The following points highlight the three types of simple permanent tissue. They are: (1) Parenchyma (2) Collenchyma and (3) sclerenchyma.
Type # 1. Parenchyma:
Parenchyma (Fig. 534) is the most common simple tissue of the plants with relatively little specialisation. It is composed of cells which are usually isodiametric in shape with intercellular spaces. The cells have active protoplast.
This tissue is universally distributed in all the plants, the softer portions like epidermis, cortex, pith, pericycle whole or part, of stems and roots, mesophyll of leaves, pulp of the fleshy fruits, embryo and endosperm of the seeds being composed of parenchyma cells.
It is called the fundamental tissue of the plant, because it really constitutes the ground substance where other tissues remain embedded. Bodies of lower plants are made of parenchyma cells. The meristematic cells are also parenchymatous in nature.
Thus parenchyma is the precursor of all other tissues. So, it is considered to be the most primitive tissue, both phylogenetically and ontogenetically.
Parenchyma occurs as homogeneous mass in many portions, but it may also be associated with other elements in complex tissues like xylem and phloem. Normally parenchyma cells are polyhedral in shape with profuse intercellular spaces.
Unspecialised cells may approach 14-sided tetrakaidecahedron in shape. In the mesophyll of the leaves they are slightly elongated. Irregular shapes as a result of folding (Fig. 534D), lobation, etc., are also not uncommon.
Parenchyma cells have usually thin cell wall made of cellulose. Primary pit fields may be present. In storage region the walls of the cells may be considerably thick due to deposition of hemicellulose, as formed in the endosperm of Phoenix (date- palm). Often thick and lignified walls are present in the parenchyma cells of xylem, particularly secondary xylem.
From the point of view of function it is a very important tissue. Due to presence of active protoplast this tissue is the seat of all essential vegetative functions like photosynthesis, storage of food matters, secretion and excretion. Parenchyma cells occurring in xylem and phloem help in the conduction of water and food matters in solution. Parenchyma with thin cellulose wall can also serve as a supporting tissue due to turgid condition of the cells, what is particularly evident in herbaceous plants.
Epidermal cells with cutinized outer walls have protective function. Parenchyma cells retain the potentialities of cell division. Secondary meristems usually originate in this type of cells. Thus they are concerned in increase in thickness and also in healing of wounds and formation of adventitious roots and buds. In some xerophytic plants they are specially adapted for storage of water.
Water-storing parenchyma cells are large with prominent vacuoles, scanty chloroplasts and thin cell wall. Mucilaginous matters are present in the cell sap which increases water-holding capacity.
The parenchymatous cells of leaves and sometimes other organs like stem contain enough chlorophyll. These cells with that all-important function photosynthesis are also called chlorenchyma.
Fairly large air cavities may be present in the parenchyma cells of aquatic plants in particular, where the volume of cavities is often greater than that of the cells. As a result they often taken up star-like or stellate or armed appearance (Fig. 534 E, F & G).
The air spaces give buoyancy to the plants in addition to normal aeration. The term aerenchyma is usually attributed to this type of parenchyma. Parenchyma with small intercellular spaces is noticed in the endosperm cells.
Specialised parenchymatous cells which produce and store tannins, oil and crystals of calcium oxalate are referred to as idioblasts. They markedly differ from normal parenchyma cells.
The term prosenchyma for the cells much longer than breadth had been used by early authors. All elongated cells with thick walls and having supporting function were put under prosenchyma.
In view of the fact that diverse types of cells may be elongated, the term prosenchyma has gone out of use.
Type # 2. Collenchyma:
It (Fig. 535) is also a simple tissue consisting of one type of elements. The cells are somewhat elongate, occurring along the long axis of the body.
The shape of the cells is variable, the short ones being more or less like parenchyma and the longer ones resembling the fibrs, which may have, tapering ends often overlapped and interlocked like fibres. They are usually polygonal in cross-section.
The cells are living with vacuolate protoplast. Chloroplasts may also be present. Though normally collenchyma cells are narrower and longer than parenchyma, the two types of tissues very often intergrade; even transitional forms may occur.
Like parenchyma it can undergo reversible changes and retain the capacity of cell division. On account of these similarities collenchyma is considered a type of parenchyma specially adapted for supporting function.
The most distinctive character of the collenchyma cells is the cell wall which becomes unevenly thickened. There are different methods of deposition, but commonly, the thickenings are confined to the corners of the cells.
Often the degrees of deposition may be so much pronounced that cells look circular in cross-section. On the basis of thickening of the cell wall and arrangement of cells three forms of collenchyma have been recognised.
They are: (i) Angular collenchyma (Fig. 535 A & B), the most common type, where deposition is-localised to the junctions between the cells. This typical collenchyma is a compact tissue consisting of irregularly arranged cells without intercellular spaces.
Due to continued thickening of the cell wall the lumen may look circular. A term annular collenchyma has been used by some Workers for this type which has lost the angular appearance.
(ii) Lacunate or tubular collenchyma is the second type in which intercellular spaces are present and thickenings are restricted to the walls of the regions bordering on spaces (Fig. 535C).
(iii) Plate or lamellar collenchyma consists of compactly arranged cells with vigorously thickened tangential walls (Fig. 535D). As a result the cells appear like plates or bands.
Though these are the three types of collenchyma recognised, it should be noted that sharp distinction between them hardly exists. In fact, all the three types may occur in the same strand or one type may merge with another.
The wall though considerably thickened is primary in nature. It is composed of cellulose and pectic materials with high percentage of water. In some plants cellulose-rich and pectin-rich layers alternate on the wall. In some cases the wall may undergo sclerification. Simple pits may be present both on the thin and thick portions of the wall.
Collenchyma occurs in the peripheral portions of rapidly elongating organs like young stems, petioles of leaves, floral stalks and the leaves. They are noticed most commonly as homogeneous bands beneath the epidermis, or they may occur as discontinuous patches.
In ribbed organs like the stem and petiole of Cucurbita and square stems of members of family Labiatae, collenchyma occurs as patches in the ribs and at the corners of square stems. In leaves they may be present on both sides of the veins or along the margin.
This tissue is normally absent in underground organs, though it may rarely occur in roots, particularly when they are exposed to light. It is usually absent in the monocotyledons, both stems and leaves.
Collenchyma is an effective mechanical tissue of the growing organs, where it can give considerable strength and elasticity. The closely packed cells with thick walls have the capacity of increasing in surface and in thickness when the organ is still growing. In growing organs it provides sufficient tensile strength till more effective mechanical tissues like sclerenchyma are differentiated.
Here it serves temporary supporting function and may be crushed afterwards. In herbaceous stems collenchyma usually continues to function permanently, because secondary increase in thickness is poor in these organs.
In leaves they give support occurring on both sides of the bundles or as isolated patches. Though chloroplasts may often be present, this tissue probably has no photo- synthetic function.
Type # 3. Sclerenchyma:
Sclerenchyma is another simple tissue nicely adapted for mechanical function. The component cells are usually ‘prosenchymatous’, a term once used to designate the cells much longer than breadth.
The walls are considerably thickened, often heavily lignified with simple pits. The presence of hard elastic secondary wall with low water-content distinguishes sclerenchyma from collenchyma possessing plastic primary walls with high percentage of water.
The shape and size of the cells constituting this tissue are variable. They may be broadly placed into two groups: very much elongate cells, called sclerenchyma fibres, and short cells, isodiametric or irregular in shape, known as sclereids or sclerotic cells.
Though these are the two forms, intermediate types are not uncommon. Moreover, both the forms may remain intermixed in the same strand to carry on the same function.
Fibres:
Fibres (Fig. 536) are very much elongate sclerenchyma cells usually with pointed needle-like ends. The fibres of ramie, Boehmeria nivea of family Urticaceae may measure as much as 55 cm. in length. These are really the longest cells in the higher plants.
Though typical fibres have acute ends, blunt and even branched ends may also be noticed. At maturity these cells lose protoplast and become dead. It has been worked out that protoplast becomes multinucleate during differentiation and ultimately disappears.
Here is another anatomical feature by which sclerenchyma fibres may be distinguished from parenchyma and collenchyma. Of course some fibres may retain protoplast even up to the permanent stage. In cross-section sclerenchyma cells look angular.
The wall is usually hard, uniformly thickened and lignified. Small pits, round or slit-like in appearance, are frequently present. Often the secondary wall is so much thickened that the central lumen is almost obliterated (Fig. 536B).
Some fibres have non-lignified walls. In fact, highly thickened secondary walls of fibres of flax, Linum usitatissimum of family Linaceae are made of pure cellulose. Their walls exhibit lamellations. Some fibres may have gelatinous walls which may swell and fill up the entire lumen. They have been called gelatinous or mucilaginous fibres.
Fibres usually occur as groups or sheets along the longer axis of the body in different parts of the plants. They have peculiarly overlapped or interlocked ends (Fig. 536G). The value of sclerenchyma as the most effective mechanical tissue is due to the interlocking of the ends and considerably thickened walls. They may also occur singly as idioblasts as in the leaflets of Cycas.
The distribution of sclerenchyma in the plant members has direct relation to the strains and stresses to which they are subjected. They are abundantly present in cortex, pericycle, xylem and phloem. As regards classification of fibres a number of systems have been in use.
Often fibres are put in two groups, viz., intraxyllary fibres or wood fibres, and extraxyllary fibres, also called bast fibres. Fibres associated with xylem differ from other fibres particularly in the occurrence of bordered pits.
These fibres are put into two groups; libiriform fibres and fibre-tracheids (Fig. 536D). Libiriform fibres actually resemble other fibres, whereas fibre-tracheids are regarded as reduced tracheids due to presence of bordered pits.
These are really something intermediate between the tracheids and fibres. Some xylem and phloem fibres develop partition walls later, e.g., Vitis, which are called septate fibres (Fig. 536E).
They are living and have been found to contain starch, oils, resin and crystals and are thus thought to have storage function. Fibres occurring outside xylem—the so-called extraxyllary fibres-are often called bast fibres.
This term is misleading, because phloem is also commonly known as bast. It is possibly desirable to designate extraxyllary fibres on the basis of their topography as cortical fibres, pericyclic fibres and phloem fibres.
All these fibres are typically elongate bodies with simple pits. They commonly occur as continuous bands or isolated strands in the cortex, in the pericycle, as caps or sheaths on and around the vascular bundles and as patches in the leaves of monocotyledons in particular.
As already stated sclerenchyma is the most effective mechanical tissue of the plants. It can very successfully withstand strains and stresses like flexion, compression and longitudinal pull in cylindrical bodies and shearing stresses in leaves. The fact that some fibres, even fibre-tracheids retain protoplasts may suggest other physiological functions apart from support.
The fibres have great commercial value. Jute, hemp, flax, ramie, etc., are common sclerenchyma fibres.
