Sclereids are a reduced form ofsclerenchyma cells with highly thickened, lignifiedcellular walls that form small bundles of durable layers of tissue in mostplants.[1] The presence of numerous sclereids form the cores ofapples and produce the gritty texture ofguavas.
Although sclereids are variable in shape, the cells are generally isodiametric, prosenchymatic, forked, or elaborately branched. They can be grouped into bundles, can form complete tubes located at the periphery, or can occur as single cells or small groups of cells withinparenchyma tissues. An isolated sclereid cell is known as anidioblast. Sclereids are typically found in theepidermis,ground tissue, andvascular tissue.[2]
The term "sclereid" was introduced byAlexander Tschirch in 1885.[3]
Sclereids are created through belatedsclerosis ofparenchyma cells or can arise from sclereid primordia that are individualized early in development. Sclerification typically involves thickening of the cell wall, increasing rigidity. In thephloem, when tissue ceases to function in conduction cells may begin sclerification. In vascular tissue, sclereids will develop fromcambial and procambial cells.[4]
In the vascular region of the stem ofHoya carnosa, a column of sclereids can be found, and in thepith of stems ofHoya andPodocarpus groups of sclereids can be found. These are sclereids with thick cell walls and numerouspits, resembling adjacent parenchyma cells. This resemblance suggests that these sclereids are originallyparenchyma cells, but are so sclerified that they are now sclereid cells rather than parenchyma cells. These sclereids are an example of brachysclereids, or stone cells.[2]
Leaves contain a variety of types of sclereids. In themesophyll, two distinct sclereid structures are found. Sclereids in a diffuse pattern are dispersed throughout the leaf tissue, and sclereids in a terminal pattern are concentrated about the tips ofleaf veins. Sclereid formations in leaves include the branched sclereids ofTrochodendron, the columnar sclereids ofHakea, and the hair-like trichosclereids that branch into air chambers within the leaves of thewater lily andyellow pond lily. Sclereids can also form part of or the entire epidermis offoliar structures such as the clove scales ofAllium sativum.[2]
Especially interesting are the sclereids inolive leaves. They are typically 1 mm (0.039 in) in length and are thus named fiberlike sclereids. These sclereids permeate the mesophyll as a dense mat.[5] During sclerification these fiberlike sclereid cells can increase by several hundred times their original size, compared to other parenchyma cells that only increase by two or three times.[4]
Sclereids in fruits vary in form and use. Inpears, sclereids from concentric clusters that grow about earlier formed sclereids. These pear sclereids, as well as sclereids withinquince fruit, often formbordered pits when the cell wall increases in thickness during sclerification. Inapples, layers of elongated sclereids form theendocarp that encloses the seeds.[2]
The hardening of seed coats during ripening often occurs through sclerification, when the secondary cell walls are thickened in the epidermis and below the epidermis.Leguminous seeds are examples of such sclerification. Larger sclereids form columns in the epidermis ofpea,bean, andsoybean seeds, and bone-shaped osteosclereids occur beneath the epidermis. In the seed coats ofcoconuts, sclereids possess numerous bordered pits.[2] These larger macrosclereids found in seed coats are ofprotodermal origin.[4]
