In terms of architectural organization, in the plant world we distinguish three basic types of organisms:
- a) without differentiated structures, such as algae;
- b) with differentiated shapes on the outside, which do not correspond to differentiated structures inside, such as mosses;
- c) with differentiated structures both externally and internally (higher plants).
In the latter case, a part of the cells becomes adult and differs according to the function to be performed, while a part remains in an embryonic state, to ensure the growth of the individual.
The fabrics .
Cells having the same function form a tissue.
Meristematic or embryonic tissues.
The phase of plant development that leads to the formation of new organs and to the plant-based structure is called primary growth. The primary growth is the result of the activity of the apical meristems, in which the cell division is followed by a progressive expansion of the cell, typically by elongation. After the completion of distension we can have secondary growth in a given region. The secondary growth implies the presence of two lateral meristems, the cribro-vascular change and the subero-fellodermic change.
Adult tissues.
The main tissues of a plant are divided into:
A) covering fabrics
Epidermis: compact tissue (without intercellular spaces), transparent (without chloroplasts, excluding stomatal cells, involved in gas exchange), consisting of living cells with cellulose wall, sometimes with deposition of waxes (waterproofing) or silicon (resistant). It is localized in the young parts of the plant. In the roots, the epidermis is replaced by the rhizoderm, and has neither coatings nor stomata (gas exchange takes place directly on the surface of the cell). Let us recall some epidermal productions: live (stinging, radical) or dead (on the underside of some leaves) hairs; the papillae on the petals of the flowers or on some fruits.
Cork: lacunose tissue (with wide intercellular spaces) composed of dead cells with suberized (impermeable) walls; stomata replaced by lenticels; present in the adult parts of the plant. Both fabrics are external and have protective functions.
B) basic supportive tissues
Collenchima: consisting of living cells, leaning against each other, with thickened walls, along their entire length or only in some points, with cellulose. Present normally under the epidermis. Function: elastic support.
Sclerenchyme: consisting of dead cells, with strongly thickened walls of lignin; present in the adult parts of the plant or in lamellar layers, or in fibers (in support of woody and cribrose vessels), or in the form of sclercides, idioblasts etc. Function: rigid support.
C) parenchymatous tissues with generally filling functions, are constituted by living cells, and assume different denominations depending on the functions they perform:
Aerifer: loose tissue rich in intercellular spaces used for the passage of gases.
Aquifer: cells cords with large spaces supported by thin lignified structures, deputy to the water reserve.
Reserve: compact fabric rich in nutrients, it is located in the areas where the plant is accumulated (marrow, root).
Chlorophyllian: in the green parts of the plant, with elongated cells (also called palisade) very rich in chloroplasts.
The cortical parenchyma near the epidermis and the rhizodermis, medullary along the woody and cribrose bundles, deserve a mention.
D) conductive fabrics
Floema (or cribro): consisting of living cells with cellulosic walls, with very reduced or even missing nucleus and protoplast, whose vital functions are carried out by the companion cells, leaning against each portion of the vessel. The cells, placed one above the other, form long cords that from the leaves carry the processed substance, called lymph, rich in sugars and nutrients (proteins, lipids etc.), to all parts of the plant. The side walls allow the diffusion of the lymph towards the outside of the vessels, thanks to the presence of plasmodesmata. The transverse walls have small perforations, which in winter get clogged, allowing the sap to stagnate in the portion of the overlying vase; they slow down the flow by regulating its distribution.
Xilema (or wood): consisting of dead cells with lignified walls, which form vases with a wider lumen than the cribrose vessels, superimposed on each other to form continuous columns that from the roots to the leaves allow the water and mineral salts to rise dissolved in it. We distinguish the tracheae, with a large lumen, where the transverse walls of the cells are completely missing, and the tracheids, with a narrower lumen with transverse walls present and perforated. If the wood is formed only by tracheids it is called omoxilo (Gymnosperme), if from both and from the others, eteroxilo (Angiosperms dicotyledons). The lignin thickenings have a different trend depending on the position of the vase. For example, in an area subject to secondary growth, they may have a spiral or overlapping ring pattern. Even today, the phenomenon of water elevation is not completely explained; we list some of the most important causes: a) capillarity; b) cohesion forces of water molecules; e) radical push (osmosis); d) suction power (stomatal transpiration).
E) Meristematic tissues: tissues that remain in the embryonic state throughout the life of the plant, consisting of living cells with thin walls (hemicellulose and poetic substances) capable of reproducing and giving rise to new cells by mitosis (equational division of the chromosomes contained in the core). Let us remember the cribrovascular change (capable of producing phloem towards the outside and wood towards the inside), the phellogen (capable of producing cork) and the apical meristems (root and stem). The peculiarity of these fabrics is to allow an "unlimited" life for some types of plants (perennials) and always the regeneration of all tissues.
