Chap. IL A.CTION OF THE BADIGLE. 6S 



Radicles. — In all the germinating seeds observed 

 by us, the first change is the protrusion of the 

 radicle, which immediately bends downwards and 

 endeavours to penetrate the ground. In order to 

 effect this, it is almost necessary that the seed should 

 be pressed down so as to offer some resistance, unless 

 indeed the soil is extremely loose ; for otherwise the 

 seed is lifted up, instead of the radicle penetrating 

 the surface. But seeds often get covered by earth 

 thrown up by burrowing quadrupeds or scratching 

 birds, by the castings of earth-worms, by heaps of 

 excrement, the decaying branches of trees, &c., and 

 ■will thus be pressed down ; and they must often fall 

 into cracks when the ground is dry, or into holes. 

 Even with seeds lying on the bare surface, the first 

 developed root-hairs, by becoming attached to stones 

 or other objects on the surface, are able to hold down 

 the upper part of the radicle, whilst the tip pene- 

 trates tlie ground. Sachs has shown* how well and 

 closely root-hairs adapt themselves by growth to the 

 most irregular particles in the soil, and become firmly 

 attached to them. This attachment seems to be 

 effected by the softening or liquefaction of the outer 

 surface of the wall of the hair and its subsequent 

 consolidation, as will be on some future occasion 

 more fully described. This intimate union plays an 

 important part, according to Sachs, in the absorption 

 of water and of the inorganic matter dissolved in it. 

 The mechanical aid afforded by the root-hairs in pene- 

 trating the ground is probably only a secondary 

 service. 



The tip of the radicle, as soon as it protrudes from 

 the seed-coats, begins to circumnutate, and the whole 



' Pnysiologie Vegetale,' 1868, pp. 199, 205. 



