In the world there are a huge variety of plants that differ in appearance, and the main feature of each plant is its leafy part. The leaves come in 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
- Basic types and forms
- Simple and complex
- According to the shape of the plate
- Around the edges
- On top
- By base
- Types of venation
- By leaf position
- The internal structure of the sheet
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, distinguishing 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 blades, petiole, stipule and base. In nature, there are also species of plants that have no stipules and petioles. 
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 sheet 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. We can speak of pinnacles 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. Complex - rowan, ash.
According to the shape of the plate
There are the following sheet plates that are in shape:
- broadly ovoid;
- rounded;
- ovoid;
- obliquely 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
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 substances that have accumulated in the plant.
The main types of venation are: arcuate, parallel, reticular or pinnate, palmate. 
As the arc venation of leaves, there are 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 the other veins are arranged in an arc-shaped 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, have many smaller fan branches.
By leaf position
The leaf location is presented in the form of whorled, alternate, rosette and opposite.
As an example of a whorl leaf arrangement, you can consider the horsetail of the forest, the next leaf location - vanilla leaves, the rosette leaf location - plantain leaves, the opposite leaf position - Rostock's eyelets.
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.
The upper part of the leaf plate is covered with skin, which is presented in the form of transparent cellular tissue. The 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.
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, which is why 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. The formation of spongy tissue occurs below; 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. Spongy tissue provides gas exchange processes.
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.
