Plant transport and mineral nutrition definitions

Xylem and the transpiration stream

Root hair cell

An epidermal cell of the root extended into a long narrow projection, giving a large surface area for the absorption of water and mineral ions.

Apoplast pathway

Movement of water through the cellulose cell walls and the spaces between them, without entering the cytoplasm of any cell.

Symplast pathway

Movement of water through the cytoplasm of the cells, passing from one to the next through plasmodesmata.

Casparian strip

A band of suberin in the walls of the endodermal cells that is impermeable to water, so the apoplast route is blocked and water must cross a plasma membrane.

Sclerenchyma fibre

A long, narrow, dead plant cell with tapering ends and a thick lignified secondary wall around a narrow lumen, whose function is support.

Vascular bundle

A strand of xylem and phloem running along a stem, root or leaf; in a young stem the bundles form a ring, each with the xylem on the inner side and a cap of sclerenchyma fibres outside the phloem.

Secondary thickening

Extra layers of cell wall laid down inside the primary wall once a cell has stopped growing, made of cellulose microfibrils at different angles in successive layers and usually impregnated with lignin.

Culture solution

A solution of mineral ions in distilled water in which plants are grown without soil, so that the ions available can be controlled.

Chlorosis

Yellowing of leaves because too little chlorophyll is made, as when nitrate or magnesium ions are in short supply.

Transpiration and stomatal control

Transpiration

The loss of water vapour from the aerial parts of a plant, mainly through the stomata, by evaporation from the mesophyll cell walls followed by diffusion out of the leaf.

Transpiration stream

The continuous movement of water from the soil, through the root and up the xylem, to replace the water lost from the leaves.

Stoma

A pore in the epidermis, bounded by two guard cells, through which gases and water vapour pass.

Circadian rhythm

A biological rhythm with a period of about twenty-four hours that continues in constant conditions, showing that it is generated by an internal clock rather than by the daily change in the environment.

Abscisic acid

A plant hormone produced under water stress that binds a receptor on the guard cell membrane and brings about stomatal closure.

Second messenger

A substance inside a cell whose concentration changes when a hormone binds a receptor on the outside, carrying the signal onwards; calcium ions do this job in the guard cell.

Phloem translocation and the mass-flow hypothesis

Sieve tube element

A living cell of the phloem, joined end to end with others into a tube; at maturity it has no nucleus, few organelles and only a thin layer of cytoplasm around the edge.

Sieve plate

The perforated end wall between two sieve tube elements, through which the sap passes.

Companion cell

A cell alongside each sieve tube element with dense cytoplasm, a nucleus and many mitochondria, connected to it by numerous plasmodesmata and responsible for loading it and keeping it alive.

Translocation

The transport of assimilates, mainly sucrose, through the phloem from a source to a sink.

Mass flow

The bulk movement of a fluid down a pressure gradient, carrying its dissolved solutes with it. In phloem it runs from the high hydrostatic pressure at a source to the lower pressure at a sink.