed, hogweed, corn. On the other hand, many herbaceous plants have very small leaves.

Leaf arrangement

There is a certain pattern in the arrangement of leaves on the stem, due to which mutual shading is largely eliminated. Distinguish another, opposite, whorled And rosette leaf arrangement(Fig. 36).

At next leaf arrangement leaves are arranged in a spiral, each node bears one leaf (in birch, linden, apple, pear). If a node contains two leaves that are opposite, the leaf arrangement is opposite(y lilac, maple, viburnum). If there are 3 or more leaves in a node, leaf arrangement whorled(y Elodea canadensis, crow's eye). On shortened shoots it is sometimes observed rosette arrangement of leaves (at dandelion, round-leaved wintergreen, hairy hawksbill).

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