334 



W. A. CANNON 



VARIATION EXPERIMENTALLY INDUCED 



Observations on the habits of species when growing under 

 natural conditions show, as has been indicated in the first section 

 of this paper, that a certain amount of variation away from type 

 is to be expected in most roots. This is greatest in roots of a 

 generahzed character, ^^^lere the soil conditions compel, such 

 roots penetrate but little although they may extend widel}^, or, 

 on the other hand, where the soil is deep the roots may also 

 attain a considerable depth. The first variation is toward the 

 root type normal to the cacti especially, where there is a marked 

 division of function into anchoring and absorbing roots. The 

 second variation is toward the root type characteristic of Zizy- 

 phus, which has few, or no superficial roots. With this capacity 

 for root variation it follows, other conditions being equal, that 

 plants with generalized roots are capable of adjusting themselves 

 to a wider range of soil conditions, than plants having other 

 types of roots. It has been shown, especially, that the tap root 

 type of root-system restricts its possessor to areas having appro- 

 priate soil depth. The case is not so clear with the opposite 

 type, how^ever, since, as the garden cultures reported in the sec- 

 ond section of this paper indicate, an increase in the soil depth 

 is not followed necessarily by a correspondingly deeper root pene- 

 tration. Under the existing natural conditions, species having 

 the superficial type of roots are wholly dependent on seasonal 

 rains for their water supply. They do not pierce to levels where 

 the soil moisture is perennial else it might be easilj^ seen that 

 their distribution might be more extensive, and very different 

 from what it is at present. However this may be, it appears 

 from the results of the garden cultures here reported that irri- 

 gation of plants with the cactus-tj'pe of roots operates in two 

 directions. In the first place, it destroj'S the differentiation which 

 is a characteristic of such roots, and, in the second place, it 

 tends to a somewhat deeper placing of the roots themselves. 

 This last is a result of the first in so far as the absorbing roots 

 do not develop. 



It became desirable, with the range and the direction of vari- 



