THE 0R0UP8 OF TISSUES, OB TISSUES rSTEMS. 41 



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

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

 protoplasmic contents. 



70. 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 disappearance 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 

 usually be readily removed as a thin transparent sheet of 

 colorless cells. 



71. The change in the form of the epidermal cells is due 

 to the mode of growth of the organ of which they form a 

 part; the lateral and longitudinal growth of an organ 

 causes a corresponding extension and consequent flattening 

 of the cells ; if the growth has been mainly in one direction, 

 as in the leaves of grasses, or if the growth in two direc- 

 tions has been regular and uniform, the cells are quite reg- 

 ular in outline; where, however, the growth is not uniform 

 the cells become irregular, often extremely so (Fig. 29, 

 page 44). 



72. The thickening of the walls is greatest in those plants 

 and parts of plants which are most exposed to the drying 

 effects of the atmosphere. It consists of a thickening of 

 the outer walls, and frequently of the lateral ones also. 



73. The outer portion of the thickened walls sometimes 

 separates as a continuous pellicle, the so-called cuticle. 



