. Botany for agricultural students . Botany. CELLULAR ANATOMY OF THE ROOT TIP 145. regions of the root increase in diameter, but almost all elongation takes place in the growth zone, as shown in Figure 129. The meristematic zone is thus so situated that the new cells formed by it may be added both to the root cap, the thickness of which is thereby maintained in spite of its being rapidly worn awaj^ on its outer surface, and to the growth zone, the older portions of which are constantly taking on the fea- tures of the absorptive zone just behind. The growth zone merges imperceptibly into the ab
. Botany for agricultural students . Botany. CELLULAR ANATOMY OF THE ROOT TIP 145. regions of the root increase in diameter, but almost all elongation takes place in the growth zone, as shown in Figure 129. The meristematic zone is thus so situated that the new cells formed by it may be added both to the root cap, the thickness of which is thereby maintained in spite of its being rapidly worn awaj^ on its outer surface, and to the growth zone, the older portions of which are constantly taking on the fea- tures of the absorptive zone just behind. The growth zone merges imperceptibly into the absorptive zone where the fol- lowing tissues become quite well defined: (1) a surface layer of cells constituting the epidermis which has most to do with absorption, the special absorptive agents being the root hairs, which, as the section shows, are merely projections of the epi- dermal cells; (2) a broad band of cells just beneath the epidermis and constitut- ing the cortex; and (3) a group of con- ductive tissues forming a central cylinder, known as the vascular cylinder. It is to be noted that the epidermis of roots, unlike that of leaves and stems, has no cutinized walls and contains no stomata or other openings for the entrance of air, although so many active cells re- quire much oxygen for respiration. How- ever, openings are not necessary, for the uncutinized walls offer practically no re- sistance to the passage of water, which usually carries in solution oxygen enough to support quite active respiration. Through the development of the root hairs the absorptive surface of the root system is much increased, and may be thereby increased from five to six times in Corn, about twelve times in Barley and as much as eighteen times in some other plants. All root hairs are able to absorb regardless of their size, which ranges from a slight bulge near the growth zone of the root to often more than an inch farther back. They live only a few days, but, as they die off behind, new o
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1919
