Movement of water in the roots

It is the function of the root system to take up water and mineral nutrients from the growing medium and it is constructed accordingly, as described in Chapter 6. Inside the epidermis is the parenchymatous cortex layer. The main function of this tissue is respiration to produce energy for growth of the root and for the absorption of mineral nutrients. The cortex can also be used for the storage of food where the root is an overwintering organ (see p92).

The cortex is often quite extensive and water moves across it in order to reach the transporting tissue that is in the centre of the root. Movement is relatively unrestricted as it moves through the intercell spaces and the lattice work of the cell wall (see p88). The central region, the stele, is separated from the cortex by a single layer of cells, the endodermis, which has the function of controlling the passage of water into the stele. A waxy strip forming part of the cell wall of many of the endodermal cells (the Casparian strip) prevents water from moving into the cell by all except the cells outside it, called passage cells. In this way, the volume of water passing into the stele is restricted. If such control did not occur, more water could move into the transport system than can be lost through the leaves. In some conditions, such as in high air humidity (see p41), more water moves into the leaves than is being lost to the air, and the more delicate cell walls in the leaf may burst. This condition is known as oedema, and commonly occurs in Pelargonium as dark green patches becoming brown, and also in weak-celled plants such as lettuce,

Xylem tissue transports the water and dissolved minerals up to the stem and leaves.

when it is known as tipburn, because the margins of the leaves in particular will appear scorched. Guttation may occur when liquid water is forced onto the leaf surface.

Water passes through the endodermis to the xylem tissue (see p 92), which transports the water and dissolved minerals up to the stem and leaves. The arrangement of the xylem tissue varies between species, but often appears in transverse section as a star with varying numbers of 'arms' (see Figure 6.11).

A distinct area in the root inside the endodermis, the pericycle, supports cell division and produces lateral roots, which push through to the main root surface from deep within the structure. Roots, as with stems, age and become thickened with waxy substances, and the uptake rate of water becomes restricted. Root anatomy is described in Chapter 6.

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