The shapes of the leaves, flowers, and roots of plants are veryvaried. Today we will talk about one of the main organs of all green plants. This is a sheet. It is located on the stem, occupies a lateral position on it. The shape of the leaves varies considerably, as does their size. For example, in duckweed, aquatic plants, they are about three millimeters in diameter. Up to a meter can reach the Victoria leaf of Amazon. In some tropical palm trees its length is 20-22 m.
General characteristics of the leaves of plants
Leafless tree is a broomvarious sizes. It is often difficult to determine its appearance in the winter when the crown is naked. Trees with foliage that fell for the winter do not grow, although they remain alive. Only after their blooming, they begin to live fully and acquire their characteristic form. A leaf is not an axial organ, but it is closely connected with the stem, which is the axis of the shoot.
In psilophytes, the most ancient land plants,There was no dismemberment of the body, 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 the leaves and their organization are very plastic. From the stem and root, these organs differ in characteristic features. The leaves of the shoot are its lateral organs. They are formed superficially (exogenously) as tubercles located in the cone of growth. However, the leaves themselves have no growth cone. They grow by the ground. There are no other leaf or axial organs directly on them. Their growth is limited to a certain length of time.
Leaf structure: rules and exceptions
Leaf blade is an extendedpart of the sheet. Petiole - its stem-shaped narrow part. It is with the help of it that a leaf plate is connected to the stem. The base is the part by which the handle is attached to the stem. At the base are stipules.
As a rule, the structure of the leaves is spinal-ventral(dorsyventral). The plane of symmetry is one, and it divides them into 2 halves, symmetrical to each other. However, there are many exceptions to these rules. For example, the leaves of the frond (ferns) grow tops. As for pine needles, it increases in size over several years. In pine needles growing interchangeable growth at the base.
Однако самыми удивительными исключениями из этих rules can be considered leaves Velvichii Mirabilis. This gymnosperm plant, which is found in South Africa (Kalahari desert). The tumbar-shaped trunk of Velvichia Mirabilis (40 cm in height and 1 meter in diameter) forms only 2 leaves. Their length reaches three meters. The shape of the leaves is belt-like, they are leathery. These leaves die off from the ends, and at the base are constantly growing. As a result, their lifespan may exceed 100 years.
How to classify the leaves?
The outer variety of leaves is so largethat a single classification system based on one or several signs cannot be created. There are several classifications, which we now tell.
Petiole classification
There are three ways by which to stalkleaves are attached. Plants are distinguished with and without petioles. In the first case, the leaves of such a plant are called petioles, and in the second - sedentary. The base of some plants grows, covering the stem above the node. In this case, the leaf is called vaginal. The stem is as if embedded in it. If a sedentary leaf of a plant goes down along the stem, it is called decaying. A typical example is the thistle. If the leaf of a plant covers a stalk, it is called stalk comprehensive.
Complex and simple leaves
We proceed to the following classification.Leaf blades can also be quite diverse in shape, size, structure, and other parameters. They may be one or more. If there is only one plate, the leaves are called simple. The shape of the leaves of the trees in this case may be oval, round, lanceolate, oblong, ovate, linear, obovoid. When there are several plates on the same stalk, we are talking about complex forms. The arrangement of the leaf plates can also be different. The shape of the leaves (complex) may be as follows: interdiscipital, threefold, double-peristosophageal, non-paristophagous, paristophysic, palmatous, triple-complex.
However, simple leaves are not so simple.Consider this on the example of many well-known monstera plants. Its sheet consists of only one leaf blade, therefore it is considered simple. However, its shape is very fanciful. Leaves of this type are called dissected. There are other types. If the dissection of the plate does not exceed a quarter of its width, the shape of the leaves of the trees is lobed. If it is cut into a third, it is called separate. It also happens that the incision reaches the main vein of the leaf. In this case, the shape of the leaves of plants dissected.
The number of cuts, the shape of the leaf blades and edges
Переходим к следующей классификации.Plants can vary in the number of cuts on the sheet. If it is divided into 3 parts, it is called troychato, if it is 5 - palchato-, if it is divided into a larger number of parts, pinnacular (dissected, divided, lobed).
Leaf blades are also classified according to shape.Many of their forms are distinguished: ovoid, rounded, lanceolate, lanceolate, linear, oblong, heart-shaped, arrow-shaped, etc. Edges can also be classified according to the same basis. The most common form of the leaf edge is whole (whole leaves). However, there are several other species. Serrated, crenate, prickly-toothed (prickly), serrated, winding leaves with the shape of an edge are distinguished.
Heterophilia
Do you know this concept?If not, then we note that the leaves on one shoot may have different shapes, colors and sizes. This phenomenon was called heterophilia. It is typical, for example, for arrowhead, buttercup, and many other species.
Veins of plants
When considering a leaf blade of a plantyou can see that there are veins on it. These are conducting vessels. Their arrangement on the sheet may also be different. Venation is a way of arranging the leaves. There are several types of it: reticular (pinnate and palmar), dichotomous, arc, parallel. For monocotyledonous plants, arc or parallel veining is characteristic, and for dicotyledons - reticulate.
We offer to review and compare the leaves of oak and maple, to determine their shape.
Oak leaves
Oak - a plant characteristic of moderateclimate. It can be found in various regions of the Northern Hemisphere. Tropical highlands - the southern limit of its growth. Its leaves are leathery. They stay on the tree in evergreen species for several years, while in other species they fall annually or they remain on the branches, gradually collapsing and drying out. The shape of the oak leaf is paddle. However, sometimes there are also whole ones. This form of oak leaf is observed in some evergreen species. In white, for example, the leaves are quite large (up to 25 cm). This type of tree has an oblong-oval leaf shape. In spring, the crown gets a bright red color, and in the summer time changes its color to bright green, while the lower part turns white. The hue of the leaves in the fall varies. It can be from deep purple to burgundy. The shape of the autumn leaves does not change.
Red Oak (otherwise called northern) istall tree (up to 25 m), having a dense crown. Its leaves are large, have pointed blades. The name of this tree was due to foliage, which has a reddish color in autumn and spring.
Maple leaves
Homeland maple is Eurasia.It is a deciduous tree with a dense, rounded, wide crown. It reaches a height of 30 meters. The tree can live under favorable conditions up to 200 years. Its leaves are large, their diameter reaches 18 cm. They have pronounced veins. The shape of the maple leaf 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 lower two are somewhat smaller. Rounded recesses are between all of them. Petioles long. As for color, it also differs depending on the time of year. In summer, the leaves are dark green above and light green below. In the fall they get brown, red, maroon and brown shades.
So, we have considered the main forms of leaves. In conclusion, tell about their role.
Meaning of leaves
The most important function is educationorganic matter. A large and flat sheet plate captures sunlight. It is in the leaves that the process of photosynthesis proceeds. With their help, the plant also evaporates water. It can change the intensity of this process, closing and opening the stomata. In addition, gas exchange takes place with the help of leaves. Carbon dioxide and oxygen are supplied through the stomata. Oxygen is needed for respiration, and carbon dioxide is necessary for the plant for the synthesis of organic substances. During leaf fall, unnecessary substances are removed, the surface of the aerial organs decreases during an unfavorable period. The plant evaporates less water, crown accumulates less snow, which means it will not break.
