THE FORMATION OF THE ROOT SYSTEM 27 



separated mechanically. While they thus aid materi- 

 ally in attaching the root to the soil, they carry on the 

 absorption of the water of the soil with the mineral 

 compounds dissolved therein. 



It is customary to consider the influences we have 

 spoken of, gravitation, contact, light, and moisture, as 

 stimuli, and to speak of the behaviour of the root as 

 response to stimulation. The power of receiving stimu- 

 lation indicates the possession of special sensitiveness, 

 and its response is to a large extent under the control 

 of the living root. The movements or alterations of 

 growth are purposeful, and lead us to look upon the 

 latter as a living sensitive organism engaged in the task 

 of making the best of its surroundings and varying its 

 behaviour as the surroundings change. 



Seeing the very purposeful behaviour of the root we 

 may pause to ask what is the most potent factor in the 

 growth, or what is the determining influence which 

 causes it to penetrate the ground. The fact that 

 stability of position is secured strikes us at once, but it 

 is doubtful if this is the first consideration. 



We may dismiss the responses to the stimulation of 

 light and contact. They are accessory to the effort of 

 the plant to come into close relatio'n with the soil, but 

 they by themselves do not minister to any of its needs. 

 The behaviour of the root suggests that it is seeking 

 something which the long experience of the race has 

 shown to be advantageous and which has now become 

 hereditary in the plant. The object of this search is 

 the water which the soil contains, which is present as 

 delicate films surrounding the particles of which it is 

 composed. Inherited experience has shown to the 

 vegetable organism that the soil is the source of water, 

 and its instinctive efforts are directed to the securing of 

 a position leading to an adequate supply. 



The stimulus of gravity, therefore, or the perception 



