38 MOVEMENTS OF SEEDLINGS. 



which causes the root to bend downwards towards 

 the centre of the earth. This bending is called 

 geotropism, from two Greek words meaning, " the 

 earth," and " to turn." 



"Sensitiveness to gravitation," says Darwin, 

 speaking of the radicle of the seedling, " resides in 

 the tip ; and it is the tip which transmits some 

 influence to the adjoining parts, causing them to 

 bend. As soon as the tip, protected by the root- 

 cap, reaches the ground, it penetrates the surface, 

 if this be soft or friable, and the act of penetration 

 is apparently aided by the rocking or circumnu- 

 tating movement of the whole end of the radicle. 1 



" After the tip has penetrated the ground to a 

 little depth, the increasing thickness of the radicle, 

 together with the root-hairs, hold it securely in its 

 place ; and now the force exerted by the longi- 

 tudinal growth of the radicle drives the tip deeper 

 into the ground. This force, combined with that 

 due to transverse growth, gives to the radicle the 

 power of a wedge. Even a growing root of mod- 

 erate size, such as that of a seedling bean, can 

 displace a weight of some pounds." 



The radicle is constantly growing in length, 

 and at the same time in thickness. Darwin says, 3 



1 p. 548. 2 p. 549. 3 p. 77. 



