THE AMERICAN BOTANIST 95 



tant, and the inDgcny of susccptibk- ])lanl^ arc susceptible. 

 What wc have to consider hero is the process of making sus- 

 ceptible plants resistant so that their descendants in later 

 generations will al>o be resistant. 



The power which a plant cell has of resisting the attack 

 of some other cell (a disease germ) is develo[)ed in the same 

 way that any other power is developetl. and that is by exer- 

 cising in a particular way tlie powers previously in existence. 

 A man gets his muscles more powerful by exercising them, 

 antl in no other way. A plant gains in its power of resisting 

 a particular disease by growing in tlie presence of and by 

 fighting that disease, and in no other way. When plants of 

 a non-resisting variety are grown on sick soil. — soil which is 

 infected with <lisease germs, — some of them die and some 

 survive to produce seeds. If seeds are taken from the sur- 

 vivors and again planted on sick soil, some resultant i)lants 

 live and some die. After a few generations of this process, 

 all of the plants left have powers of resisting the particidnr 

 disease involved, and few or none die when raised in presence 

 of that disease. 



It is said that in this process the weaker plants are killed 

 and the more resistant ones survive. This process is called 

 "selection." and the tlieory of selection is based on the assump- 

 tion that some of the plants of the "non-resisting" variety have 

 more resistance than others. It is also based on certain othor 

 assumptions which we will consider later. But I think that 

 before we get through, it will be seen that the selection theory 

 in the production of disease resistance is an illusion, and that 

 selectif^n has nothing to do with the matter. .\lsi>. that the 

 art of man is not capable of causing selectif)n to ha\e any part 

 in the i)roduction of disease resistance. 



