thumb|Fresh mount of a sclereid in a banana fruit
Sclereids are a reduced form of sclerenchyma cells with highly thickened, lignified cellular walls that form small bundles of durable layers of tissue in most plants. The presence of numerous sclereids form the cores of apples and produce the gritty texture of guavas.
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 within parenchyma tissues. An isolated sclereid cell is known as an idioblast. Sclereids are typically found in the epidermis, ground tissue, and vascular tissue.
The term "sclereid" was introduced by Alexander Tschirch in 1885.
Origin
Sclereids are created through belated sclerosis of parenchyma cells or can arise from sclereid primordia that are individualized early in development. Sclerification typically involves thickening of the cell wall, increasing rigidity. In the phloem, when tissue ceases to function in conduction cells may begin sclerification. In vascular tissue, sclereids will develop from cambial and procambial cells.
Stem sclereids
In the vascular region of the stem of Hoya carnosa, a column of sclereids can be found, and in the pith of stems of Hoya and Podocarpus groups of sclereids can be found. These are sclereids with thick cell walls and numerous pits, resembling adjacent parenchyma cells. This resemblance suggests that these sclereids are originally parenchyma 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. 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.