206 Plant Life and Evolution 



some liverworts the dorsiventral character of the 

 thallus can be inhibited by subjecting all parts of 

 the developing plant to equal illumination. 



Relation of Photosynthetic Organs to Light 



In a general way the development of leaves, or the 

 corresponding photosynthetic organs, is directly as- 

 sociated with the intensity of light, which up to a 

 certain optimum is more and more efficient as the 

 intensity increases. However, many plants are ex- 

 posed to an intensity of light, and with it usually 

 a degree of heat, which is in excess of the optimum, 

 and, therefore, these plants have developed devices 

 for protecting the delicate assimilating tissues from 

 injury which might result from excessive illumina- 

 tion. The leaves may be covered with a very thick 

 epidermis, which is sometimes supplemented by a 

 sub-epidermal tissue of such character as to inter- 

 cept much of the light and heat; or there may be 

 special pigments developed, or the surface of the 

 leaf may be covered with masses of hairs or scales 

 which cover the exposed surfaces with a gray or 

 white film. 



In many tropical trees, like the mango, the 

 young leaves are limp, and hang vertically, while 

 they are colored pink or purple, both their position 

 and the development of the special pigments being 

 supposed to be methods of neutralizing the effect 

 of the powerful sun's rays upon the delicate as- 

 similative tissues of the young leaves. It is possi- 

 ble that the deep red or purple color of the young 



