TISSUE. 



93 



ii8. The epidermis is the external, compact layer oi 

 cells, rarely containiug chlorophyll, destitute of intercel- 

 lular spaces, and with their outer walls more or less thick- 

 ened (Figs. 203, 210). When the walls are much thick- 

 ened, the outer portion becomes cuticularized (impregnated 

 with cutin) and impervious to water. Often the outermost 

 portion becomes separated as a continuous pellicle, and is 

 called the cuticle. A waxy, or resinous matter, often 

 forms on the cuticle, and constitutes the bloom found on 

 some leaves and fruits. When the walls become much 

 thickened, the protoplasm disappears 

 from the cells. But in aquatic plants, 

 and the roots of ordinary plants, there 

 is but slight modification from the 

 other cells. The epidermis at first 

 consists of but one layer of cells, but 

 later may split into two or more la}'- 

 ers. The inner layers may resemble 

 the outer ones, as in the Oleander ; or 

 they may be thin-walled cells, with 210 



watery contents (called aqueous tissue), as in the Begonia. 



119. The outgrowths of the epidermis are called tri- 

 chomes (Gr. thriches, hairs), and may have the form of 

 hairs, scales, bristles, prickles, etc. (Fig. 210). They are 

 at first enlargements, or protrusions, of an epidermal cell. 

 They elongate and may remain one-celled, or become many- 

 celled, and take on an endless variety in shape, as may be 

 seen by examining these structures on many diiFerent plants. 

 Those elongated, unicellular hairs on the young roots of 

 plants, called root-hairs, have very thin walls, and absorb 

 the plant food which is dissolved in the water of the soil. 



Fig. 210. Epidermis and hairs from the ovary of the Squash flower. 



