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A TEXTBOOK OF BOTANY 



[Ch. V, 7 



the quantities of moisture coming from different directions ; 

 it transmits a suitable influence to the growth zone; this 

 zone swings the tip over towards the moister side until the 



stimulus is even all around; 

 then the root continues its 

 growth in that direction. It 

 is important to note that 

 leaves and stems, neither of 

 which absorb any water, are 

 not in the least hydrotropic. 



A third self-adjustment of 

 roots takes them towards air, 

 — aerotropism. Other things 

 being equal, roots grow 

 towards the places in the soil 

 where air is most plentiful. 

 At first sight it would seem 

 that hydrotropism and aero- 

 tropism must neutralize one 

 another, since in general much 

 water in the soil means little 

 air, and vice versa. In fact, 

 however, a complete satura- 

 tion of the soil gives more 

 water than plants can make 

 use of, just as we have found 

 that full summer sunlight gives 

 more light than can be used 

 by leaves (page 56) ; and it 

 is towards the optimum com- 

 bination of water and air, best 

 for root life, that roots are 

 guided by their aerotropism, 

 when their hydrotropism is 

 satisfied. It is because this 

 optimum combination of water 



Fig. 174. — Masses of roots, of a 

 Willow, which filled drain pipes. 



The rod between them is a meter 

 stick, cross-marked in decimeters. 

 The apparent cross folds mark the 

 collars where the tiles were joined. 

 (Drawn from a photograph.) 



