Endodermis
Encyclopedia
The endodermis is the central, innermost layer of cortex in some land plants. It is made of compact living cells surrounded by an outer ring of endodermal cells that are impregnated with hydrophobic
substances (Casparian Strip
) to restrict apoplastic
flow of water to the inside. The endodermis is the boundary between the cortex and the stele
.
In many seedless vascular plants, the endodermis is a distinctly visible layer of cells immediately outside the vascular cylinder (stele) in roots and shoots. In most seed plants, especially woody types, an endodermis is absent from the stems but is present in roots.
The endodermis helps regulate the movement of water, ions and hormones into and out of the vascular system. It may also store starch, be involved in perception of gravity and protect the plant against toxins moving into the vascular system.
, a water-impermeable, waxy substance which in young endodermal cells is deposited in distinctive bands called Casparian strips. These strips vary in width but are typically smaller than the cell wall on which they are deposited. If the endodermis is likened to a brick cylinder (e.g. a smokestack), with the bricks representing individual cells, the Casparian strips are analogous to the mortar between the bricks. In older endodermal cells, suberin may be more extensively deposited on all cell wall surfaces and the cells can become lignified, forming a complete waterproof layer.
Some plants have a large number of amyloplasts (starch containing organelles) in their endodermal cells, in which case the endodermis may be called a starch sheath.
Endodermis is often distinctly visible with certain stains like phloroglucinol due to the phenolic and lipid nature of the Casparian strips or the abundance of amyloplasts.
Passage cells are endodermal cells of older roots which have retained thin walls and Casparian strips rather than becoming subarized and waterproof like the other cells around them, to continue to allow some symplastic flow to the inside. For the most part, however, old roots seal themselves off at the endodermis, and and only serve as a passageway for water and minerals taken up by younger roots "downstream".
Endodermal cells may contain starch granules in the form of amyloplasts. These may serve as food storage, and have been shown to be involved in gravitropism in some plants.
Hydrophobe
In chemistry, hydrophobicity is the physical property of a molecule that is repelled from a mass of water....
substances (Casparian Strip
Casparian strip
In plant anatomy, the Casparian strip is a band of cell wall material deposited on the radial and transverse walls of the endodermis, which is chemically different from the rest of the cell wall. It is used to block the passive flow of materials, such as water and solutes into the stele of a plant...
) to restrict apoplastic
Apoplast
Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant....
flow of water to the inside. The endodermis is the boundary between the cortex and the stele
Stele (biology)
In a vascular plant, the stele is the central part of the root or stem containing the tissues derived from the procambium. These include vascular tissue, in some cases ground tissue and a pericycle, which, if present, defines the outermost boundary of the stele...
.
In many seedless vascular plants, the endodermis is a distinctly visible layer of cells immediately outside the vascular cylinder (stele) in roots and shoots. In most seed plants, especially woody types, an endodermis is absent from the stems but is present in roots.
The endodermis helps regulate the movement of water, ions and hormones into and out of the vascular system. It may also store starch, be involved in perception of gravity and protect the plant against toxins moving into the vascular system.
Structure of the Endodermis
The endodermis is developmentally the innermost portion of the cortex. It may consist of a single layer of barrel-shaped cells without any intercellular spaces or sometimes several cell layers. The cells of the endodermis typically have their primary cell walls thickened on four sides (radial and transverse) with suberinSuberin
Suberin is a waxy substance found in higher plants. Suberin is a main constituent of cork, and is named after the Cork Oak, Quercus suber.-Anatomy and physiology:...
, a water-impermeable, waxy substance which in young endodermal cells is deposited in distinctive bands called Casparian strips. These strips vary in width but are typically smaller than the cell wall on which they are deposited. If the endodermis is likened to a brick cylinder (e.g. a smokestack), with the bricks representing individual cells, the Casparian strips are analogous to the mortar between the bricks. In older endodermal cells, suberin may be more extensively deposited on all cell wall surfaces and the cells can become lignified, forming a complete waterproof layer.
Some plants have a large number of amyloplasts (starch containing organelles) in their endodermal cells, in which case the endodermis may be called a starch sheath.
Endodermis is often distinctly visible with certain stains like phloroglucinol due to the phenolic and lipid nature of the Casparian strips or the abundance of amyloplasts.
Function of the Endodermis
The endodermis prevents water, and any solutes dissolved in the water, from passing through this layer via the apoplast pathway. Water can only pass through the endodermis by crossing the membrane of endodermal cells twice (once to enter and a second time to exit). Water moving into or out of the xylem, which is part of the apoplast, can thereby be regulated since it must enter the symplast in the endodermis. This allows the plant to control to some degree the movement of water and to selectively uptake or prevent the passage of ions or other molecules.Passage cells are endodermal cells of older roots which have retained thin walls and Casparian strips rather than becoming subarized and waterproof like the other cells around them, to continue to allow some symplastic flow to the inside. For the most part, however, old roots seal themselves off at the endodermis, and and only serve as a passageway for water and minerals taken up by younger roots "downstream".
Endodermal cells may contain starch granules in the form of amyloplasts. These may serve as food storage, and have been shown to be involved in gravitropism in some plants.