Leaf fall is characteristic of many trees and shrubs, as well as some grasses. When falling leaves the plant leaves. It happens in a certain period of the year. So in temperate climates the foliage falls in the fall, in the tropics and subtropics - in the period of drought. From this we can conclude that leaf fall precedes climatic conditions unfavorable to plants. So, the leaf fall serves as some kind of adaptation to survival in harsh conditions (cold or drought).
What is the main function of the leaves? This is photosynthesis, i.e. the synthesis of organic matter from inorganic. What is needed for photosynthesis? Water, carbon dioxide and light energy. However, in winter, little water enters the plant, since the water is in a frozen state. Water is also insufficient during the drought period (this applies to plants of hot climates). In addition, in winter, the length of the day is substantially less, and the sun is lower, that is, there is little light energy for photosynthesis. It follows that in winter the leaves of the plant are not particularly needed, if they can not perform their main function.
However, a temporary decrease in the possibility of photosynthesis does not mean that the foliage should be dropped. Why don't the trees leave old leaves intact until spring? After all, in the spring to form new ones, you need to spend the nutrients. It is not rational. The fact is that through the leaves a large amount of water evaporates. And in winter it is not enough. To avoid dehydration, the plant has to shed its foliage.
There are plants whose leaves have adaptations to reduce evaporation. Leaves of such plants small sizes, have a wax coating. In this case, leaf fall is not observed. Such evergreen plants include many conifers (their leaves are turned into needles), cranberries, lingonberries, etc. Of course, the foliage of these plants also falls and a new one grows in its place. But this happens as the specific foliage naturally grows old. All together, at a certain season of the year, the leaves of such plants do not fall off.
Fall foliage also prevents trees from breaking branches due to the snow that fell on them. After all, if large leaf blades remained, large masses of snow would accumulate on them. Under its weight could break off whole branches.
Plants that have leaf fall are also “learned” to solve their other problems with it. The plants do not have an excretory system (while most animals have it). In plant cells, many harmful substances accumulate in the vacuoles (although there are not only harmful, but also spare substances). To get rid of the harmful substances of the plant before the leaf fall, transport them from the vacuoles of the cells and the cells themselves to the foliage. During leaf fall, harmful substances leave the body.
Before the leaves fall, the leaves turn yellow and red. This is due to the fact that chloroplasts destroy the green pigment chlorophyll, and the chloroplasts themselves are reborn into chromoplasts. Therefore, the color of the leaves is determined mainly by the remaining and newly synthesized other pigments. And they are mostly yellow, orange, reddish flowers.
To make the sheet opal, a special cork layer forms in the bark of the tree. From the outer side of the cork at the junction of the leaf, the cells become close and the leaf falls off.
It is believed that the trees determine when it is necessary to prepare for leaf fall, along the length of daylight hours.
Fallen leaves gradually rotten, organic substances eventually decompose to inorganic (mineral). In this process, organisms-saprophytes (worms, bacteria, etc.) are actively involved. Thus, with leaf fall, many mineral substances that are so necessary for plants return to the soil. In other words, leaf fall contributes to the circulation of substances in nature.
The development of all living things in nature is a year-round process, where certain seasonal changes occur during each season. In the spring, nature wakes up, all the processes necessary for growth awaken, which prepare the plants for the summer season of the year. In the fall, plant growth slows down, the whole animal and plant world begins preparatory actions for the long and cold season of the year winter. Birds leave their homelands in poikah warmer places, sand animals collect food for a long wintering, and the trees are covered with gold, starting from the top of the head and the forest in the gusts of the autumn wind rustles the leaves. So begins the period, which is called the golden autumn of nature. At this time, the trees shed their leaves, forming a golden carpet around them.
In the autumn, due to the less frequent occurrence of the sun in the sky and cooling, chlorophyll is destroyed in the leaves, which gives them a green color. But besides chlorophyll, they also contain red and yellow substances. Since their share in the sheet is not very high, they are invisible in spring and summer. And in the autumn the decomposition of chlorophyll from the cold red and yellow dyes are preserved and become visible. Therefore, in autumn the leaves become bright colors. During the summer, a large number of substances harmful to the plant accumulate in the cells of the leaves, and by the autumn the leaves grow old. Also, during a cold snap, water supply is hampered and water evaporation slows down. And in order to overwinter, the plants shed their foliage.
Along with the leaf fall, the accumulated harmful substances come out of the plants. If this process would not have happened in the fall, in winter trees and shrubs would have died from lack of moisture. Plant roots are not adapted to suck up cold water from the soil in winter. And without leaves, they naturally need a lot less water to survive. However, in nature there are plants that retain green leaves in winter. This so-called evergreens. Their very small and dense leaves almost do not evaporate moisture, which allows them to remain under the snow. These include plants such as heather, cranberries, lingonberries. And although they are called evergreen, their leaves eventually fall off, having lived for several years. And in the place of the fallen young leaves appear again.
In this autumn period, especially, it is pleasant to walk, enjoying the bright colorful colors of the forest, as if picking up pictures of golden autumn on the desktop and you do not know which one is better to choose. Nature is so beautiful in the most beautiful season.
Autumn leaf fall is akin to poetry. Many poets draw inspiration from fall foliage. It is then that love poems are born when the soul sings and every poet has a need to write his own works.
18. Leaf life. Leaf fall and its biological significance.
The life span of green leaves that have developed on a shoot is different for different plant species and varies from 2-3 weeks to 20 years or more. In general, it should be noted that the leaves of perennial plants, compared with the stem and root, have the shortest longevity. This is apparently due to the fact that the leaf tissues, once formed, are no longer renewed, and on the other hand, the leaves are very active during their relatively short life. There are types of plants deciduous and evergreen. The former are characterized by the fact that they are leafless for some period every year, and this period usually coincides with unfavorable environmental conditions. The lifespan of their leaves is about 5 months. (This is birch, oak, alder, hazel, wild rose.) For example, most of our trees and shrubs do not have leaves in winter. Evergreens are characterized by the presence of green leaves throughout the year. But this does not mean that the sheet they have maintained and functions forever, throughout the life of the individual. Evergreens also have leaf fall, but older leaves fall off the plant and leaves that are formed at a later date are always preserved. In Moscow region evergreens - spruce and pine - the leaves are stored for 5-7 (by spruce) and 2-4 (by pine) years. The life span of the spruce needles is longer for plants growing in the Kola Peninsula and the Subpolar Urals, where it reaches 12–16, and in some cases, 18 (22) years. The needles of Tien-Shan spruce in Zailiysky Ala Tau, where leaves of 26-28 years of age were found, have long been preserved. (According to the data of I. G. Serebryakov (1952)).
Thus, the life span of a leaf is determined by the processes of growth and development of the plant organism as a whole. Leaf fall - The process of biological, due to the development of the plant organism and its vital activity. Falling leaves are a natural process of separating leaves from stems. In winter, it saves plants from water loss when its supply from the soil almost stops. In addition, leaf fall free plants from substances harmful to them, which accumulate by the autumn in the leaf cells. Without leaf fall, a lot of snow would linger on the leaves of the plants, and under its weight could break branches and stems.
Leaf fall is preceded by leaf aging: the intensity of life processes occurring in its cells (photosynthesis, respiration) decreases, the content of ribonucleic acid, nitrogen and potassium compounds decreases. Hydrolysis prevails over the synthesis of substances; final decomposition products (for example, calcium oxalate crystals) accumulate in cells. The most valuable mineral and plastic compounds leave the leaves. Their outflow usually coincides either with the formation and growth of new organs, or with the deposition of spare substances into ready-made storage tissues. In the experiments, it was possible to prolong the life of the leaves by removing the buds or other formations on the plant, which can receive plastic and mineral substances from the leaves. The transfer of substances to their reuse is considered as one of the causes of aging and abscission of leaves. Most of the trees and shrubs in the period of leaf fall leaves change color and become yellow or purple. Their yellow color is due to plastid pigments (carotene and xanthophyll) and cell sap (flavones). The reddish purple color of the leaves is provided by the accumulation of pigment in the cell sap anthocyaninwhich changes its color depending on the environment. In an alkaline environment, anthocyanin takes on a bluish-blue color, and in an acidic environment it is pink-purple. In some plants (alder, lilac) leaves retain green color until they die. In addition to the biochemical transformations that occur in the leaves before they fall, anatomical changes are also observed at the base of the leaf. Perpendicular to the longitudinal axis of the petiole near the stem, the cells of the separating layer are laid. The intercellular substance connecting these cells becomes close together and the cells separate from each other. At the place of separation of the leaf from the side of the stem, by this time, layers of cells are formed, the membranes of which are crushed. The resulting cork layer protects the internal stem tissue in place of the separated sheet. After the formation of a separating layer and the breaking of the connection between the cells, the leaf continues to remain on the tree for some time due to the conducting bundles connecting the leaf with the stem. But there comes a moment of violation and this connection, and the leaves fall.
Biological significance of leaf fall:
Improving value for a plant and protecting it from excessive evaporation of moisture in the fall and winter;
Protection against freezing of plant roots and fallen seeds;
Fall leaves is a good mineral and organic fertilizer.
Introduction
When I was still quite small, when I saw autumn leaf fall for the first time, I thought it was the wind that was tearing off the already “ripe” leaves from the trees and dropping them to the ground. Why the leaves for me were “ripened” in the autumn - because I thought that they, like the fruits on the trees, ripened with the onset of autumn.
When I went to school in class " The world“I began to realize that dropping leaves in the fall in the trees is their preparation for winter. But I still thought that it was not entirely correct, because trees and shrubs all spring and summer grow their leaves, expend energy, and then they take all this wealth from them. “What if the trees remain with leaves in the winter?” I thought at the time.
The time has passed. I began to study at the school "Biology", and only then from the textbook and from the teacher's stories did I understand what leaf fall is and why it happens. I did not find enough information about leaf fallings that I learned in class, and I wanted to learn more about the meaning of leaf fall for trees, using additional literature for this purpose.
So, taking as a basis the information from the textbook "Biology 6 class", supplementing them with the information of the Internet journal (http://awesomeworld.ru/), information from the sitehttp://ru.wikipedia.org/wiki/listopad , from the book of Graubin Georgy Rudolfovich “Why is leaf fall in the fall?” and from the book by Kozhevnikov A.V. “Leaf Fall” (chapter “Spring and Autumn in Plant Life”), I conducted my research.
The object of my research is leaf fall.
Hypothesis - leaf fall - is a biological adaptive phenomenon in the life of deciduous plants.
Objective: to study the biological significance, causes and mechanism of leaf fall.
To achieve the goal, the following tasks were set:
consider the causes of leaf fall;
determine the mechanism of leaf fall;
to draw conclusions about the biological significance of leaf fall for plants and the surrounding nature.
Used research methods:
Reading educational, popular science and reference books on the problem of the study.
Search for information in the global computer networks.
What is leaf fall?
Fall foliage refers to the natural process of separating leaves from branches of trees or shrubs, which was caused by a seasonal change in climatic conditions (cooling, drought), the internal rhythm of plant development, the defeat of harmful insects, diseases, chemicals, or poorly fertilized soil. This process occurs in absolutely all plants, including those that are considered evergreen: their leaves fall off gradually, simultaneously being replaced by new ones. Foliage may fall off at a time in a certain period of the year, like in deciduous trees, and maybe, like in evergreens, gradually, over a long time. It is worth noting that in tropical forests, plants stand without leaves, usually only a few days, whereas in temperate latitudes this period can last from eight to nine months.
Listopad - the biological process of dropping foliage by plants. In a temperate climate in winter, many plants lack water. The water in the frozen ground is in a state of ice and can not penetrate into the cells of the roots. At the same time, evaporation from the surface of the leaves does not stop (although it naturally decreases as it depends on the air temperature). If trees and shrubs, as well as some herbaceous plants did not shed their leaves, they would dry out. A similar phenomenon is observed in the subtropical zone. The reason there is not winter, but annual drought. Coniferous plants, such as spruce and pine, tolerate dry periods much better, so they are evergreen in temperate zones. The amount of water that is evaporated by hardwoods is 6-10 times the amount of water evaporated by conifers. This, on the one hand, is associated with a smaller evaporation surface, on the other, with differences in structure. Birch in terms of 100 g of leaves during the summer evaporates about 80 liters of water, for pine this figure is about 9 liters. Larch is intermediate between deciduous and coniferous species. The second reason for dropping leaves is protection against mechanical damage in winter against the mass of adhering snow. In addition, leaf fall cleanses the body of plants from harmful substances. Scientists have found that the leaves in the fall contain much more minerals than in spring and summer. This explains the fact that in the tropical zone with a uniform climate throughout the year, leaf fall still exists. There it happens not in short time, and distributed over the entire year and therefore less noticeable. Seasonal leaf fall in different latitudes different. In central Russia, the process of actively dropping leaves by plants begins in the second half of September and ends mostly by mid-October.
Causes fall foliage
What are the causes of leaf fall? What causes our deciduous trees and shrubs to shed their foliage every year in order to put them on again at the end of a harsh winter? To answer this question, it is first necessary to find out whether leaf fall is a biological phenomenon caused by the life of the plant, or is it caused by a fall in temperature and the onset of autumnal bad weather. If in the summer we transplant some young tree, for example, oak or maple, into the pot with the ground, and put it in the room or in the greenhouse, in the fall it will inevitably throw off its leaves, despite the best care. Autumnal bad weather does not penetrate into the room or behind the glass of the greenhouse, there are no frosts here, however, the leaf fall will show up here with sufficient accuracy. This indicates that the fall leaves are not a direct consequence of adverse conditions. It, along with the winter dormancy period, enters the very cycle of plant development.
Fall foliage is the adaptation of plants to the conditions of winter - not only cold, but also a dry season. If our deciduous trees remained in their green dress for the winter, they would inevitably die as a result of a lack of moisture, since the evaporation of water from their leaves would not stop, and the flow of water into the plant could almost completely stop. In many tropical and subtropical countries, where the temperature throughout the year is high enough, but the humidity is subject to strong fluctuations, every year, when drought occurs, the trees shed their leaves. The trees of the African savannas are exposed for several months, the grasses of which are also burned by the sun, until the heavy rain again revives the vegetation of the savannas.
Speaking here about the significance of leaf fall in the life of our trees, one cannot help but note that by dropping foliage, they thereby protect themselves from mechanical damage under the weight of snow. Often in winter one can observe how even in the leafless state large branches of trees break under the pressure of snow; a wide leafy surface, on which a lot of snow would settle, would make this phenomenon catastrophic. The above is far from being limited to the biological significance of leaf fall. In the life of trees, he also plays another role. It helps to remove waste, various mineral salts, a large amount of which accumulates in the leaves in autumn and becomes harmful to the plant. If you take the leaves of a tree and examine how much ash they contain in spring, in the middle of summer and autumn, before the leaves fall, then a sharp increase in the ash with the age of the leaves will be revealed. How does the accumulation of such a significant amount of mineral substances in the leaves occur during the summer? The fact is that the leaf throughout its life strongly evaporates water. In place of this evaporated moisture, a new one is continuously being fed into it, which is sucked in by the roots of the soil. However, as we know, the plant receives from the soil not pure water, but solutions of various salts. These salts, passing with the water through the whole plant, fall into the leaves. Some of them go to feed the plant, while the part that remains unused is deposited in the leaf cells. As a result of this, by the autumn the leaves become, as it were, mineralized, richly soaked in salts, the deposits of which in some cases can even be viewed under a microscope. A large amount of mineral salts deposited in the leaves by the fall disrupts their normal operation and becomes harmful to the plant; therefore, dropping old leaves is a prerequisite for its normal functioning. Since the deposition of mineral salts in the leaves is a result of evaporation, it is clear that the more moisture is able to evaporate the leaves, the more they are mineralized by autumn. The need to get rid of the accumulated in the leaves of harmful waste causes leaf fall in trees in a humid tropical climate. At first it was believed that in tropical areas, where the climate throughout the year remains more or less uniform, leaf fall does not exist at all. However, more careful observations, made on the island of Java in the famous tropical botanical garden in Bütenzorg and in India, showed that in the tropics, leaf fall is a most common phenomenon. True, the abscission of leaves in different trees does not occur here at the same time, and even different specimens of the same species fall into different leaves. time. As a result, a dormant period in a humid tropical climate often lasts only a few days for a tree or part of a tree. The plant discards old leaves that have become unnecessary ballast and immediately puts on a new green outfit. These facts indicate that leaf fall depends not only on external, but also on internal causes, that is, it becomes necessary as a result of the vital activity of the plant itself.
This is how the reasons for leaf fall are described in the book of Georgy Rudolfovich Graubin, “Why is leaf fall in the fall?”: “Although our deciduous trees live for dozens, often hundreds of years, their leaves“ work ”for only one season. And during this time, still wear out quickly. After all, the "work" of the leaves is very intense. In the green leaf, the entire lower surface, covered with transparent skin, is dotted with small openings — the stomata. Under the influence of ambient temperature and humidity, they open and close. Like window vents in homes. The water that the root sucks from the soil rises along the trunk to the branches and leaves. When the stoma openings are open, moisture evaporates from the leaves, and new portions of water are pulled through the barrel to the crown. The sun heats the leaves, and evaporation cools them, does not allow to overheat. Attach a leaf to your cheek - it cools. A green leaf torn from a tree dries quickly. And on the tree the leaves are juicy, fresh - the cells of the living leaf are always filled with water. Trees need a lot of water. During the summer, big birch, for example, evaporates about 7 tons of water. In winter, so much moisture from the soil will not get. Winter for trees is not only cold, but also, most importantly, a dry season. Losing leaves, the trees are protected from the "winter drought." No tree leaves - and there is no such abundant evaporation of water. In addition, leaf fall trees and for medicinal purposes. Together with the water, the tree draws various mineral salts from the soil, but does not fully use them. The excess accumulates in the leaves, like ash in the furnaces of the furnaces. If the leaves did not fall, the tree could poison itself. In cities, the air is heavily polluted by smoking pipes in factories and plants. The smallest particles of soot deposited on the leaves, clog the stomata. Evaporation slows down. Therefore, in cities, some trees have to change foliage twice a year. And the case is known when the poplar changed it five times! There is a third reason for leaf fall: to protect the thin fragile branches of a tree from the severity of the snow. Once I saw such a sad sight. The snow fell, and the trees have not yet dropped foliage. And all the birches that stood along the road were bent into an arc. They were so crushed by snow that the peaks fell to the ground. Many years later. I again saw these birches - many trunks remained similar to rocker arms. So, these trees are not quite healthy, the movement of the juices in them is disturbed. After all, it is along the trunk that nutritious juices rise to the leaves. Falls foliage trees for winter ... ".
How leaves fall
Since we live in latitudes, where leaves flying in the wind in autumn are common and absolutely not surprising, few people think that the process that led to this is rather complicated, difficult and begins long before we can see autumn leaf fall.
In August, shrubs and trees begin to prepare for dropping foliage. During this period, a partition (cork layer) appears at the base of the leaves, the cells of which break the connection between the leaf plate and the stem, gradually separating them from each other. The leaf doesn’t come off right away: for some time the water-bearing vessels hold it, but it is worthwhile to hit a gentle breeze, like leaves flying in the wind over a short period of time cover the earth’s surface with a bright carpet.
A visible sign that there will soon be significant changes in plant life is yellowing or redness of the leaves. This happens because the plants are given a green color by small chlorophyll grains, in which carbonic acid decomposes. Under the influence of the sun's rays, these grains constantly decay and are re-created (this process cannot occur without light). In addition to chlorophyll, in the foliage there are pigments of orange and yellow colors (the main among them are xanthophyll and carotene). In the summer, they are disguised as the color of chlorophyll, although they are present, but they are absolutely not visible. But when a separating layer appears on a sheet, the formation of chlorophyll slows down first and then stops altogether. As a result, the sheet plate loses its green color, while the yellow pigments do not disappear anywhere and show up clearly. This is the reason why the leaves are green much longer in rainy and cloudy autumn, and when the weather is replaced by clear sunny days (Indian summer), the leaves become bright golden colors in a very short period of time. The leaves of some trees become crimson in color, for example, maple, aspen, spruce, wild grapes. This happens because of the presence of anthocyanin dissolved in the cell sap in plants. When the air temperature decreases, its quantity increases and the leaf acquires a brown shade, which is also promoted by the fact that at the same time the supply of nutrients to the leaf plate starts to linger or stops altogether.
Conclusion
Leaf fall is the adaptation of deciduous deciduous plants to the onset of adverse conditions, both in temperate latitudes and in hot climates. Leaf fall allows plants to, first, slow down evaporation and save moisture, the amount of which is significantly reduced. Secondly, leaf fall, together with the fallen leaves, removes from the plant's body unnecessary substances that have accumulated during the vegetative period. Thirdly, by shedding leaves, the plants protect their branches from mechanical damage in winter from the mass of adhering snow. And the fallen leaves protect the roots of the plant from severe frosts and enrich the soil with organic matter, increasing its fertility.
The nature of leaf fall is a very wise decision, it allows plants to gain strength for the next season.
List of used sources and literature
1. Graubin G. R. Why fall leaf fall? - ACT. 2015 - 48 seconds
2. Kozhevnikov A. V. Fall leaves // Spring and autumn in plant life. - M .: Publishing house MOIP, 1950. - 239 p.
3. Online Journal: http://awesomeworld.ru/
4. http://ru.wikipedia.org/wiki/listopad