The stem is the axial part of the shoot. It performs various functions: axial, conductive, support, storage. The external and internal structure of the stem is determined by the functions that it performs in the life of the plant.
On a cross-section of a branch or cut of a tree, it is easy to distinguish the following areas: bark, cambium, wood and core.
Young (annual) stems are covered on the outside with a skin, which is then replaced by a cork consisting of dead cells filled with air.
The skin and cork are integumentary tissues. They protect the stem cells located deeper from excessive evaporation, various damages, from the penetration of atmospheric dust with microorganisms that cause plant diseases.
In the skin of the stem, as well as in the skin of the leaf, there are stomata through which gas exchange takes place.
Lentils develop in the cork – small tubercles with holes, clearly visible from the outside (especially in elder, oak and bird cherry). Lentils are formed by large cells of the main tissue with large intercellular spaces. Gas exchange is carried out through them.
Under the skin and cork are bark cells formed by different tissues. The outer part of the cortex is represented by layers of cells of the integumentary and mechanical tissues with thickened membranes and thin-walled cells of the main tissue, which may contain chlorophyll. The inner layer of the cortex, which contains many cells of conducting tissue, is called bast.
The bast contains sieve tubes, thick-walled bast fibers and groups of cells of the main tissue.
Sieve tubes are a vertical row of elongated living cells, in which the transverse walls are pierced with holes (like a sieve), the nuclei in these cells are destroyed, and the cytoplasm is adjacent to the membrane. It is a conductive bast tissue, along which solutions of organic substances move.
Bast fibers, elongated cells with destroyed contents and lignified walls, represent the mechanical tissue of the stem.
Example: in the stems of flax, linden and some other plants, bast fibers are especially well developed and very strong. Linen is made from the bast fibers of flax, and bast and matting are made from the bast fibers of linden. The densest, widest layer, lying deeper, is wood – the main part of the stem. Wood is formed by cells of various shapes and sizes: vessels of conductive tissue, wood fibers of mechanical tissue and cells of the main tissue. The cambium lies between bark and wood. It consists of narrow, long cells of educational tissue with thin membranes. Due to their division, the stem grows in thickness and the formation of annual rings on it.
In the center of the stem is a looser layer – the core, which consists of large cells of the main tissue with thin membranes, in which nutrient reserves are deposited. From the core in the radial direction, core rays pass through the wood and bast. They also consist of cells of the underlying tissue and perform storage and conduction functions.
Example: the pith is clearly visible, for example, in aspen, elderberry and some other plants. In birch and oak, it is very dense, and it is difficult to see the border with wood. In some plants, there are large intercellular spaces between cells. This core is very loose.