The below mentioned article provides a study on the sclereids and sclerenchyma in plant cell with diagram.
Sclereids or sclerotic cells (Fig. 537) are non-prosenchymatous cells, usually isodia- metric or irregular in shape (Fig. 537C). That is a marked point of distinction between
sclereid and sclerenchyma; but it has to be taken into account that sclereids originate from parenchyma cells by continued thickening and lignification of the wall and sclerenchyma fibres from meristematic cells.
But some of them may be slightly elongated (Fig. 537 B & D), specially those occurring in seed and fruit coats. Normally sclereids become dead with maturity. Cases have been recorded where they may retain protoplasts.
They may occur singly or in groups or patches in different parts of plants, more commonly in the cortex and phloem. As they vary considerably in size, shape and nature of the cell wall, sclereids may be put broadly into a few groups.
Brachysclereids, also called stone cells (Fig. 537A), are usually isodiametric ones like parenchyma cells. They are abundantly present in the soft parts of the plants like cortex, pith, phloem and flesh of fruits. The gritty texture of pears is due to presence of this type of sclereid. Macrosclereids are rather rod-shaped elongated cells which form a palisade-like epidermal layer on the seed-coats in Phaseolus (Fig. 537D), Pisum and other members of pulse family.
Osteosclereids are bone-like sclereids consisting of columnar cells, more or less dilated at the ends (Fig. 537F). They occur in the leaves of some dicotyledons and seed-coats.
Astrosclereids have irregularly branched arms and thus attain star-like or stellate appearance (Fig. 537G). This type is common in many dicotyledonous leaves. This classification does not really cover all the types. Very often sclereids develop into idioblasts, which are distinctly different from surrounding parenchyma cells.
In aquatic plants like Nymphaea armed idioblastic sclereids are found to project into the intercellular spaces. These are also called ‘internal hairs’ or trichoblasts or tri– chosclereids. Some parenchyma cells have thick lignified secondary walls. They are called sclerotic parenchyma.
The wall is typically hard and lignified. Strong secondary wall with distinct pits is the characteristic feature. The wall often blocks the central lumen and exhibits lamellations. The pits are simple. Ramiform canal-like cavities and branches are frequently formed due to fusion of pits as a result of pronounced thickening of the wall.
As already stated sclereids occur singly or in groups in different parts of the plants, but are most abundant in the soft tissues like cortex, pith, phloem, flesh of fruits, leaves and also in seed-coats and fruit wall. Solid layers of sclereids are present in the epidermis of some protective scales, as in Allium sativum (Fig. 537E). They may also occur in xylem and phloem often intergrading with fibres. It is an important mechanical tissue.
