Plant vascular tissues
1. Structure of plant roots
The outer layer of the root (as in leaves and stems) is the epidermis and vascular tissue is concentrated in a central stele (vascular cylinder). The single layer of cells immediately outside the stele is the endodermis. The layer of undifferentiated cells between the epidermis and endodermis is the cortex. Cells in the cortex typically have small air spaces between them and the cells themselves may be rich in starch grains.
The stele is mainly composed of xylem tissue with a smaller amount of phloem tissue.
The cells that transport most of the water and ions are called xylem vessels. In these cells, a secondary wall, impregnated with lignin, is formed inside the primary cellulose wall. Lignin is impermeable to water, so mature xylem vessels are dead. They have no end walls, no cell contents and are dead when fully formed. A column of vessels produces a long continuous tube up the plant, ideal for water transport.
There are different patterns of lignification in xylem vessels. In the first-formed xylem (protoxylem), produced in the growing regions behind the root and shoot tips, an annular or spiral pattern is produced. These patterns allow the vessels to elongate along with other tissues in the growth regions.
In the xylem vessels in more mature parts of the plant (metaxylem…