. An introduction to vegetable physiology. Plant physiology. FlQ. 25.—COLLBNOHYMA TJITDEE THE Epidermis of Petiole. X 60. latter is worn away. Certain cells become speciaKsed and form layers of cork (fig. 23), which arise successively at gradually increasing distances from the exterior, and in the case of trees finally lead to the construction of a bark (fig. 24). The corky formations are supple- mented by masses or sheaths of hardened or sclerenchymatous parenchyma, or even by scleren- chyma itself. In forms which are intermediate in requirements, such as the petioles of leaves, layers of coU
. An introduction to vegetable physiology. Plant physiology. FlQ. 25.—COLLBNOHYMA TJITDEE THE Epidermis of Petiole. X 60. latter is worn away. Certain cells become speciaKsed and form layers of cork (fig. 23), which arise successively at gradually increasing distances from the exterior, and in the case of trees finally lead to the construction of a bark (fig. 24). The corky formations are supple- mented by masses or sheaths of hardened or sclerenchymatous parenchyma, or even by scleren- chyma itself. In forms which are intermediate in requirements, such as the petioles of leaves, layers of coUenchyma are de- veloped below the epidermis (fig. 25). Sometimes sheaths or layers of sclerenchyma are developed instead of cork; this condition occurs especially among the stouter Monocotyledons. The protective mechanisms developed by roots also show a good deal of variety. The outermost layer does not at first take the form of an impervious membrane; this would be inconvenient in view of the necessity for the existence of root-hairs. In some cases the second layer undergoes modification, its cells fit closely together, and the radial walls become cuticularised where they are in contact with each other (fig. 26, ex) ; it then constitutes the exodermis. Later the corky change extends to all the cell-walls of this layer. Other sheathing layers are also found more deeply seated, while eventually the pericycle becomes the place of formation of corky tissue. The second prominent differentiation which presents itself is the formation of a system of cells and vessels for the trans- port of water through the plant and the circulation of nutri- tive and other materials. We may speak of this as the. —Section oi? Outer Rebiou of Root, show- IHO Exodermis, Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original
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