All plant tissues are divided into two large groups: educational (or meristematic) and permanent. The latter are already differentiated groups of cells that perform certain functions (conducting, mechanical, etc.). Permanent tissues form the basis of the plant, but all of them are formed from meristematic cells, without which neither embryonic development nor growth is possible. One of these meristems is the cambium.
General characteristics of cambium
Since the main function of the cambium is the formation of conductive plant tissues, it, along with procambium, is referred to as vascular meristem (from lat. Vascularis - vascular). Cambium is of secondary origin and is present in the axial organs of plants (stems, roots). At the place of localization, this tissue is referred to as lateral (lateral) meristems.
Cambium cells are living, highly vacuolated and thin-walled. They have the ability to actively share without wasting a layer of the meristem, in contrast to procambium, which over time completely differentiates into vascular tissue. They have a prosenchymal shape with pointed ends and are elongated along the surface of the axial organ. The flat and wide sides of the cells face the conductive tissues, and the remaining walls are in contact with each other.
When dividing cambium, 2 types of cells are formed:
- initial - retain the ability to divide, completely identical to maternal cambial cells;
- differentiating - able to share a limited number (usually 2-3) with the formation of derivatives in the form of cells.
The initial layer of cambium is preserved throughout the life of the plant, serving as a source for the formation of a conducting system. The meristem is formed directly from Procambium or from secondary permanent tissues.
The cambium contains not only spindle-shaped, but also short radiation cells that give rise to radial conductive elements - heart-shaped rays, which are also called bast-wood. They provide the movement of substances in the horizontal direction.
Cambia functions
Unlike its predecessor (procambium), cambium gives rise to secondary conducting tissues (xylem and phloem) and therefore is characteristic mainly of gymnosperms and dicotyledonous angiosperms (both grassy and woody forms). In monocotyledons, only primary conducting tissues are present (metaxylem and metaphloem), which are formed from procambium. An increase in the diameter of the stem of the plant due to the formation of xylem and phloem from the cambium is called secondary thickening. Thus, the meristem promotes lateral growth of the axial organ.
As a result, 4 main functions of cambium can be distinguished:
- formation of a secondary conductive system;
- an increase in the number of vascular structures as the plant develops;
- secondary thickening of the stem;
- an increase in the diameter of the axial organs.
In plants with dying shoots, the cambium functions for one year, and for perennials - throughout life. The growth of the stem in thickness is seasonal in nature, since in the cold months the division of the meristem cells freezes. In tree trunks, this leads to the formation of the so-called annual rings.
The functions of cambium in the stems of herbaceous and woody plants differ only in the number and nature of the derivatives. In both cases, the work of the initial layer leads to the postponement of the phloem (bast) to the periphery, and xylem - inward. The difference lies in the fact that in the tree trunk the elements of vascular tissues are more rigid, powerful and are more compact than in herbaceous plants.
How does secondary thickening occur?
Secondary thickening is preceded by the formation of cambium, which can occur in three scenarios:
- The formation of a continuous cambial ring from a continuous procambium, which produces continuous layers of vascular tissue.
- The appearance of a beam (formed from the procambium inside the primary conducting bundles) and inter-beam species of cambium, which join in a continuous ring and begin to lay down xylem and phloem.
- The meristem laying occurs similarly to the second type, but the inter-beam cambium does not form conductive tissues, as a result of which the growth of the xylem and phloem is limited to the bundle zone.
In the latter case, the function of the cambium, which forms the jumper between the bundles, is the formation of mechanical elements or parenchyma, and the secondary conductive tissues do not form a continuous ring. This design is typical for short-lived plants with limited thickening.
Cambium in the trunk of a woody plant
The main function of cambium in a tree is the formation of secondary conductive tissues of the trunk - wood and bast. The meristem is laid before the differentiation of the primary vascular system. The cambium arising from the precursor tissue forms the initial layer, giving rise to the mother cells of wood and bast, located within the cambial zone. The latter grow in the radial direction, differentiating into elements of the vascular system.
Since xylem and phloem cells of woody plants die over time, the function of the cambium is to maintain the conduction system of the trunk. The working vascular zone is located near the vascular meristem, and the old layers of wood and bast play a supporting-mechanical and protective role.