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 many 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- 
