Depending on the environmental conditions in most plants, the nature of the elements of which they are composed changes. At the same time, tissues are redistributed, most of which pass through all the organs of the plant continuously. However, they are modified in different parts in accordance with their functions.
In the initial period of development in the stem of a woody and herbaceous dicotyledonous plant, the primary bark, central cylinder and core are most often isolated.
Stem
The primary bark of the stem is its outer part. It is covered by the epidermis and extends to the central cylinder. It includes the main parenchyma, assimilation, mechanical, excretory, storage, secretory and other tissues. It is mainly formed by a multilayer tunic of a growth cone. During the transition to the structure of the stem of the secondary type, the primary cortex is deformed and, according to the results of the activity of the phallogen, is rejected into the cortical layer.
The structural features of the primary cortex
Between two adjacent tissues: the epidermis and endoderm, this cortex is enclosed. For different groups of plants, the cytological properties of this part of the stem are not the same.
The primary cortex in its composition, in addition to two adjacent tissues, has:
- subepidermal layer - the hypodermis, which for the most part consists of living cells with green plastids;
- mechanical tissues, the most common of which is collenchyma (fibers and scleroids are also found);
- the main parenchyma.
Functions
The primary cortex performs the following functions:
- protects the stele;
- promotes selective absorption of substances from the soil and their transportation to the stela;
- contributes to the download of xylem;
- is the custodian of water reserves (asparagus root cones);
- fungal hyphae that form mycorrhiza also develop in it.
Endoderm
In all organs of the plant, the endoderm is present as the inner layer of the bark. It is most differentiated in the roots and represented in the stem mainly by a single-row, narrow layer of cells, which are very compact.
At the first stages of development, the endoderm differentiates in the ontogenesis of the plant and has a common beginning with the cells of the cortex, so it would be fair to call it the deepest layer of the cortex.
Endoderm Stages
The meristematic phase of the endoderm is called the proendoderm, or embryonic endoderm. A typical endoderm can be discussed only after a thickened strip of a different chemical composition appears on the smallest cellulose walls of its cells. In the cross section, this strip is clearly visible. It encircles the transverse and radial walls of the cells. The strip is called Caspari in honor of the scientist who first described it in detail. The first stage of development of the endoderm is a cell with such a band.
The second stage is due to the appearance on the cell walls of the suberin plate, which is uniformly formed along the entire wall. The mechanism of suberin formation is not fully explained, however, it is known that the cause of its occurrence is the oxidation and condensation of phenols and unsaturated fatty acids with the assistance of the enzymatic system.
Numerous layers of cellulose are gradually superimposed on the secondary wall in the third stage of the endoderm. In most cases, these layers are visible under a microscope without prior treatment. They are lignified and can contain all kinds of inclusions.
Which plants have an endoderm?
The endoderm is widespread among various groups of plants. Only in psilophytes (the lowest forms of fossils without leaves) is it absent. In pteridophytes, the endoderm in the first and second stages, with some exceptions, is located in the root, petioles of the waya, the stem and leaves of the cirrus leaf, that is, it passes through the entire body of the plant. The endoderm is also located in the roots of gymnosperms, where it quickly crosses the first stage and passes into the second, but never reaches the third. Also, it does not occur in gymnosperm stems, only more or less deeply enters the hypocotyl in conifers.
The endoderm in the angiosperm roots has a very correct structure. Depending on the type of plant, the first, second or third stage can be maintained over a long root. The stem organs and roots of aquatic plants are characterized by a prolonged continuation of the first stage of the endoderm.
Typically, a typical endoderm is absent in the aerial organs of the angiosperms. However, a distinctive feature of the inner layer of the cortex from other cells is that it contains in large quantities large starch grains. This layer is considered a homolog of the endoderm, as it takes its place.
Older sites are occupied by the usual bark parenchyma, but it also happens that the starchy vagina, also called the inner layer of the primary cortex, is delimited as a typical endoderm with Caspari bands.
Periderm
Primary bark in woody plants is short-lived. The periderm (secondary covering tissue) is laid in different layers of the cortex of different plants on the branches of the first year of life. All tissues that were outside the periderm die off soon, since they are isolated from the central cylinder and living tissue of the cortex. Due to the fact that fellogen contributes to the deposition of cork tissue, the volume of primary cortex tissue will gradually decrease. When the laying of the pellogen occurs, it will be pushed outward by layers of cork into the endoderm or into the pericycle, where it will soon dry.
At the same time, significant changes occur in the central cylinder due to the activity of the cambium.
Typically, the secondary bark, wood, and core are distinguished in the secondary structure of the stem.
Concepts such as the primary and secondary cortex are not homologous. The latter differs from the first in composition, function, and origin and is a collection of tissues that lie outside of the cambium, including hard and soft bast.
If the remains of the primary cortex are preserved, then they are called secondary integumentary tissues. In this way, tissues of different functional significance and origin enter the secondary cortex.