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

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

s.Evert, Ray F; Eichhorn, Susan E. Esau's Plant Anatomy: Meristems, Cells, and Tissues of the Plant Body: Their Structure, Function, and Development. Publisher: Wiley-Liss 2006. The presence of numerous sclereids form the cores of

apple An apple is a round, edible fruit produced by an apple tree (''Malus'' spp.). Fruit trees of the orchard or domestic apple (''Malus domestica''), the most widely grown in the genus, are agriculture, cultivated worldwide. The tree originated ...

s and produce the gritty texture of

guava Guava ( ), also known as the 'guava-pear', is a common tropical fruit cultivated in many tropical and subtropical regions. The common guava '' Psidium guajava'' (lemon guava, apple guava) is a small tree in the myrtle family (Myrtaceae), nativ ...

s. 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 upright=1.6, Lung parenchyma showing damage due to large subpleural bullae. Parenchyma () is the bulk of functional substance in an animal organ such as the brain or lungs, or a structure such as a tumour. In zoology, it is the tissue that ...

tissues. An isolated sclereid cell is known as an idioblast. Sclereids are typically found in the

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...

ground tissue The ground tissue of plants includes all tissues that are neither dermal nor vascular. It can be divided into three types based on the nature of the cell walls. This tissue system is present between the dermal tissue and forms the main bulk of t ...

, and

vascular tissue Vascular tissue is a complex transporting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. T ...

. The term "sclereid" was introduced by Alexander Tschirch in 1885.

Origin

Sclereids are created through belated sclerosis of

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 Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is ...

, 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 Pith, or medulla, is a tissue in the stems of vascular plants. Pith is composed of soft, spongy parenchyma cells, which in some cases can store starch. In eudicotyledons, pith is located in the center of the stem. In monocotyledons, it ex ...

of stems of '' Hoya'' and ''

Podocarpus ''Podocarpus'' () is a genus of conifers, the most numerous and widely distributed of the podocarp family, the Podocarpaceae. ''Podocarpus'' species are evergreen shrubs or trees, usually from tall, known to reach at times. The cones have ...

'' 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

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.

Leaf sclereids

Leaves contain a variety of types of sclereids. In the mesophyll, 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 of leaf veins. Sclereid formations in leaves include the branched sclereids of '' Trochodendron'', the columnar sclereids of ''

Hakea ''Hakea'' ( ) is a genus of about 150 species of flowering plants in the family ''Proteaceae'', endemic to Australia. They are shrubs or small trees with leaves that are sometimes flat, otherwise circular in cross section in which case they are s ...

'', and the hair-like trichosclereids that branch into air chambers within the leaves of the

water lily Water lily or water lilies may refer to: Plants * Members of the family Nymphaeaceae Nymphaeaceae () is a family of flowering plants, commonly called water lilies. They live as rhizomatous aquatic herbs in temperate climate, temperate ...

and yellow pond lily. Sclereids can also form part of or the entire epidermis of foliar structures such as the clove scales of ''

Allium sativum Garlic (''Allium sativum'') is a species of bulbous flowering plants in the genus ''Allium''. Its close relatives include the onion, shallot, leek, chives, Allium fistulosum, Welsh onion, and Allium chinense, Chinese onion. Garlic is native to ...

''. Especially interesting are the sclereids in

olive The olive, botanical name ''Olea europaea'' ("European olive"), is a species of Subtropics, subtropical evergreen tree in the Family (biology), family Oleaceae. Originating in Anatolia, Asia Minor, it is abundant throughout the Mediterranean ...

leaves. They are typically in length and are thus named fiberlike sclereids. These sclereids permeate the mesophyll as a dense mat. 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.

Fruit sclereids

Sclereids in fruits vary in form and use. In

pear Pears are fruits produced and consumed around the world, growing on a tree and harvested in late summer into mid-autumn. The pear tree and shrub are a species of genus ''Pyrus'' , in the Family (biology), family Rosaceae, bearing the Pome, po ...

s, sclereids from concentric clusters that grow about earlier formed sclereids. These pear sclereids, as well as sclereids within

quince The quince (; ''Cydonia oblonga'') is the sole member of the genus ''Cydonia'' in the Malinae subtribe (which contains apples, pears, and other fruits) of the Rosaceae family. It is a deciduous tree that bears hard, aromatic bright golden-yel ...

fruit, often form bordered pits when the cell wall increases in thickness during sclerification. In

s, layers of elongated sclereids form the

endocarp Fruits are the mature ovary or ovaries of one or more flowers. They are found in three main anatomical categories: aggregate fruits, multiple fruits, and simple fruits. Fruitlike structures may develop directly from the seed itself rather th ...

that encloses the seeds.

Seed sclereids

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 of

pea Pea (''pisum'' in Latin) is a pulse or fodder crop, but the word often refers to the seed or sometimes the pod of this flowering plant species. Peas are eaten as a vegetable. Carl Linnaeus gave the species the scientific name ''Pisum sativum' ...

bean A bean is the seed of some plants in the legume family (Fabaceae) used as a vegetable for human consumption or animal feed. The seeds are often preserved through drying (a ''pulse''), but fresh beans are also sold. Dried beans are traditi ...

, and

soybean The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean. Soy is a staple crop, the world's most grown legume, and an important animal feed. Soy is a key source o ...

seeds, and bone-shaped osteosclereids occur beneath the epidermis. In the seed coats of

coconut The coconut tree (''Cocos nucifera'') is a member of the palm tree family (biology), family (Arecaceae) and the only living species of the genus ''Cocos''. The term "coconut" (or the archaic "cocoanut") can refer to the whole coconut palm, ...

s, sclereids possess numerous bordered pits. These larger macrosclereids found in seed coats are of protodermal origin.

Origin

Stem sclereids

Leaf sclereids

Fruit sclereids

Seed sclereids

References

Further reading