In the world there is a huge variety of plant species, which differ in appearance, and the main feature of each plant is its leafy part. The leaves are of different sizes, shapes and colors, but these features are formed due to the unique cellular structure.
Therefore, today we consider the external and internal structure of the sheet, as well as its main types and forms.
What are the leaves: 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 foliage that distinguishes this part of the plant from others. This type of sheet is not without reason, since due to the flat form, it ensures maximum contact with air and light. This plant organ is bounded by leaf blade, petiole, stipule and base. In nature, there are also species of plants that have no stipules and petioles. 
Did you know? The world's hottest plates are putang. 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 are the types of green plates by types and shapes, 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 solid or dissected. To determine the nature of dissection, it is necessary to take into account how the protruding portions of the plate are distributed, depending on the main vein and petiole. It is possible to speak about feathering, if the parts that project beyond the base of the plate are symmetrical to the main vein. But if they protrude dottedly, from a certain place, they are called fingerprints.
The names of complex varieties are consonant with the simple ones, but the word "complex" is added to them. These are palmate-complex, peristosis, triple and others. 
To make it easier to understand simple and complex leaves, you can consider a few examples of plants.
Examples of simple are birch, maple, oak. Difficult - rowan, ash.
According to the shape of the plate
There are the following sheet plates that are in shape:
- broadly ovoid;
- rounded;
- ovoid;
- obscenely ovoid;
- elliptical;
- obovate;
- linear;
- oblong;
- obliquely ovoid;
- lanceolate;
Around the edges
The edges of the plant can be:
- entire;
- notched;
- wavy;
- prickly;
- notched;
- double-toothed;
- serrated;
- crentate;
On top
Plate tops can be:
- spiky;
- pointed;
- spinous;
- dull;
- notched;
- rumpled;
- rounded.
By base
The bases of the green plates can be of the following forms:
- rounded;
- rounded wedge-shaped;
- sphenoid;
- kidney-shaped;
- swept;
- spear-shaped;
- notched;
- truncated;
- drawn by
Did you know? There is a tree, each leaf of which has its own unique pattern. This Malaysian plant is anxiety.
Types of venation
When the study of the appearance of the considered part of the plant takes place, 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 elimination of organic matter accumulated in the plant.
The main types of venation are: arcuate, parallel, reticular or pinnate, palmate. 
As the arc venation of leaves, one can cite examples of such plants: lilies of the valley, plantain, which have a large venation, presented in the form of one central flat vein, around which all other veins are arranged in an arcuate manner. As a parallel venation, we can consider examples of maize and wheat plants.
As examples of mesh venation are sheets of aspen, oak, birch. They have a main vein, which is surrounded by many small, creating a kind of grid.
As an example of the finger-like venation, a platanovo maple, caustic buttercup, presented in the form of large veins, which diverge in a fan-shaped manner, has many smaller fan branches.
By leaf position
The leaf layout is presented in the form of whorled, alternate, rosette and opposite.
As an example of a whorl leaf layout, you can consider the horsetail of the forest, the next leaf position - vanilla leaves, the rosette leaf position - plantain leaves, the opposite leaf position - Rostok's eyebright.
The internal structure of the sheet
If we talk about the internal structure, it can be noted that it will be a question of its cellular structure. In order to most accurately characterize the cellular structure of the sheet, resort to considering its cross-section.
We advise you to get acquainted with the beneficial and harmful properties of the leaves: blueberries, raspberries, burdock and beets.
The upper part of the leaf plate is covered with skin, which is presented in the form of transparent cellular tissue. Skin cells are very closely spaced with each other, which ensures maximum protection of the 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 the inner part of the sheet.
The lower part of the leaf is presented in the form of stomata - green cells with slits. They may diverge or converge, open or close the gap. Due to the stomata, evaporation of moisture and gas exchange occurs.
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 a stomata on the surface of the leaf plate, for example, cabbage. Some aquatic plants, such as a water lily, do not have stomata on the inside of the leaf at all, as they are on the surface of the water, and evaporation by the lower parts of the plate is impossible.
The inside of the leaf is filled with pulp consisting of cells that contain innumerable chloroplasts, due to which they have a green color. In this part of the process of photosynthesis, so that the formation of organic substances. 
The pulp of the hardwood part is distinguished by the duplicity of cells. The 1st type is represented by columns that are on the surface of a hardwood plate, under the skin, and is called a columnar fabric. Below is the formation of spongy tissue, the cells of which are distinguished by friability, have between themselves sufficient air space.
The main part of organic matter is formed in the columnar tissue - this is due to the better illumination of the surface of the leaf plate, which contributes to the intensive process of photosynthesis. The spongy tissue provides gas exchange processes.
Important! If the plant is constantly in a well-lit place, several layers of columnar tissue are formed. Plants that are in the shade, have only one layer. At the same time, if one part of the tree is in the shade, and the second is in the sun, then leaves can be formed on one tree with one or several layers of columnar fabric.
To learn to distinguish between types of leaves, it is necessary to pay attention not only to the shape of the leaf plate, but also to its internal and external structure, which are described in detail in the article.
