Anatomy of the Root
On the longitudinal section of young growing root, there are different horizontal layers, zones: root cap covering division zone, elongation zone, absorption zone, and maturation zone (Fig. 5.25). The root cap protects the root apical meristem (RAM), which is a group of small regularly shaped cells. A small, centrally located part of the RAM is the quiescent center where initial cells divide and produce all other cells of root. Root cap is responsible for the geotropic growth, if the root tip comes into contact with a barrier, root cap will feel it and will grow on a different direction to go around it.
The elongation zone is where the cells start to elongate, giving it length. The absorption zone is where the rhizodermis tissue (root hairs) develops and wherewater and nutrients are absorbed and brought into the plant. Within the matu-ration zone, root hairs degrade, many cells start to acquire secondary walls andlateral roots develop (Fig. 5.25).
On the cross-section of the root made within absorption zone, the first tissue is the rhizodermis, which is also known as the root epidermis, then cortex, which segregates external exodermis and internal endodermis one-cellular layers, and vascular cylinder (Fig. 5.26). Typically, roots have no pith. In some cases (for example, in orchids), cortex may give multi-layered velamen (see above), another absorption tissue.
Vascular cylinder is located in the center of the root, it contains the pericycle which is made of mostly parenchyma and bordering endodermis. Pericycle cells may be used for storage, they contribute to the vascular cambium, and initi-ate the development of lateral roots (consequently, lateral roots are develop-ing endogenously and break tissues located outside). Root phloem is arranged in several strands whereas xylem typically has a radial, sometimes star-shaped structure with few rays (Fig. 5.27). In the last case, phloem strands are located between rays of xylem.
Root tissues develop in the way similar to stem, RAM gave rise to ground meris-tem, procambium, and the protoderm, which in turn make all primary tissues mentioned above. Later, pericycle develops into lateral roots or the vascular cambium which in turn produces into the secondary xylem and phloem. The secondary root is similar to secondary stem.
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