What is a Root? 



manures of such substances, and the part they play (if present) 

 in organic manures, invocation of growth-substances in such 

 manures can be little more than insecurely-based sales talk. 

 Vitamin B^ may be responsible for some of the growth-promot- 

 ing effect of organic manures. (See p. 68). 



Many workers have found that synthetic grov/th-substances, 

 and even auxin, are actually toxic under certain conditions to 

 normal roots growing in solution-cultures. Some workers, 

 such as Grace, have produced methods of application that 

 are claimed to minimize the toxic effect, or even to convert it 

 into a real stimulation of normal roots in sand or soil. 



Some of the reported toxic effects are discussed in Chapter 

 XI. It is not possible to sum them up, and the reader is 

 therefore left to form his own conclusions about the paradox. 

 It may resolve itself as knowledge grows. 



" 



