66 PROTECTION FROM INJURIES AND LOSS OF WATER 



cant in its defensive characters in comparison with the epidermis 

 of the fruit which is to last through a much longer period and 

 endure greater hardships (Fig. 32). 



The Radial and Inner Walls of the Epidermis. — The radial 

 and inner walls are usually thinner than the outer. Cutinization 

 sometimes extends for some distance into the radial walls, but 

 it seldom involves the whole of the radial wall or any part of the 

 inner wall. It has already been stated that the epidermis remains 

 alive so long as it is not cut off from water by the formation of 

 cork beneath it, and the relative thinness and non-cutinization 

 of the radial and inner walls permit the inflow of water and the 

 interchange of materials necessary to all living cells. 



The Cell-contents of the Epidermis. — The cell cavity is 

 usually quite clear and is evidently serving as a reservoir for 

 surplus water. The protoplast lines the wall as a very thin film, 

 and although so well exposed to the light it seldom contains 

 chloroplasts excepting in the guard cells of the stomata, and in 

 the case of some plants, particularly Monocotyledons, of shady 

 habitats. Leucoplasts are frequenlty present, and in flowers 

 and fruits they often become transformed into chromoplasts, and 

 produce the yellow, orange, and some of the red colors. Not 

 infrequently blue, violet, and some qualities of red pigments occur 

 in solution in the sap of the epidermal cells of flowers and fruits, 

 young leaves in the spring, the upper epidermis of some alpine 

 and tropical plants where absorption of a part of the intense sun- 

 light before it reaches the chlorophyll apparatus may be of use, 

 and of the lower epidermis of some shade-loving plants where it 

 may be of advantage in absorbing more of the sun's energy before 

 it escapes at the lower surface. We assume that the pigments 

 in fruits and flowers attract insects and other animals that may 

 be of use in pollination and in dissemination of seeds. The use 

 of pigment in the cell-sap of young leaves seems to be to protect 

 the chlorophyll against the destructive chemical changes which 

 are induced by strong light. In addition to pigments tannins 

 not infrequently occur in the epidermal cells, where they may 

 be of use in warding off attacks of animals and fungous parasites. 



