196 SUMMARY OF CHAPTER. CHAP. III. 



when their tips were slightly cauterised on the lower 

 side. 



Finally, the several co-ordinated movements by 

 which radicles are enabled to perform their proper 

 functions are ^admirably perfect. In whatever direc- 

 tion the primary radicle first protrudes from the seed, 

 geotropism guides it perpendicularly downwards ; and 

 the capacity to be acted on by the attraction of 

 gravity resides in the tip. But Sachs has proved* 

 that the secondary radicles, or those emitted by the 

 primary one, are acted on by geotropism in such a 

 manner that they tend to bend only obliquely down- 

 wards. If they had been acted on like the primary 

 radicle, all the radicles would have penetrated the 

 ground in a close bundle. We have seen that if 

 the end of the primary radicle is cut off or in- 

 jured, the adjoining secondary radicles become geo- 

 tropic and grow vertically downwards. This power 

 must often be of great service to the plant, when the 

 primary radicle has been destroyed by the larvae of 

 insects, burrowing animals, or any other accident. The 

 tertiary radicles, or those emitted by the secondary 

 ones, are not influenced, at least in the case of the 

 bean, by geotropism ; so they grow out freely in all 

 directions. From this manner of growth of the various 

 kinds of radicles, they are distributed, together with 

 their absorbent hairs, throughout the surrounding soil, 

 as Sachs has remarked, in the most advantageous 

 manner ; for the whole soil is thus closely searched. 



Geotropism, as was shown in the last chapter, 

 excites the primary radicle to bend downwards with 

 very little force, quite insufficient to penetrate the 

 ground. Such penetration is effected by the pointed 



* l Arbeiten Hot. Institut., Wiirzburg,' Heft iv. 1874, pp. 605-631. 



