158 PART II. VEGETABLE HISTOLOGY. 



Hairs, also are mostly modifications of the epidermis. They 

 may be thin-walled, like ordinary parenchyma, or become cutin- 

 ized or hardened by deposit of mineral matters, such as silica 

 and calcium carbonate, or by sclerosis or excessive thickening of 

 the walls of their component cells, they may become hardened 

 into prickles. In the latter case, they are commonly strength- 

 ened by an outgrowth of cells from tissues underlying the 

 epidermis. 



Epidermal cells usually contain a nucleus and watery, trans- 

 parent and commonly colorless protoplasm. In some cases, 

 however, the cells are tinted by coloring matters in solution in 

 the cell-sap. Ordinary epidermal cells are usually destitute both 

 of chlorophyll-bodies and starch-grains, but exceptions occur in 

 the leaves of some thin-leaved plants growing in shady places 

 and in the leaves and stems of many aquatics. The guard-cells 

 of the stomata nearly always ^contain chlorophyll-bodies, and are 

 rich in protoplasm. Hairs, when mature, very often lose their 

 protoplasmic contents and become filled with air. 



(5) Endodermal Tissue. This tissue consists of a single layer 

 of compactly arranged cells which surround and form a protecting 

 sheath to either single fibro-vascular bundles or, more rarely, to 

 groups of them. The cells composing it are usually elongated, 

 four-sided prisms, with square or oblique ends and more or less 

 cutinized cell-walls. The cutinization is usually most decided in 

 the radial portion of the wall, or that which is common to adja- 

 cent cells of the tissue. This portion of the wall is also usually 

 seen to be more or less wrinkled or folded when examined under 

 a high power. See Figs. 435, 437, 438 and 441. 



(6) Cork or suberous tissue consists of parenchyma cells, the 

 walls of which have undergone the suberous modification. It is 

 commonly formed beneath the epidermis of woody stems and 

 roots, and ordinarily soon replaces the latter. It is also formed 

 over the surface of wounds during the healing process. The 

 cells are usually tabular in form, very compactly arranged in 

 radial rows, and at maturity lose their protoplasm, becoming 

 filled with air, Fig. 410. 



II. — Prosenchymatous Series. To this series belong those 

 tissue elements, or cell derivatives, which, at maturity, lose their 

 nuclei and protoplasmic contents, and therefore their distinctively 



