GROUPS OF TISSUES, OR TISSUE.SYSTEMS. 35 
ally be readily removed as a thin transparent sheet of col- 
orless cells. 
69. 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 directions has 
been regular and uniform, the cells are quite regular in 
outline; where, however, the growth is not uniform the 
cells become irregular, often extremely so (Fig. 24, page 38). 
70. 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. 
71. The outer portion of the thickened walls sometimes 
separates as a continuous pellicle, the so-called cuticle, 
which extends uninterruptedly over the cells, and may be 
readily distinguished from the other portions of the outer 
epidermal walls. It is insoluble in concentrated sulphuric 
acid, but may be dissolved in boiling caustic potash. 
Treated with iodine it turns a yellow or yellowish-brown 
color. A waxy or resinous matter is frequently developed 
upon the surface of the cuticle, constituting what is called 
the bloom of some leaves and fruits. 
72, The protoplasm of the epidermal cells generally dis- 
appears in those cases where there is much thickening of 
the walls; it is always present in young plants and parts 
of plants; it is also frequently present in older portions, 
which are not so much exposed to the drying action of the 
atmosphere, as in roots, and the leaves and shoots of aquatic 
plants and of those growing in humid places. In few 
