VEGETATIVE ORGANS OF PLANTS. 



257 



its smallest pores, or rifts. It is likewise the reason that 

 a root, which has been cut off in transplanting or other- 

 wise, never afterwards extends in length. 



Although the older parts of the roots of trees and of 

 the so-called root-crops acquire a considerable diameter, 

 the roots by which a plant feeds are usually thread-like 

 and often exceedingly slender. 



Spongioles. The tips of the rootlets have been 

 termed spongioles, or spongelets, from the idea that 

 their texture adapts them especially to collect food for 

 the plant, and that the absorption of matters from the 

 soil goes on exclusively through them. In this sense, 

 spongioles do not exist. The real living apex of the 

 root is not, in fact, the outmost extremity, but is situ- 

 ated a little within that point. 



Root-Cap. The extreme end of the root usually con- 

 sists of cells that have become loosened and in part 

 detached from the proper cell-tis- 

 sue of the root, which, therefore, 

 shortly perish, and serve merely 

 as an elastic cushion or cap to 

 protect the true termination or 

 living point of the root in its act 

 of penetrating the soil. Fig. 36 

 represents a magnified section of 

 part of a barley root, showing the 

 loose cells which slough off from 

 the tip. These cells are filled 

 with air instead of sap. 



A striking illustration of the 

 root-cap is furnished by the air- 

 roots of the so-called Screw Pine 

 (Pandanus odoratissimus), exhibited in natural dimen- 

 sions, in Fig. 37. These air-roots issue from the stem 

 above the ground, and, growing downwards, enter the 

 soil, and become roots in the ordinary sense, 



17 L 



Fig. 36. 



