Small plants (especially aquatic ones) need a thin cellulose membrane surrounding the cells to maintain the strength and shape of the body. Large land 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 rarer, but plays a crucial role in maintaining the vegetative parts of a developing plant. The term itself comes from the Greek word "kolla" - 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.
The plasticity of cell membranes is provided by two factors:
- lack of lignification;
- reducing the elasticity of the shell due to the release of the protoplast (living cell content).
Collenchyma consistsfrom elongated parenchymal or prosenchymal cells up to 2 mm long. Their shells are characterized by uneven thickening, which gives the tissue a peculiar shape. A notable feature is the absence of a visible boundary between the primary and secondary walls.
The thickened areas 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 wall.
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, collenchyma ceases to perform its functions when 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 the ability to divide for a long time.
Toughness rating
In terms of mechanical strength (ability to resist tearing and bending), collenchyma exceeds the characteristics of cast aluminum, but is significantly inferior to sclerenchyma. In older parts of plants, collenchyma cells may undergo secondary thickening and lignification, which increases tissue strength but makes it more brittle.
Special property - high value of the modulus of elasticity (comparable to lead). This means that the fabric well restores its original structure after the cessation of mechanical stress.
Differences
Sclerenchyma is a more "rigid" mechanical tissue. Its cells not only lose the ability to divide, but also completely die due to thick lignified walls that block communication with the external environment.
Sclerenchyma differs from collenchyma in the following ways:
- death of protoplasts;
- uniform thickening of the shells with their subsequent lignification;
- cell walls are impervious to water and electrolytes;
- higher strength;
- inability of shells to stretch.
Sclerenchyma acts as a skeletal frame in the already formed parts of the plant. To a greater extent, this tissue is present in stems with secondary thickening. Sclerenchyma can be either primary or secondary, while collenchyma is only primary.
They perform their functions only in conjunction with other plant tissues.
Functions of collenchyma
Its main purpose is to ensure plant resistance to various mechanical loads (both static and dynamic). In addition, due to its good elasticity, this fabric forms the flexibility of stems and leaves.
Despite its relatively low strength, 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
According to the nature of the thickening of the cell wall, there are 3 main types of collenchyma:
- lamellar (typical for young stems of woody plants and sunflower);
- corner (pumpkin, buckwheat, sorrel);
- loose (highlander amphibian, belladonna, coltsfoot).
In the corner collenchyma, the 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 three- or pentagons (if you look at the cross section of the fabric). The thickened sections of the membranes in the lamellar collenchyma are arranged in parallel layers, and the cells themselves are elongated along the stems.
Loose collenchyma is a tissue with developed intercellular spaces, which are formed between the thickened areas of the membranes. It is characteristic of plants that develop aerenchyma (air-bearing tissue) as an adaptation to environmental conditions.
Distribution in the plant body
Collenchyma is a tissue characteristic mainly of dicotyledonous plants, young shoots, as well as vegetative structures that do not undergo secondary thickening (for example, leaf blades).
It can be located:
- in the zone of primary stem thickening;
- in petioles;
- in leaf blades of cereal plants;
- under the epidermis;
- extremely rare in roots (cabbage is an example).
In the stems, collenchyma is most often located on the periphery, close to the surface (sometimes immediately under the epidermis). This distribution provides good resistance to bending and fracture.
In leaves at the microstructural level, the arrangement of collenchyma elements, as well as other supporting tissues, resembles the design of an I-beam, in which a vertical one stands between two horizontal blocks, which does not allow them to sag under mechanical action.
