Chap. XII. CONCLUDING EEMAEKS. 549 



the seed itself, unless it be a heavy one, is displaced 

 or lifted up by the continued growth and elongation 

 of the radicle. But in a state of nature seeds often 

 get covered with earth or other matter, or fall into 

 crevices, &c., and thus a point of resistance is afforded, 

 and the tip can more easily penetrate the ground. 

 But even with seeds lying loose on the surface there 

 is another aid : a multitude of excessively fine hairs 

 are emitted from the upper part of the radicle, and 

 these attach themselves firmly to stones or other ob- 

 jects lying on the surface, and can do so even to glass ; 

 and thus the upper part is held down whilst the tip 

 presses against and penetrates the ground. The 

 attachment of the root-hairs is effected by the lique- 

 faction of the outer surface of the cellulose walls, and 

 by the' subsequent setting hard of the liquefied matter. 

 This curious process probably takes place, not for 

 the sake of the attachment of the radicles to superficial 

 objects, but in order that the hairs may be brought into 

 the closest contact with the particles in the soil, by 

 which means they can absorb the layer of water sur- 

 rounding them, together with any dissolved matter. 



After the tip has penetrated the ground to a little 

 depth, the increasing thickness of the radicle, togethei 

 with the root-hairs, hold it securely in its place ; and 

 now the force exerted by the longitudinal 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 moderate size, such as that 

 of a seedling bean, can displace a weight of some 

 pounds. It is not probable that the tip when buried 

 in compact earth can actually circumnutate and thus 

 aid its downward passage, but the circumnutating 

 movement will facilitate the tip entering any lateral 



