EXHAUSTIVE NESS. 65 



exclusion of the other two hypotheses me- 

 chanical contact and arrest of growth they 

 confined themselves to the last one, sensitive- 

 ness to contact. This would have been a fine 

 field for a discussion of the known facts fol- 

 lowed by a necessary inference. Of the three 

 possible hypotheses, two had been excluded, 

 and the third must be true. It would seem 

 quite clear that the case was logically proved. 

 But instead of making this the end, they made 

 it the beginning of their work. 



They "thought that any small hard object 

 affixed to the tip of a radicle freely suspended 

 and growing in damp air, might cause it to 

 bend if it were sensitive, and yet would not 

 offer any mechanical resistance to its growth." 

 The results of their experiments proved remark- 

 able. When approaching the subject they made 

 a preliminary trial with seven beans at a rather 

 cool temperature, and six radicles curved. To 

 quote again, "These six striking cases almost 

 convinced us that the apex was sensitive, but 

 of course we determined to make many more 

 trials." As they had noticed that radicles grew 

 much more quickly when subjected to con- 

 siderable heat, and as they imagined that heat 

 would increase their sensitiveness, they made 

 five or six dozen trials on more than two dozen 

 5 



