30 



General Botany 



Fig. 20. The vein system of a skeletonized sassafras leaf. The leaf was prepared by placing 

 it in water and allowing bacteria to digest the epidermis and mesophyll. 



parts become green if they are exposed to the hght. This is why 

 potatoes that grow at the surface of the soil are likely to be green. 



Seedlings of pine, spruce, lemon, and lotus develop green color 

 even in the dark. One occasionally finds in lemons, for example, 

 green sprouted seeds that have developed inside the fruit in 

 complete absence of light. Evidently there are substances in 

 some plants which make possible the formation of chlorophyll 

 without light. 



The veins. The veins in a leaf branch again and again, forming 

 a fine meshwork through all its parts. ^ Each vein is composed of 

 a bundle of water-conducting and food-conducting tissues sur- 

 rounded by a bundle sheath. The water-conducting tissues are 

 located in the upper side of the vein. These tissues are made up 



^ The venation, or arrangement of the veins of leaves, is of three general types : 

 parallel, extending more or less parallel from the base to the apex ; dichotomous or 

 forked, when the veins divide at intervals into two smaller veins; and net-veined, 

 when the veins form an irregular network throughout the blade. The principal 

 veins may be arranged either palmately, as in maple leaves, or pinnately, as in oak 

 leaves 



