Sclereids are either oval, rounded or elongated cells with thick Iignified walls that occur singly or in groups in plants.
Distribution of Sclereids Cells:
Sclereids occur singly or in groups and in association with xylem and phloem in all plant organs. They occur in the aerial roots of Monstera and in the roots of Nymphaea. The stems and petioles of Hoya, Nymphaea etc. have sclereids in their parenchymatous pith and cortex.
They are also present in the leaves of Camellia, Trochodendron, Nymphaea, and Monstera etc. The flesh of fruits of Pyrus, the seed coats of Phaseolus, Pisurn etc. contain sclereids. In the seed coats they may be arranged in a complete layer or occur as idioblasts, i.e. as cells that are readily distinguishable from the neighbours by their shape, cell contents and wall thickness.
The distribution of sclereids in the mesophyll tissues of leaves varies. They are apparently random or irregular in distribution (termed diffuse sclereid) in the leaves of Trochodendron, Nymphaea, and Olea etc. They may also occur at the veinlet endings (termed terminal sclereids) as observed in the leaves of Capparis spinosa, Arthrocnemum etc. Sclereids also occur near the margin of leaves, e.g. Camellia.
Origin of Sclereids Cells:
The origin of sclereids varies and it depends on the position in which they occur. The diffuse sclereids, which are irregularly distributed, originate from parenchyma cells, whose walls are secondarily thickened. The cortical parenchyma gives rise to sclereids that occur in the cortex (e.g. Nymphaea, Monstera etc.).
The terminal sclereids, which occur at the veinlet endings, have common origin with the veinlet. The same meristem gives rise to terminal sclereids and the procambium that gives rise to veinlet. Astrosclereids, present in the leaves (e.g. Nuphar) develop from mesophyll cells.
Structure and Content of Sclereids Cells:
Sclereids are thick-walled Iignified cells and thickening may not be uniform. The sclereids in the bark of Cinnamomum are strongly thickened on their tangential walls. The cell wall of sclereids may have numerous simple pits with round apertures and narrow lumen. Ramiform pits are also present.
Usually mature sclereids are described as dead cells. But it has been observed that sclereids can retain living protoplast. The stone cells of pear also retain living protoplast for several years.
Shape of Sclereids Cells:
Sclereids are of various shapes and accordingly Tschirch in 1889 distinguished four types (brachysclereid, macrosclereid, osteosclereid and astrosclereid) and Bloch (1946) added one more type —the trichosclereid.
Types of Sclereids (Fig. 8.5):
These sclereids are more or less isodiametric and resemble parenchyma cells. These are also commonly known as stone cell due to hard walls. They are also called grit cells and the gritty texture of Pyrus fruit is attributable to these cells, e.g. flesh of fruits of Pyrus, phloem of Hoya, Cinnamomum stem, barks of Alstonia etc.
These are elongated, rod-like or columnar in shape. The testa of many leguminous seeds is built entirely of macrosclereids, e.g. seed coat of Pisum (pea), Phaseolus (bean) etc.
The shapes of osteosclereids are columnar —the end of which may be lobed or branched or simply enlarged like a narrow bone, e.g. the seed coats of Pisum, the leaves of Hakea etc.
It is stellate cell, i.e. the cell is deeply lobed or branched in such a manner that it resembles stars, e.g. leaves of Thea (tea), Olea, and petiole of Nymphaea etc.
It is hair like, very much elongated cell with branches, which extends into the intercellular spaces, e.g. leaves of Olea, Nymphaea, and aerial root of Monstera etc.
The sclereids present in the leaf of Olea europaea are very much elongated, fibre-like and about one millimetre in length. Arzee (1953) termed these as filiform sclereids.
Function of Sclereids Cells:
(i) Sclereids are mechanical cells and support the tissues in which they occur;
(ii) Sclereids, which form a continuous layer at the periphery, protect the inner tissues.