Sclereids are a reduced form of

sclerenchyma The ground tissue of plants includes all tissues that are neither Epidermis (botany), dermal nor Vascular tissue, vascular. It can be divided into three types based on the nature of the cell walls. # Parenchyma cells have thin primary walls and ...

cells with highly thickened, lignified cellular walls that form small bundles of durable layers of tissue in most

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...

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 an edible fruit produced by an apple tree (''Malus domestica''). Apple fruit tree, trees are agriculture, cultivated worldwide and are the most widely grown species in the genus ''Malus''. The tree originated in Central Asia, wh ...

s and produce the gritty texture of

guava Guava () 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), native to Mexico, Central America, the ...

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 Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the word π ...

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 hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...

, ground tissue, and vascular tissue. The term "sclereid" was introduced by

Alexander Tschirch Alexander Tschirch (17 October 1856 – 2 December 1939) was a German-Swiss pharmacist born in Guben. He received pharmacy training in Dresden and at the ''Berner Staatsapotheke'' (Bern state apothecary). From 1878 to 1880 he studied at the U ...

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, 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 ''Hoya carnosa'', the porcelainflower or wax plant, is an asclepiad species of flowering plant in the dogbane family Apocynaceae. It is one of the many species of ''Hoya'' that are native to Eastern Asia and Australia. It is a common house plant ...

'', a column of sclereids can be found, and in the pith of stems of ''

Hoya Hoya may refer to: Places *Hoya, Germany, a city in Lower Saxony, Germany *County of Hoya, a former state in present Germany * Hoya, Tokyo, now incorporated within Nishi-tokyo, Tokyo, Japan * Hoya, Hpruso, a place in Hpruso Township, Kayah, Myanma ...

'' 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

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 A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ste ...

, 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 A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...

. Sclereid formations in leaves include the branched sclereids of '' Trochodendron'', the columnar sclereids of ''

Hakea ''Hakea'' ( ) is a genus of about 150 species of 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 sometimes d ...

'', 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 family Nymphaeaceae * Formerly, members of the genus ''Nelumbo'' (the genus to which lotus belongs) * Some members of the genus ''Nymphoides'' Other uses * ''Water Lilies'' (Monet ...

and

yellow pond lily ''Nuphar advena'' (spatterdock or cow lily or yellow pond-lily) is a species of ''Nuphar'' native throughout the eastern United States and in some parts of Canada, such as Nova Scotia.Flora of North America''Nuphar advena''/ref> It is similar to ...

. Sclereids can also form part of or the entire epidermis of

foliar A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ste ...

structures such as the clove scales of ''

Allium sativum Garlic (''Allium sativum'') is a species of bulbous flowering plant in the genus ''Allium''. Its close relatives include the onion, shallot, leek, chive, Welsh onion and Chinese onion. It is native to South Asia, Central Asia and northeastern ...

''. Especially interesting are the sclereids in

olive The olive, botanical name ''Olea europaea'', meaning 'European olive' in Latin, is a species of small tree or shrub in the family Oleaceae, found traditionally in the Mediterranean Basin. When in shrub form, it is known as ''Olea europaea'' ...

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 the Northern Hemisphere in late summer into October. The pear tree and shrub are a species of genus ''Pyrus'' , in the family Rosaceae, bearing the p ...

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 also contains apples and pears, among other fruits) of the Rosaceae family (biology), family. It is a deciduous tree that bears hard ...

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

s, layers of elongated sclereids form the endocarp 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 The pea is most commonly the small spherical seed or the seed-pod of the flowering plant species ''Pisum sativum''. Each pod contains several peas, which can be green or yellow. Botanically, pea pods are fruit, since they contain seeds and d ...

bean A bean is the seed of several plants in the family Fabaceae, which are used as vegetables for human or animal food. They can be cooked in many different ways, including boiling, frying, and baking, and are used in many traditional dishes th ...

, 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, which has numerous uses. Traditional unfermented food uses of soybeans include soy milk, from which tofu an ...

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

coconut The coconut tree (''Cocos nucifera'') is a member of the palm tree 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, the seed, or the ...

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

Origin

Stem sclereids

Leaf sclereids

Fruit sclereids

Seed sclereids

References

Further reading