CH. V, 7] SELF-ADJUSTMENTS OF ROOTS 249 



and air, with mineral matters, oecurs commonly in drain 

 pipes that roots are prone to enter and fill them. The 

 adaptive explanation of aerotropism is of course very plain ; 

 it is found in the need that all roots have for air (i.e. oxygen), 

 indispensable to their respiration, which underlies all of 

 their growth and work. 



Aerotropism is really but one phase of CHEMOTROPISM, or 

 self -adjustment to particular chemical substances, of which 

 several forms are known. Thus, some roots grow towards 

 a greater supply of the mineral substances they specially 

 absorb, though their behavior in this respect is not always 

 consistent, nor is it well understood. They show also several 

 minor "tropisms, " of which TRAUMATROPISM, or a turning 

 away from injurious contacts or substances, is best known. 



All of these tropisms are typical cases of irritability, the 

 equivalent of reflex action in animals (pages 55, 176). The 

 response is not forced by the moisture, air, etc., but simply 

 guided thereby, the work of turning being done by the plant. 

 These phenomena in roots are especially interesting because 

 the place of perception of the guiding stimulus is usually 

 the growing point, while the place of response is the growth 

 zone just behind it. Thus we have an arrangement com- 

 parable with that in animals, where special sense organs 

 receive the stimuli, and a separate muscular system makes 

 the responses. 



In most plants the young main root is so strongly geotropic 

 that it can be deflected only a limited amount from the 

 vertical position by other influences, but the tertiary and 

 later formed roots have so weak a geotropism that they 

 respond to other stimuli very freely. And here must 

 naturally arise this question : What happens in cases where 

 two or more different stimuli act simultaneously upon the 

 same root from different sides? In some few cases the 

 stimuli seem to influence one another's action, but in general 

 the root attempts to respond to them all. The position that 

 is actually taken is then a resultant, depending upon the 



