THE CONTROL OF LIFE 99 



of substances promote growth to an 

 astonishing degree. Thus peat, inocu- 

 lated with certain bacteria produces 

 substances which greatly stimulate the 

 growth of crops. 



Certain marine organisms will not 

 grow in artificial sea water until a few 

 drops of natural sea water are added. 

 The growth of certain fungi is greatly- 

 increased by minute amounts of certain 

 metals. 



In all these cases the growth-promoting 

 substance acts at a very low concentra- 

 tion: often a mere trace suffices. In the 

 future we shall be compelled to pay more 

 attention to substances which act at 

 great dilution. 



The form of the plant may be con- 

 trolled in a great variety of ways. One 

 of the most potent substances for influ- 

 encing development is water. This is 

 illustrated by the behavior of a potato 

 lying on a dry table-top without food or 

 water. Under these circumstances a 

 potato can grow for a long time. It de- 



