Tissues of the root

The layer with the root hairs, the epidermis, is comparable with the epidermis of the stem; it is a single layer of cells which has a protective as well as an absorptive function. 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 p79).

The cortex is often quite extensive and water must move across it in order to reach the transporting tissue that is in the centre of the root. This central region, called 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.

Water passes through the endodermis to the xylem tissue, 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'. Phloem tissue is responsible for transporting carbohydrates from the leaves as a food supply for the production of energy in the cortex.

Figure 6.10 Bark of silver birch showing lenticels
Ranunculus Root Cross Section

Phloem

Figure 6.11 Cross-section of Ranunculus root showing thickened outer region, large area of cortex and central vascular region or stele enclosed in an endodermis.

Thickened epidermis

Cortex

Endodermis

Xylem

Pericycle

Phloem

Figure 6.11 Cross-section of Ranunculus root showing thickened outer region, large area of cortex and central vascular region or stele enclosed in an endodermis.

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 age and become thickened with waxy substances, and the uptake rate of water becomes restricted.

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