THE FORMATION OF THE ROOT SYSTEM 25 



with the vertical. So long as they are intact, they 

 gradually modify their growth so as to make their apices 

 again point vertically downwards (Fig. 8). 



If we study the behaviour of the roots under various 

 conditions we soon find that they manifest other forms 

 of sensitiveness, all of which are brought to bear upon 

 the problem of establishing themselves in the soil. 

 When a root enters the latter and passes between the 

 particles which compose it, it must sooner or later 

 come into contact with some of them, and not improb- 

 ably such contact will hinder the advance of the root 

 in a straight or nearly straight 

 line. The growth of the root is 

 achieved by its advancing in a 

 kind of corkscrew fashion, the 

 tip describing a spiral rather 

 than a straight line. This no 

 doubt tends to push aside slight 



obstacles which may meet the FIG. 8. Geotropic curvature in 



advancing tip. If we experi- root and shoot of mustard. 



ment upon a seedling bean, size '> (After 



which we have seen can be culti- 

 vated in moist air, we can imitate the conditions met 

 with in the soil by attaching some small piece of a hard 

 substance to one side of the root tip, using a little gum 

 as the attaching medium. By this treatment we can 

 ensure that the contact shall be prolonged, and hence 

 the struggle between the root and the obstacle will be 

 carried to such a point as to exhibit very striking effects. 

 After a short time the growing region of the root, which 

 is some little distance behind the apex, will be observed 

 to curve in such a way as to turn the tip from the object 

 touching it. As the pressure is not removed under the 

 conditions of the experiment, this curvature will become 

 very pronounced and after a day or two the root will be 

 curled into a loop. In the soil so pronounced a curva- 



