34 BOTANY. 



but in most higher plants it frequently attains some degree 

 of complexity, and is sharply separated from the underly- 

 ing ground-tissues. 



65. In the simpler epidermal structures of the lower 

 plants the cells are generally darker colored, smaller, and 

 more closely approximated than they are in the subjacent 

 mass; in some of the higher fungi a boundary tissue may 

 be easily separated as a thickish sheet, but probably in such 

 case a portion of the underlying mass is also removed. In 

 many lower plants there is absolutely no differentiation of 

 an epidermal portion. 



66. The epidermal systems of ferns and flowering plants 

 consist usually of three portions: (1) a layer of more or less 

 modified parenchyma — the epidermis proper — bearing two 

 other kinds of structures which develop from it, viz., (2) 

 hairs, and (3) breathing-pores. 



67. Epidermis. — The differentiation of parenchyma in 

 the formation of epidermis, when carried to its utmost ex- 

 tent, involves three modifications of the cells, viz., change 

 of form, thickening of the walls, and disappearance of the 

 protoplasmic contents. 



68. These may occur in varying degrees of intensity; 

 they may all be slight, as in m.any aquatic plants and in the 

 young roots of ordinary plants; or the cells may change 

 their form, while there may be little thickening of their 

 walls, as in other aquatic plants and some land-plants 

 which live in damp and shady places; or, on the other hand, 

 the change of form of the cells may be but little, while 

 their walls may have greatly thickened, resulting in a dis- 

 appearance of their protoplasm, as may be seen in parts of 

 some land-plants which grow slowly and uniformly. When 

 the differentiation of epidermis is considerable, it can usu- 



