THE CELL AS A MEMBER OF A GROUP. 37 



These cells are thought to regulate evaporation by* 

 their expansion. (Fig. 12,) 



Hairs are epidermal structures, composed of one 

 or more cells. Under this general term may be in- 

 cluded prickles, scales, stinging-hairs, and glands. 

 Some secrete volatile oil, and others, as the nettle, an 

 acrid fluid. Fig. 13 exhibits some of their forms. 



Fig. 13. 



7. Next to the epidermis we find the cortex, or 

 bark, often composed of cells containing starch or 

 chlorophyll. Yessels containing latex (Sec. 4) and 

 glands, as well as sap-passages between cells, may also 

 occur in it. In some plants, masses of cork may be 

 found in the bark or beneath it. In such the outer 

 parts die and the bark peels off. 



8. Beneath the bark is the formative layer or 



