BIOLOGY Volume 1 - A Guide to General Biology - 2004

6. HISTOLOGY

6.1. Simple Plant Tissues (tissues composed of cells of a single type)

6.1.3. Sclerenchyma

The sole function of sclerenchyma is to provide structural support for plant Organs and impart mechanical strength. The distribution of this tissue within a plant depends on the mechanical stresses to which individual organs are subjected. Unlike collenchyma Cells, mature sclerenchyma cells are dead; they are incapable of elongation, and consequently, their maturation occurs only after the elongation of surrounding living cells has ceased.

Structure

Two MAIN TYPES OF sclerenchyma cells are distinguished: fibres, which are elongated in shape, and sclereids, or stone cells, which are more spherical; it should be noted, however, that both the shape and size of these cells vary considerably. The structure of fibres and sclereids is illustrated in Figs. 6.6 and 6.7, respectively. In both Cell types, The Cell wall is heavily thickened by deposits of Lignin, a complex polymer that increases hardness as well as compressive and tensile strength. High tensile strength implies The ability to undergo considerable stretching without breaking, while high compressive strength provides adequate resistance to bending.

Class="center">

Fig. 6.6. Structure of sclerenchyma fibres. A. Transverse section showing polygonal cells. B. Longitudinal section showing elongated cells (length varies considerably: typically over 1 mm, and occasionally reaching 250 mm). C. General view. D. Sclerenchyma in a transverse section of sunflower (Helianthus) stem. E. Sclerenchyma in a Cytology/practical/54.html">Longitudinal section of Helianthus stem.

Fig. 6.7. Structure of sclereids. A. Transverse or longitudinal section showing isodiametric cells. B. Stone cell from macerated pear flesh, ×400.

Lignin is deposited On the surface of the primary Cellulose cell wall and within the microcellular spaces. As The Cell walls thicken, the living cell contents are lost; mature sclerenchyma cells are dead. The thickened cell walls of both fibres and sclereids contain simple pits. These are regions where lignin is not deposited on The surface of the primary cell wall; instead, these areas are traversed by groups of plasmodesmata (cytoplasmic strands that pass through microscopic pores in adjacent cell walls, interconnecting neighbouring cells). Each group corresponds to a single pit. They are called simple pits because each one forms a straightforward channel of uniform diameter. The diagram in Fig. 6.8 illustrates how such pits are formed.

Fig. 6.8. Formation of simple pits in sclerenchyma fibres and sclereids.

Functions and Distribution of Fibres

Each individual sclerenchyma fibre is structurally robust due to its lignified cell walls. However, when these cells are grouped together in Tissues to form strands and layers extending over considerable longitudinal distances, their collective strength is greatly enhanced. This overall strength is further reinforced by the overlapping of cell ends within the tissue, allowing the cells to interlock securely with one another.

Fibres are found in the pericycle of stems, where they form strands adjacent to the vascular bundles in dicotyledons (see Fig. 6.1). Frequently, fibres are located in the cortex just beneath the epidermis of stems or roots as a discrete layer, much like collenchyma—i.e., forming a hollow cylinder enclosing the remaining cortex and vascular tissue. Fibres also occur, either singly or in groups, within the xylem and phloem (Section 6.2).

Functions and Distribution of Sclereids

Sclereids are scattered either singly or in groups throughout almost the entire plant body, though they are particularly abundant in the cortex, pith, and phloem, as well as in fruits and seeds.

Sclereids impart strength or rigidity to the structures in which they reside, with these properties depending on both the number and arrangement of the sclereids. In pear fruits, for example, sclereids occur in small clusters, which accounts for the characteristic gritty texture of the flesh. In other cases, sclereids form exceptionally tough and dense layers, such as in nut shells or the woody endocarp (stone) of drupes. In seeds, they typically increase the hardness of the testa (seed coat).