[Chap. XXI INTERRELATIONS OF THE PARTS OF A PLANT 191 



Sassafras may grow as a weed in pastures in certain parts of the coun- 

 try, but not when sheep are present in abundance. Almost pure forests 

 of hemlock, of pine, or of hickory have been known to develop in pas- 

 tures in which seeds of many other trees were present. In a forest the 

 kinds of seedling trees that grow to maturity may depend in part on the 

 presence of browsing animals, such as deer. 



On the roots of the plants of the clover family there are nodules result- 

 ing from the presence and activities of nitrogen-fixing bacteria. When 

 the clover plant is exposed to dense shade, or when its roots are in soils 

 containing an abundance of salts of nitrogen, there are fewer nitrogen- 

 fixing bacteria and nodules than there are when the clover tops are ex- 

 posed to an abundance of light, or when the roots are in soils in which 

 the supply of nitrogen salts is low. 



Many plants propagate vegetatively if one merely cuts off the leafy tip 

 of a stem ( a cutting ) and inserts its basal end into moist soil. In a short 

 time roots will grow from the base of the stem and the cutting will con- 

 tinue to grow and become a complete individual plant. Many persons 

 remove the leaves from cuttings, but the new roots will appear sooner, 

 be more abundant, and grow more rapidly if some of the leaves are not 

 removed. Exact observations may be made more readily if the cuttings 

 are placed in water in a beaker instead of in soil ( Fig. 68 ) . 



Fig. 



68. The initiation and the growth of roots from cuttings are dependent upon 

 leaves. Photo by F. H. Norris. 



