SENSITIVENESS. 61 



part tends towards the centre of gravity. In almost every 

 case we can clearly perceive the final purpose or advantage 

 of the several movements. Two or perhaps more of the 

 exciting causes often act simultaneously on the tip, and 

 one conquers the other, no doubt, in accordance with its 

 importance for the life of the plant. The course pursued 

 by the radicle in penetrating the ground must be deter- 

 mined by the tip ; hence it has acquired such diverse kinds 

 of sensitiveness. It is hardly an exaggeration to say that 

 the tip of the radicle thus endowed, and having the power 

 of directing the movements of the adjoining parts, acts 

 like the brain of one of the lower animals, the brain being 

 seated within the anterior end of the body, receiving im- 

 pressions from the sense-organs, and directing the several 

 movements." 



Practical Inferences. It will be obvious, then, from 

 what has been before said, that for cultural purposes, such 

 as the various operations connected with tillage, the nature, 

 quantity, and time of application of manure, and the like, the 

 character of root-action in general must be studied in con- 

 nection with the nature and properties of the soil. The 

 special form and characteristics of the root in the particular 

 crop it is wished to cultivate tap-rooted, fibrous-rooted, 

 fleshy, surface-rooting, or deep-rooting, &c. must also be 

 taken into consideration in the same relation. 



Action of Gravitation on Leaves. The tendency of 

 leaves during their growing period so to place themselves 

 that their upper surface looks to the heaven, their lower to 

 the earth, is a matter of every-day observation. Scarcely 

 less familiar are the turns and twists which the leaves or 



