544 6ENS1T1VKNESS TO GEAVIIATIOJS Chap. XI 



part which is sensitive to geotropism in the members of 

 such distinct families as the Leguminos£e, Malvaceaj, 

 Cucurbitaceaj and Graminete, we may infer that this 

 character is common to the roots of most seedling 

 plants. Whilst a root is penetrating the ground, the 

 tip must travel first ; and we can see the advantage of 

 its being sensitive to geotropism, as it has to deter- 

 mine the course of the whole root. Whenever the tip 

 is deflected by any subterranean obstacle, it will also 

 be an advantage that a considerable length of the root 

 should be able to bend, more especially as the tip 

 itself grows slowly and bends but little, so that the 

 proper downward course may be soon recovered. But 

 it appears at first sight immaterial whether this were 

 effected by the whole growing part being sensitive to 

 geotropism, or by an influence transmitted exclusively 

 from the tip. We should, however, remember that it 

 is the tip which is sensitive to the contact of hard 

 objects, causing the radicle to bend a\vay from them, 

 thus guiding it along the lines of least resistance in 

 the soil. It is again the tip which is alone sensitive, 

 at least in some cases, to moisture, causing the 

 radicle to bend towards its source. These two kinds 

 of sensitiveness conquer for a time the sensitiveness 

 to geotropism, which, however, ultimately prevails. 

 Therefore, the three kinds of sensitiveness must often 

 come into antagonism ; first one prevailing, and then 

 another ; and it would be an advantage, perhaps a 

 necessity, for the interweighing and reconciling of 

 these three kinds of sensitiveness, that they should 

 be all localised in the same group of cells which have 

 to transmit the command to the adjoining parts of 

 the radicle, causing it to bend to or from the source of 

 irritation. 



Finally, the fact of the tip alone being sensitive to 



