Endoderme

A endoderme é unha capa interna do córtex, que rodea ao cilindro vascular nalgunhas plantas terrestres (ou ao periciclo nas raíces). A endoderme marca o límite entre o córtex e a estela. Está feita de células vivas ben compactadas coas veciñas; a súa capa externa de células estan impregandas de substancias hidrófobas, que forman a chamada banda de Caspary, que impide o fluxo apoplástico de auga cara ao interior do cilindro central.

En moitas pteridófitas, a endoderme é unha capa perfectamente distinguible situada xusto por fóra do cilindro vascular (estela) de talos e raíces. Na maioría das plantas con sementes, especialmente as leñosas, a endoderme está ausente dos talos pero presente nas raíces.

A endoderme axuda a regular o movemento de auga, ións e hormonas cara ao interior e exterior do sistema vascuar. Pode tamén almacenar amidón, estar implicada na percepción da gravidade e protexe a planta das toxinas impedindo que entren no tecido vascular.

Estrutura da endoderme

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 suberin, 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.

Funcións da endoderme

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. Experimental evidence suggests that passage cells function to allow transfer of solutes such as calcium and magnesium into the stele, in order to eventually reach the transpiration system. For the most part, however, old roots seal themselves off at the endodermis, 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.

Notas

• Gifford, Ernest M. & Foster, Adriance S. (1988). Morphology and Evolution of Vascular Plants, (3rd ed.). New York: W. H. Freeman and Company. ISBN 0-7167-1946-0.

Véxase tamén

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