BIOLOGY Volume 3 - A Guide to General Biology - 2004

22. GROWTH AND DEVELOPMENT

22.4. Growth and development in flowering plants

22.4.5. Primary root growth

The Structure of a typical ROOT apical meristem is shown in Fig. 22.19. At the very tip of the apical meristem lies the quiescent centre — a group of initials (meristematic Cells) that ultimately give rise to all other root cells, yet divide significantly slower than their derivatives in the surrounding apical meristem. External to these lie the Cells of the root cap; they develop into large parenchymatous cells that protect the apical meristem as the root pushes down into the soil. These cells are continually worn away and replaced by new ones. They also serve an important additional function as gravity receptors: they contain large starch grains that act as statoliths, settling to the "bottom" of the cells under METABOLISM/18.html">The Influence of gravity. The Role of these grains is discussed in detail in section 16.2.2.

Just behind the quiescent centre, one can observe regular rows of cells and meristematic zones already described for the SHOOT, namely the protoderm, ground meristem, and procambium (Fig. 22.19). In the root, the entire central cylinder is referred to as the procambium, even though in its mature state it contains non-conducting pericycle and pith Tissues, if a pith is present.

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Fig. 22.19. Apical meristem of a typical root (longitudinal section). Xylem differentiation is shown on the right, and phloem differentiation on the left. In reality, the xylem and phloem alternate, positioned along different radii around the root circumference. Furthermore, the zone of elongation in a root is actually longer.

The zone of Cell Division typically occupies 1—2 mm behind the root tip and slightly overlaps with the zone of cell elongation. Root tips provide convenient material for studying mitosis, and one of the Methods used for this purpose is described in section 23.3. Behind the division zone, growth occurs primarily through cell elongation; the cells increase in size much like those described for the shoot and shown in Fig. 22.18. The zone of cell enlargement is located about 10 mm from the root tip, and the elongation of these cells pushes the root tip downward, forcing it to penetrate deeper into the soil.

Some Cell Differentiation begins even within the division zone with The Development of the first phloem sieve elements (Fig. 22.19). Longitudinal sections of the root tip clearly reveal longitudinal rows of sieve elements that become progressively more mature with distance from the root tip, until they transform into fully formed sieve tubes. Phloem development proceeds centrifugally from the outside inwards toward the central axis of the root.

Further away from the root tip, within the zone of elongation, xylem vessels begin to differentiate, also centripetally (exarch xylem), in contrast to centrifugal differentiation in the stem (endarch xylem). As in the stem, protoxylem vessel elements form first, undergoing the same lignification and stretching as the surrounding cells grow. Their Functions are then taken over by the metaxylem, which develops later and matures in the differentiation zone after cell elongation has ceased. The xylem often extends to the central axis of the root, in which case no pith is formed.

Development is easier to study in roots than in shoots. In shoots, procambial strands leading to the leaves complicate the distribution of developing tissues. The development of xylem is particularly well observed in squash preparations of root apices from thin roots, such as cress seedlings, using appropriate staining.

Differentiation concludes when all cells cease elongation. By this time, the development of root hairs from the epidermis is also complete.