For small plants (especially aquatic), a thin cellulose membrane surrounding the cell is sufficient to maintain the strength and shape of the body. Large terrestrial plants require a more advanced support system, represented by two types of mechanical structures: collenchyma and sclerenchyma. Otherwise, these fabrics are called supporting or reinforcing.
Collenchyma is much less common, but plays a crucial role in maintaining the vegetative parts of a developing plant. The term itself comes from the Greek word "calla" - glue.
Structure and properties
Despite its mechanical function, collenchyma is a living plant tissue capable of photosynthesis. Its protoplasts do not die, and the walls are elastic and able to stretch.
Plasticity of cell membranes is provided by two factors:
- lack of lumbering;
- a decrease in the elasticity of the membrane due to secretions of the protoplast (living cellular contents).
The collenchyma consists of elongated parenchymal or prosenchymal cells up to 2 mm long. Their shells are characterized by uneven thickening, which gives the fabric a peculiar shape. A notable feature is the lack of a visible boundary between the primary and secondary walls.
Thickened sections consist of alternating layers, some of which contain mainly cellulose, while others contain hemicellulose, pectin and a large amount of water. The total content of the latter is 60-70% of the mass of the cell membrane.
Uneven thickening of the cell wall contributes to its plasticity, and also regulates osmosis (thin sections allow water and electrolytes to pass through). For the same reason, the collenchyma ceases to fulfill its functions when the turgor disappears. An example is the wilting of leaves and grass as a result of water loss.
Collenchyma is a derivative of the main meristem. The cells of this mechanical tissue retain their ability to divide for a long time.
Strength characteristic
In terms of mechanical strength (the ability to withstand tearing and bending), the collenchyme exceeds the characteristics of cast aluminum, but is significantly inferior to sclerenchyma. In the old parts of plants, collenchyma cells can undergo secondary thickening and lignification, which increases the strength of the tissue, but makes it more fragile.
A special property is the high value of the elastic modulus (comparable to lead). This means that the fabric restores its original structure well after the cessation of mechanical stress.
Differences
Sclerenchyma is a more “stiff” mechanical tissue. Its cells not only lose their ability to divide, but also die completely due to the thick lignified walls that block communication with the external environment.
Sclerenchyma from collenchyma differs in the following ways:
- protoplast death;
- uniform thickening of the shells with their subsequent lignification;
- cell walls do not pass water and electrolytes;
- higher strength;
- inability of the shells to stretch.
The sclerenchyma plays the role of a skeletal skeleton in the already formed parts of the plant. To a greater extent, this tissue is present in the stems with a secondary thickening. Sklerenchyma can be of either primary or secondary origin, while collenchyma can be only primary.
They perform their functions only in conjunction with other plant tissues.
Functions of the Collenchyma
Its main purpose is to provide plant resistance to various mechanical loads (both static and dynamic). In addition, due to good elasticity, this fabric forms the flexibility of the stems and leaves.
Despite the relatively low strength, the collenchyma, due to its plasticity, is the only tissue suitable for young developing shoots, since the appearance of a rigid sclerenchyma would limit their growth.
Varieties
By the nature of thickening of the cell wall, 3 main types of collenchyma are distinguished:
- lamellar (typical for young stems of woody plants and sunflower);
- corner (pumpkin, buckwheat, sorrel);
- loose (highlander amphibian, belladonna, coltsfoot).
In the angular collenchyma, thickening of the membranes occurs at the corners of the cells (from which the name came). At the junction with each other, these zones merge, forming a pattern in the form of tri- or pentagons (if you look at the cross section of the fabric). Thickened sections of the membranes in the lamellar collenchyma are arranged in parallel layers, and the cells themselves are elongated along the stems.
The loose collenchyma is a tissue with developed intercellular spaces that form between thickened sections of the membranes. It is characteristic of plants in which aerenchyma (airborne tissue) develops as an adaptation to environmental conditions.
Distribution in the body of the plant
Collenchyma is a tissue characteristic mainly of dicotyledonous plants, young shoots, and also vegetative structures that do not undergo secondary thickening (for example, leaf blades).
It can be located:
- in the zone of primary thickening of the stems;
- in petioles of leaves;
- in leaf blades of cereal plants;
- under the epidermis;
- extremely rare in the roots (for example, cabbage).
In the stems, the collenchyma is most often located on the periphery, close to the surface (sometimes immediately below the epidermis). This distribution provides good resistance to bending and kink.
In the leaves at the microstructural level, the arrangement of the elements of the collenchyma, as well as other supporting tissues, resembles the design of an I-beam, in which there is a vertical beam between two horizontal blocks, which does not allow them to bend under mechanical action.