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 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 excl ...

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 trees are cultivated worldwide and are the most widely grown species in the genus '' Malus''. The tree originated in Central Asia, where its wild ancest ...

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, t ...

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 � ...

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 rel ...

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. # Parenchyma cells have thin primary walls and usually remain alive after they beco ...

, and

vascular tissue Vascular tissue is a complex conducting 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. The ...

. 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 c ...

, 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 ext ...

of stems of '' Hoya'' and ''

Podocarpus ''Podocarpus'' () is a genus of conifers, the most numerous and widely distributed of the podocarp family, the Podocarpaceae. The name comes from Greek πούς (poús, “foot”) + καρπός (karpós, “fruit”). ''Podocarpus'' species ...

'' 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, ...

, 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 ''Trochodendron'' is a genus of flowering plants with one living species, '' Trochodendron aralioides'', and six extinct species known from the fossil record. It was often considered the sole genus in the family Trochodendraceae, though botanist ...

'', the columnar sclereids of '' Hakea'', and the hair-like trichosclereids that branch into air chambers within the leaves of the water lily and yellow pond lily. 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 (biology), family Rosacea ...

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. It is a deciduous tree that bears hard, aromatic bright ...

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

s, layers of elongated sclereids form the

endocarp Fruit anatomy is the plant anatomy of the internal structure of fruit. 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. Aggr ...

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 A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock for ...

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 ...

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 f ...

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