Leaves and Their Structures 



23 



TifT* 



^^mMSM 



or yellowish instead of green. 



Light is usually necessary to 



the development of chlorophyll. 



The white sprouts on potatoes 



in a dark cellar, the blanching 



of celery when the lower part of 



the leaves is covered, and the 



whitening of grass under a board 



are familiar evidences of this 



fact. In the inner tissues of 



plants and in the underground 



parts, the plastids are usually 



colorless ; but in many plants 



these parts become green if they 



are exposed to the light. This is why potatoes that grow at 



the surface of the soil are likely to be green. 



Fig. 17. Part of a moss leaf that is 

 composed of a single layer of cells. 



