GROWTH BY THE BOOT. 13 



leaves ; the other is to give off such superfluous mat- 

 ter as it is necessary for its - health that the plant 

 should part with. 



24. But although there can be no doubt that the 

 spongelets act as absorbents with more force than any 

 other part of the root, yet it is equally certain that 

 the whole surface of young roots also possesses an 

 absorbing property, only in a more limited degree. 

 It is not until their tissue is solidified that roots be- 

 come incapable of passing fluid through their sides ; 

 and when very young and soft, there is probably but 

 little difference between their action and that of the 

 spongelets themselves; for it is to be remembered 

 that the latter are not special organs, but are only the 

 very youngest part of the root. 



25. The absorbent power of the spongioles must be 

 much greater than would have been supposed, if we 

 consider that it is almost entirely through their action 

 that the enormous waste of fluid, which takes place 

 in plants by perspiration, is made good ; and hence 

 their importance to plants, and the danger of destroy- 

 ing them, become manifest. 



26. The spongioles and youngest parts of roots are 

 found to be rich in nitrogen, a principle once sup- 

 posed to be unknown in the vegetable kingdom ; and 

 it seems that a supply of this gas is indispensable to 

 their healthy condition. 



27. Boots being furnished with the power of per- 

 petually adding new living matter to their points, are 

 thus enabled to pierce the solid earth in which they 

 grow, to insinuate themselves between the most 



