Forms of the base of the leaf blade. Separate sheet - a sheet with a plate, dissected up to ½ the width of the half-sheet
In the world there are a huge variety of varieties that differ in appearance, and main feature each plant is its leafy part. Leaves are different size, shapes and colors, but these features are formed due to the unique cellular structure.
Therefore, today we will consider the external and internal structure sheet, as well as its main types and forms.
What leaves are made of: external structure
The green plate in all cases is located on the side of the shoot, in the node of the stems. The vast majority of plants have flat-shaped foliage that distinguishes this part of the plant from others. This type of sheet is not without reason, since due to the flat shape, maximum contact with air and light is ensured. This plant organ is limited by the leaf blade, petiole, stipules 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 shaving device.
Basic types and forms
Consider what types of green plates exist by type and shape, how they differ from each other.
Simple and complex
The leaves of most plants are simple because they contain only one plate, but there are other species that consist of many plates, so they are called complex.
A simple variety has a leaf plate, which can be whole or dissected. To determine the nature of 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 pinnate 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 palmate.
The names of complex varieties are consonant with simple ones, but the word "complex" is added to them. These are palmate, pinnate, ternary and others. 
To make it easier to understand simple and complex leaves, you can consider a few examples of plants.
Examples of simple ones are oak. Complex -,.
There are the following sheet plates, which are in shape:
- broadly ovoid;
- rounded;
- ovoid;
- back broadly ovoid;
- elliptical;
- obovate;
- linear;
- oblong;
- reverse narrow ovoid;
- lanceolate;
The edges of the plant can be:
- whole;
- notched;
- wavy;
- prickly;
- jagged;
- double-toothed;
- serrated;
- crenate;
Top
The upper parts of the plate can be:
- pointed;
- pointed;
- spinous;
- blunted;
- notched;
- clipped;
- rounded.
Based on
The bases of green plates can be of the following shapes:
- rounded;
- round-wedge-shaped;
- wedge-shaped;
- kidney-shaped;
- swept;
- spear-shaped;
- notched;
- truncated;
- drawn.
When the appearance of the part of the plant in question is studied, the veins, which are small bundles, 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, palmate. 
As an arc venation of leaves, examples of such plants can be given: plantain, which have a large venation, presented in the form of one central even vein, around which all other veins are arranged in an arcuate manner. As parallel venation, consider examples of corn and wheat plants.
As examples of reticulated venation, sheets,. They have a main vein, which is surrounded by many small ones, creating the appearance of a grid.
As an example of palmate venation, one can consider plane-shaped, caustic, presented in the form of large veins that diverge in a fan-shaped manner, have many smaller fan-shaped branches.
By leaf arrangement
The leaf arrangement is presented in the form of whorled, alternate, rosette and opposite.
As an example of a whorled leaf arrangement, one can consider a forest one, another leaf arrangement - vanilla leaves, rosette leaf arrangement - plantain leaves, opposite leaf arrangement - Rostkov's eyebright.
The 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 accurately characterize cellular structure sheet, resort to the consideration of its cross section.
The upper part of the leaf plate is covered with a skin, which is presented in the form of a transparent cellular tissue. Skin cells are very closely spaced between each other, which provides maximum protection of internal cells from mechanical stress and drying. Due to the fact that the skin is transparent, it contributes to a better penetration of sunlight into inner part sheet.
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 the gap. The stomata allow moisture to evaporate and gaseous exchange.
Important!If there is a lack of moisture, the stomata are in a closed position.
At least 100 stomata are located on one leaf plate. Some plants have stomata on the surface of the leaf plate, such as cabbage. Some aquatic plants, such as a water lily, do not have stomata on the inside of the leaf at all, since they are on the surface of the water, and evaporation by the lower parts of the plate is impossible.
Isn't it paradoxical that, speaking about the world around us, we, without thinking about it, perceive it as green?
This is easily explained: as long as there are green plants on Earth that create organic matter from carbon dioxide with the help of light - the basis of life for everyone else - we also live ...
But why are plants green?
All objects we see only due to the fact that they reflect the rays of light falling on them. For example, a sheet of clean paper, perceived by us as white, reflects all parts of the spectrum. And an object that seems black to us absorbs all the rays. It is easy to understand that if the fibers of the fabric are impregnated with a substance that absorbs all the rays of light, except for red ones, then we will perceive the dress sewn from this fabric as red.
Similarly, chlorophyll - the main plant pigment - absorbs all rays except green ones. And it does not just absorb, but uses their energy in its own interests, especially actively - the red part of the spectrum, opposite to the green one.
And yet the leaves of plants are not always green. That is what will be the subject of my story. Of course, I will state many things in a very simplified way (may the professionals forgive me). But it seems to me that every person who is seriously involved in their cultivation should have an idea of \u200b\u200bthe reasons for the change in the color of the leaves of plants.
Non-green greens
Several pigments are constantly present in the tissues of any living plant. Of course, the main one is green - chlorophyll, which determines the basic color of the leaves.
But there is also anthocyanin, which actively absorbs green rays and completely reflects red ones.
Pigment xanthosine absorbs all rays except yellow, and carotene reflects a whole group of rays and seems to us orange-carrot.
There is also a pigment called betulin which stains plant tissue White color(but it is found only in birch; and then - not in the leaves, but in the bark, and therefore we will not talk about it).
We see all additional leaf pigments only after the death of chlorophyll. For example, on the leaves of plants with the arrival of autumn cold weather or as a result of leaf aging, as happens with popularly beloved codiaums.
Bright variegated leaves, being its only decoration, in fact, are dead and no longer give anything to the plant. Breeders only chose clones that can keep these useless but beautiful old leaves for as long as possible.
Probably, many flower growers had to observe the reddening of the leaves of plants exposed to excessively bright sunlight. In everyday life, this phenomenon is called "sunburn". But when we sunbathe, for protection from exposure ultraviolet radiation The skin produces a special pigment called melanin. In plants, no new pigments are produced, but on the contrary, chlorophyll is destroyed; then the anthocyanin previously present in the tissues becomes visible. It is clear that such reddening of the leaves is an alarm for the plant owner.
By the way, the leaves of some plants (y - stems) with an excess of light sometimes acquire a bluish color. This is due to the development of a wax layer on the surface of the fabric, which very effectively reflects all the rays of light, but especially actively - blue and blue.
It is very interesting to solve the problem of maximizing the use of light by plants living in conditions of its constant deficiency. For example, under the canopy of a tropical forest.
Many paid attention to the leaves, in which the upper surface of the leaf is dark green, and the lower is deep red. It is clear that the destruction of chlorophyll in this case speech is not being made.
The fact is that the rays of light when passing through a thin sheet plate are far from completely absorbed: part of the light passes through the leaf and is lost by the plant. It is this problem that the lower surface of the leaf stained with anthocyanin solves. It reflects especially valuable red rays back into the leaf, i.e. causes them to re-pass through the chloroplasts. It is clear that the efficiency of the use of light rays in such a sheet increases significantly.
An important function of the accessory plant leaf pigments is to capture photons in the yellow-green part of the spectrum, which is not used by chlorophyll. As a result, the overall efficiency of photosynthesis increases.
I will give as an example passionflower three-lane(Passiflora trifasciata). Among the huge variety this species worth it especially. Perhaps this is the only passion flower grown exclusively for decorative leaves. Their red-violet color, which changes depending on the illumination, is due to the presence of additional pigments that actively use all parts of the incident light spectrum. In addition, a silver stripe runs through the center of each leaf blade. In general, the color of the leaves of this passionflower resembles the elegant coloring of the leaves of royal begonias.
However, in bright light, the leaves of passionflower with a three-stripe become simply green, and from the stripes in best case separate silvery specks remain. The fact is that the silvery stripes are nothing more than an accumulation of air-filled cells, which, in equally refract all rays of light passing through them. Some of them are reflected, and therefore we perceive them as silver-white, and most of them are directed inside the sheet plate. In other words, these hollow cells act like lenses, greatly increasing the efficiency of photosynthesis. It is clear that in plants with sufficient illumination, the need for this adaptation of the leaves disappears, and then the hollow cells are filled with chlorophyll.
The program that directs the plant to produce chlorophyll is written down at the gene level. More than a hundred genes are known to be involved in this process. But this complex mechanism sometimes fails - plants appear in which either part of the leaf plate or individual leaves are completely devoid of chlorophyll. Then the cells of the leaf can be filled with additional pigments (in this case, the leaf acquires the appropriate color) or simply become hollow, and therefore appear white.
Of course, from the point of view of healthy physiology, such plants must be considered inferior. But in practical floriculture, they are especially decorative, they are readily grown.
When dealing with such plants, it should be borne in mind that they are much more capricious than their green counterparts and therefore are especially demanding on. After all, the lack of chlorophyll in the leaves in the first place entails a decrease in plant nutrition. Therefore, with insufficient lighting, their leaves quickly lose their former brightness and variegation of color, become faded and oppressed.
In addition, lovers of such plants should remember that excess nitrogen in the soil can lead to the disappearance of leaf spot due to the accumulation of chlorophyll.
And one more thing: during the reproduction of such plants, the inheritance of the variegated color of the leaves is possible only in cuttings. Seedlings (and sometimes leaf cuttings) turn into normally colored, green specimens.
tricky leaves
Special mention should be made of the unusual leaves of some members of the Mesembryanthemum (Aizoon) family, and first of all, Lithops.
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lesson type - combined
Methods: partially exploratory, problem presentation, reproductive, explanatory-illustrative.
Target:
Students' awareness of the significance of all the issues discussed, the ability to build their relationship with nature and society based on respect for life, for all living things as a unique and priceless part of the biosphere;
Tasks:
Educational: to show the multiplicity of factors acting on organisms in nature, the relativity of the concept of "harmful and beneficial factors", the diversity of life on planet Earth and the options for adapting living beings to the entire range of environmental conditions.
Developing: develop communication skills, the ability to independently acquire knowledge and stimulate their cognitive activity; the ability to analyze information, highlight the main thing in the studied material.
Educational:
Formation of an ecological culture based on the recognition of the value of life in all its manifestations and the need for a responsible, careful attitude to the environment.
Formation of understanding of the value of a healthy and safe lifestyle
Personal:
education of Russian civil identity: patriotism, love and respect for the Fatherland, a sense of pride in their homeland;
Formation of a responsible attitude to learning;
3) Formation of a holistic worldview, corresponding to the current level of development of science and social practice.
cognitive: ability to work with various sources information, convert it from one form to another, compare and analyze information, draw conclusions, prepare messages and presentations.
Regulatory: the ability to organize independently the execution of tasks, evaluate the correctness of the work, reflection of their activities.
Communicative: Formation of communicative competence in communication and cooperation with peers, older and younger in the process of educational, socially useful, teaching and research, creative and other activities.
Planned results
Subject: know - the concepts of "habitat", "ecology", " environmental factors» their influence on living organisms, «connections between living and non-living things»;. Be able to - define the concept of "biotic factors"; characterize biotic factors, give examples.
Personal: make judgments, search and select information; analyze connections, compare, find an answer to a problematic question
Metasubject:.
The ability to independently plan ways to achieve goals, including alternative ones, to consciously choose the most effective ways solving educational and cognitive problems.
Formation of the skill of semantic reading.
Form of organization learning activities - individual, group
Teaching methods: visual and illustrative, explanatory and illustrative, partially exploratory, independent work with additional literature and textbook, with DER.
Receptions: analysis, synthesis, conclusion, transfer of information from one type to another, generalization.
Goals: to form an idea of the functions of the leaves, to reveal their significance for the plant as a whole; continue the formation of knowledge about the process of photosynthesis; to acquaint with various modifications of leaves as a result of adaptability to different conditions a habitat.
Equipment and materials: houseplants, herbarium various plants with modified leaves, the table "Modification of leaves", the scheme of the process of photosynthesis, a fragment of the video film "Modification of the leaf".
Key words and concepts: photosynthesis, evaporation of water, storage role of the leaf, leaf fall, excretion harmful substances, separating layer, cork layer, protective function of the leaf, spines, devices to reduce evaporation, hairs, attachment to a support, antennae, organs for catching insects.
During the classes
Knowledge update
Frontal survey
What is fabric?
What tissues make up the leaf blade?
What tissue is the skin of a leaf?
What is its main function?
Why are leaf skin cells transparent?
What is the biological meaning of this?
What tissues are represented by the pulp of the leaf?
What shape are the cells of the columnar tissue?
What is their main function?
What shape are spongy tissue cells?
What is their main function?
What is a stomata?
Due to what cells do they open and close?
What is the function of stomata?
What are vessels?
What is their function?
What are sieve tubes?
What kind of cells are they?
What is their function?
What is photosynthesis?
Under what conditions is photosynthesis possible?
What is released and what is taken in during photosynthesis?
Under what conditions do plants respire?
What is released and what is absorbed during respiration?
Learning new material
Teacher's story with elements of conversation
In previous lessons, we have repeatedly talked about photosynthesis.
Remember what it is.
Photosynthesis- the main function of the green leaf. This is the process by which a plant produces organic matter from inorganic matter using the energy of sunlight. In the process of photosynthesis in green leaves, carbon dioxide and water are formed organic matter (mainly carbohydrates) and oxygen.
Where does a plant get its carbon dioxide from?
How does water get from the roots to the leaves?
Where are the organic substances formed in the leaves during photosynthesis spent?
Where does the oxygen go?
The thing is that oxygen is, as it were, a by-product of photosynthesis and, accordingly, is removed from the leaf. But it is necessary to remember that in the dark the plant breathes, absorbing oxygen and releasing carbon dioxide.
It is possible to make sure that plants in the light absorb carbon dioxide and release oxygen, and breathe at night, absorbing oxygen, based on the results of the experiment. To do this, under a glass cap located on a glass surface, put green plant in a pot and placed live mouse. The place of contact of the glass cap with the glass surface is covered with petroleum jelly to completely exclude the penetration of air from external environment. The cap is placed in a lighted place. A day later, the mouse was alive. We know that animals (including mice) take in oxygen and release carbon dioxide when they breathe. There was a limited amount of oxygen under the hood. So where did he come from? It was in the process of photosynthesis that the plant absorbed carbon dioxide and released oxygen, which is necessary for the respiration of the animal.
If, however, in this experiment only one condition is changed - to put the cap not in a lighted place, but in darkness, the animal will die. This proves that plants breathe in the dark, that is, they absorb oxygen and emit carbon dioxide.
Another function of the leaf is the evaporation of water. The main purpose of evaporation is to cool the plant. This is especially important for plants in hot and dry climates. In addition, due to evaporation, a constant influx of water from the roots with the necessary substances dissolved in it is maintained. If there was no evaporation, there would be no constant flow of water to the leaves.
In addition, many leaves play a storage role.
Remember the structure of the bulb.
What is the main function of the bulb?
In what part of the bulb is the supply of nutrients?
Nutrients are stored in the fleshy modified leaves of the bulb. In this way, many plants of arid regions store water, as, for example, some types of stonecrops, aloe and agave.
Leaves can accumulate waste substances in themselves - slags, and then remove them from the plant in the process. leaf fall. This leaf function can be described as release of harmful substances. The leaves turn yellow or red at first.
What do you think causes the color of the leaves to change? (Answers from students.)
Chloroplasts are destroyed, and other plastids, chromoplasts, become visible. Then, between the petiole of the leaf and the stem, a special separating layer the cells of which begin to separate from each other due to the mucus of the intercellular spaces. On the stem at the point of attachment of the leaf is formed cork layer, therefore, after the leaf falls, there is no wound left on the stem.
Modified leaves of some plants help the stem to attach, cling for support.
What plants do you think in question? (Modified pea leaves, ranks, antennae help to cling to the support)
The leaves of some plants are modified spines like, for example, barberry.
What do you think the function of these leaves is? (They are protective.)
A spines And hairs cacti are necessary to adapt the leaves to reduction in evaporation.
In addition, the leaves of some plants have turned into special organs for catching insects.
Do you know such plants? (Student answers.)
These are, for example, the Venus flytrap and the sundew. The modified leaves of these plants secrete droplets of juice that attract small insects, and when the insect sits down, the leaf closes or rolls up and the insect is trapped. The leaf secretes digestive juices and then absorbs nutrients that were contained in the insect.
In addition to these functions, the leaves of some plants may also be involved in vegetative reproduction.
Give examples of such plants. (For example, begonia, room violet.)
Consolidation of knowledge and skills
Using the text of the textbook, as well as additional literature, fill in the table.
Creative task. Draw a diagram showing all the processes that occur in the leaves in the light and in the dark.
A task for students interested in biology. In additional literature, find information about what is a signal for leaf fall in plants middle lane.
Public lesson on the topic: "The meaning of a leaf in the life of a plant"
The leaf is part of the escape. Meaning of a leaf for a plant.AVI
Jobstomatasheetplants
transpiration
Resources:
I.N. Ponomareva, O.A. Kornilov, V.S. Kuchmenko Biology: Grade 6: a textbook for students of educational institutions
Serebryakova T.I., Elenevsky A. G., Gulenkova M. A. et al. Biology. Plants, Bacteria, Fungi, Lichens. Trial textbook grades 6-7 high school
N.V. Preobrazhenskaya Biology workbook for the textbook by V. V. Pasechnik “Biology Grade 6. Bacteria, fungi, plants
V.V. Pasechnik. Manual for teachers of educational institutions Biology lessons. 5th-6th grades
Kalinina A.A. Lesson developments in biology Grade 6
Vakhrushev A.A., Rodygina O.A., Lovyagin S.N. Checking and test papers To
textbook "Biology", 6th grade
Presentation Hosting
The leaf is a very important plant organ. This is the part of the shoot whose main functions are transpiration and photosynthesis. The structural features of the leaf are its high morphological plasticity, great adaptive capabilities and variety of forms. 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. Stipules are attached to the petiole or free, they can be shifted to inside, and are then called axillary.
External leaf structure
Leaf blades vary in size: they can be from a few millimeters to ten to fifteen meters, and for palm trees - even as much as twenty meters. The structure of the leaf determines the life span of the vegetative organ, it is usually short - no more than a few months, although for 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 bilateral symmetry. It usually consists of a petiole (with the exception of 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, and complex - with several plates.
The leaf cushion (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, connecting the stem and the leaf blade with its base. It orients the leaf relative to the light, acts as a place where the intercalated educational tissue is located, due to which the growth of the vegetative organ occurs. In addition, the petiole weakens the impact on the leaf during rain, wind, hail.
Leaf blade - 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 sheet is unfolded, they may fall off or remain. Designed to protect the axillary lateral kidneys and insert educational tissue.
Compound and simple leaves
The structure of a leaf is considered simple if it has one leaf blade, and complex if there are several or many plates 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 completely.
Whole leaves in shape can be lobed, separate or dissected. In a bladed leaf, cuts along the edge of the plate are up to 1/4 of its width. A separate organ is characterized by a larger depression, its lobes are called lobes. The dissected leaf has cutouts along the edges of the plate, reaching almost to the midrib.
If the plate is elongated, with triangular segments and lobes, the leaf is called plow-shaped (for example, in a dandelion). If the lateral lobes decrease towards the base, are uneven in size, and the final lobe is round and large, a lyre-shaped external organ of the plant is obtained (for example, in a radish).
The structure of a sheet with several plates is significantly different. Allocate palmate, ternary, pinnate organs. If a complex leaf includes three plates, it is called trifoliate, or trifoliate (for example, maple). A leaf is considered palmately complex when its petioles are attached to the main petiole at one point, and the plates 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 pinnate.
Forms of whole plates
In different plants, the forms of leaf blades are not the same in terms of the degree of dissection, outline, type of base and top. They can have round, oval, triangular, elliptical and other outlines. The plate is elongated, and its free end can be blunt, pointed, sharp or pointed. The base is attenuated and narrowed towards the stem, it can be heart-shaped or rounded.
Attachment to the stem
Considering the structure of the leaf of a plant, 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 (downward leaf), and it happens that the shoot pierces through the plate (pierced leaf).
Internal structure. Skin
The epidermis (upper skin) is an integumentary tissue located on the reverse side of a plant organ, often covered with cuticles, 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 to internal tissues and other adverse effects.
Skin cells are alive, they are different in shape and size: some are transparent, large, colorless, tightly adjacent to each other; others are smaller, with chloroplasts giving them 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 open, and when the fluid drains, it closes.
The anatomical structure of the leaf is such that air enters the inner cells through the stomatal gaps and gaseous substances come out through them. When plants are not sufficiently provided with water (this happens in hot and dry weather), the stomata close. So representatives of the flora protect themselves from drying out, since with closed stomatal crevices, water vapor does not go outside and is stored in the intercellular spaces. Thus, during the dry period, plants retain water.
Main fabric
The internal structure of the leaf cannot do without columnar tissue, the cells of which are located in the upper side facing the light, tightly adjoin each other, and have a cylindrical shape. All cells have a thin shell, nucleus, chloroplasts, cytoplasm, vacuole.
Another main fabric is spongy. Its cells are round in shape, located loosely, between them there are large intercellular spaces filled with air.
What will be the structure of the leaf of the plant, how many layers of spongy and columnar tissues are formed, depends on the lighting. In leaves grown in the light, the columnar tissue is much more developed than in those that grew in dark conditions.
Sheet- part of the escape. Outwardly, the leaves of different plants vary greatly, but there is much in common between them. The leaves of most plants are green in color and consist of leaf blade And petiole by which they are connected to the stem.
In some plants, the veins are parallel to one another. Such venation called parallel. It is found in many monocot plants. Arc venation is also characteristic of monocot plants.
In dicotyledonous plants, the veins branch many times and form a continuous network. This mesh venation.
But there are exceptions. For example, in a monocot plant raven eye leaves have reticulate venation. 
If there is one leaf blade on the petiole, the leaf is called simple.
A leaf consisting of several leaf blades connected to a common petiole by small petioles is called difficult. In such leaves, each plate usually falls off independently of the others.
Let's get acquainted with the internal structure of the leaf blade. The leaf blade consists of many cells of different sizes and shapes, that is, it has a cellular structure. On the upper and lower sides, the leaf is covered with more or less identical cells, tightly adjoining one to the other. These are the skin cells that cover the leaf and protect it from damage and drying out. Skin- one of the types of integumentary tissue of a plant. The skin cells are colorless and transparent, but among the colorless cells there are green cells arranged in pairs. guard cells. There is a gap between them. These cells and the gap between them are called stomata. Air enters the leaf through the stomatal opening and releases water vapor, oxygen and carbon dioxide into the atmosphere.
In most plants, stomata are found only in the skin of the underside of the leaf blade.
Under the skin are cells of the pulp of the leaf. The leaf pulp consists of several layers of cells. One of the layers is directly adjacent to the upper skin. Its cells resemble fairly equal columns. They are especially rich in chloroplasts. Deeper lie more rounded or irregular shape cells; they are close to each other. The spaces between cells are called intercellular. The intercellular spaces are filled with air. The pulp cells are green because their cytoplasm contains green plastids - chloroplasts. The color of chloroplasts is explained by the presence of chlorophyll in them - a green-green pigment. Chlorophyll in chloroplasts is formed only in the presence of light. The chloroplasts of flowering plants are sometimes called chlorophyll grains because of their shape.
If we examine the internal structure of the leaf blade under a microscope, it can be seen cut across veins. They contain transverse sections of cells - vessels, sieve tubes and fibers. Thus, the veins are conductive bundles sheet. Strongly elongated cells with thick walls - fibers- give strength to the sheet. Vessels move water and minerals dissolved in it. sieve tubes, unlike vessels, are formed by living long cells. The transverse partitions between them are pierced with narrow channels and look like sieves. Solutions of organic substances move through the sieve tubes from the leaves.
Plants capture light mainly through their leaf blades. In some plants with short stems, the leaves are collected in rosettes, and sunlight hits every sheet. The leaf petioles of many plants are able to bend, turning the plate towards the light. This allows better absorption Sun rays. For example, in ivy, the leaves are always turned towards the light, and if the plant is turned, after a while the leaf blades will also turn towards the light and arrange themselves in the form of a leaf mosaic, almost without shading each other.
Water evaporates from the leaf surface. Inside the leaf, water vapor passes through the intercellular spaces to the stomata and evaporates mainly through them. Especially young leaves evaporate a lot of water. different plants evaporate different amounts of water. Evaporation depends on environmental conditions and the condition of the stomata. If the plants have enough water, the stomata are open day and night. In some plants, the stomata are open only during the day, and close at night. Thus, evaporation is regulated by the opening and closing of stomata.
Plants of humid tropical forests - ficuses, begonias, philodendrons - have large leaves that evaporate a lot of moisture. Appearance plants of arid regions is also peculiar. The leaves of these plants are small. Sometimes they, like cacti, are replaced by thorns. The leaves of many plants in dry places are adapted to reduce evaporation. This is a dense pubescence, a wax coating, a relatively small number of stomata and other adaptations. For example, aloe, agave leaves are fleshy and juicy. They store water.
Leaves can also be modified because they play some other role that is not characteristic of typical leaves. For example, in barberry, some leaves turn into thorns. They evaporate less moisture and protect the plant from being eaten by animals. In peas, the upper parts of the leaves are turned into antennae. They serve to keep the stem of the plant upright.
interesting leaves carnivorous plants. Grows in peat bogs small plant sundew. Sundew leaf blades are covered with hairs that secrete a sticky liquid. Brilliant as dew, sticky droplets attract insects. On the leaf, insects get stuck in a sticky liquid. First, the hairs, and then the leaf blade, are bent and cover the victim. When the plate and hairs turn around again, only its integuments remain from the insect. All living tissues of the insect will be digested and absorbed by the leaf.
In autumn, the leaves gradually turn yellow and red due to the destruction of chlorophyll. By autumn, substances that are unnecessary for plants, and sometimes harmful to them, accumulate in the cells of the leaves. The leaf fall begins. Leaf fall is also an adaptation of plants to reduce evaporation in autumn and winter.
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