What is the stem?
A stem is the organ of vascular plants that supports the leaves and reproductive structures. Along with the roots and leaves, the stems are a fundamental part of the body of a vascular plant and also have functions in the conduction of water and nutrients from the soil to the aerial portion of the plants.
Phylogenetically speaking, the stem corresponds to the most “primitive” portion of plant structures, which has been evidenced in the most primitive vascular plants, since both leaves and roots derive from it.
Although the main photosynthetic organs of plants are the leaves, the epidermis of the stems also has cells capable of photosynthesizing, although in a much smaller proportion compared to the leaves.
The stem of a plant has an apical zone, which is the one that corresponds to the youngest tissue. Leaves are formed from this area and, eventually, reproductive structures. In the case of flowering plants, the apical meristems of the stems differentiate into flowers.
Stem characteristics and morphology
The set of leaves, branches and stems is known as a stem and is what represents the aerial portion of all vascular plants. The branches and leaves are, in reality, modifications or specializations of the stem in the most evolved plants.
Usually, a stem is a cylindrical structure composed of several concentric layers of cells that fulfill specific functions. The stems differ from the roots by the presence of nodes, internodes and axillary buds.
The nodes are the insertion sites of the leaves, the internodes correspond to the space between successive nodes and the axillary buds are clusters of “dormant” cells that are located on the surface of the angles formed by the petioles of the leaves and the stems; These buds have the ability to form a new branch.
Like the root, the stems have a set of totipotential or pluripotent “stem” cells at their apex (more distal to the ground) in charge of the control of many physiological aspects of plants and the production of new cells during growth.
Primary structure of a stem
Stem growth occurs thanks to the differentiation of cells originating from the apical meristem of the stem. The primary growth of the stem is the one that gives rise to the primary structure of vascular tissue, which runs through the interior of the stem throughout its length.
There are many variations regarding the vascular structure of the stem among plant organisms. In higher plants such as angiosperms and gymnosperms, the vascular tissue is organized in the form of “independent bundles”, which correspond to “bands” of xylem and phloem.
The xylem is the set of “channels” through which the water passes, while the phloem constitutes the pipe for the transport of photoassimilates and other nutritive substances.
The primary structure of a stem consists of concentric layers of cells. From the outside in these layers are:
- The epidermis : that covers the stem and protects it.
- The cortex : formed by parenchymal tissue and that is the one found in the external part of the vascular ring.
- The vascular channels (which may be independent or beams) usually are arranged so that the phloem is disposed toward the “outer” face, closer to the cortex and the xylem to face more “internal” nearest the medulla.
- The medulla : also composed of parenchymal tissue and it is a fundamental tissue.
Secondary stem structure
Many plants undergo what is known as “secondary growth,” through which their stems and roots thicken. This growth occurs thanks to the activity of the meristems represented in the vascular cambium, which produces secondary vascular tissue (xylem and phloem).
Stems are vital structures for all vascular plants. The aerial growth of plants, photosynthesis, the formation of flowers and fruits (in Angiosperms), the transport of water and nutrients, among other things, depend on them.
For many types of plants, stems are propagation and / or storage structures for nutritional substances.
The stems produce leaves, which are the most important plant organs from a photosynthetic point of view.
Huge amounts of water and sap are transported from the roots to the aerial portion through the xylem and phloem of the stem (vascular tissue). Water hydrates and performs various cellular functions in plant tissues, and substances that are the product of photosynthetic assimilation are transported through the sap.
There are different types of stems, but the most common classification is regarding their function or their anatomical modifications.
Before describing the different modifications that this part of the body of a plant can undergo, it is important to note that stems can be differentiated not only by the arrangement of their leaves (phyllotaxis) but also by their environmental adaptations.
The most common modifications in the stems are: stolons, rhizomes, tubers, bulbs and tendrils.
Stolons are modified stems produced by many vegetatively reproducing plants. These are stems that grow horizontally, it could be said that they are parallel to the surface of the soil (they are creeping).
At the points of these stems where contact with the ground occurs, the production and / or differentiation of adventitious roots is triggered, allowing the stem to be attached to the substrate and the “independence” of this portion of the plant from the one that gave rise to it. since from this structure other plants identical to the mother plant are formed.
These stems are produced from an axillary bud on the main stem of a plant, which is activated and produces a horizontally elongated structure. When the adventitious roots settle part of the stolon to the ground, then the apex of this acquires a vertical position, thickens its structure and begins to produce leaves and flowers (when appropriate).
A typical example of stolon-producing plants is that of strawberries, which increase the size of their populations thanks to asexual reproduction through these structures.
Rhizomes are stolon-like structures, but are horizontally growing stems and are underground. Some authors highlight as typical examples of these modified stems the rhizomes of lilies, from which new leaves and stems can develop periodically.
Tubers and bulbs
Tubers are modified stolons, in which the most distal portion of the stolon “swells” and forms specialized starch storage structures. A potato or potato is a tuber and its “eyes” represent the axillary buds of the stolon that was modified.
Bulbs, on the other hand, are modified stems in which starch accumulates on the thick, fleshy leaves that are attached to the main stem.
There are “solid” bulbs, such as those of gladioli (which are modified reserve underground stems and correspond to stem growth and not leaves), and there are soft bulbs, such as onion bulbs, in which secondary growth It is from the leaves that cover it and not from the stem.
The tendrils are modifications found in some legume plants. It is a modification of the terminal portion of the stem that functions in the subjection of plants to vertical surfaces (to climb or ascend) and its corresponding support.