The part of the root just behind the root cap is called the subapical region. This region is traditionally divided into three regions: the zones of cell division, cell elongation, and cell maturation (fig. 7.7). These zones intergrade with one another and are not sharply defined. In general, however, the zone of cell division consists mainly of the apical meristem of the root, where new root cells are produced. These cells divide every 12 to 36 hours; in some plants, the meristem produces almost 20,000 new cells each day. In the zone of cell elongation, the newly formed cells elongate by as much as 150-fold, primarily by filling their vacuoles with water. Cell elongation in this zone shoves the root cap and apical meristem through the soil at rates as high as 4 centimeters per day. Cells behind the elongating zone do not elongate. In the zone of cell maturation, the immature, elongated root cells begin to take on a specific function, and root hairs develop on the outside of the root (see figs. 1.18 and 7.7b). Root hairs form only in the maturing, non-elongating region of the root; thus, roots have no lateral appendages at their tips. What would be the disadvantage of producing root hairs at the tip of the root?
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