40 PBOPAGAXION OF PLAITTS. 



Boots, like stems and branches, lengthen at their points 

 only, and while the absorption of moisture and nutriment 

 is principally through the newly-formed cells near the 

 ends of the rootlets, stiU they continue to take in liquids 

 through all parts until they have become hardened and 

 enclosed in a cortical layer of cells. It is no doubt true, 

 as Mr. Darwin claims, that sensitiveness to moisture 

 resides specially in the tip of the root, but it can readily 

 be shown that the absorbing property is not wholly con- 

 fined to this point, by removing the tip of the rootlet. 

 The absorbing powers of the young fibers must be far 

 greater than could possibly exist in the extreme point, in 

 order to supply the loss of moisture through rapid evapo- 

 ration from the leaves and young twigs of many kinds of 

 herbs and trees during dry and hot weather. But at 

 ■what age or period of growth the rootlet loses its 

 power of absorbing nutrients has not been determined, 

 but it is probably variable in different kinds of plants. 

 The epiphytes, and especially the larger species of Or- 

 chids, with coarse, fleshy, aerial roots, retain the power 

 of absorbing liquids through their side cells for a much 

 longer period than those of plants growing in the earth, 

 having roots so minute that they can readily pass between 

 small particles of soil while seeking sustenance. They 

 not only add new cells to their points, but the cells in 

 the rear are continually throwing off new branches, there- 

 by enabling the plant to occupy and gather food from 

 new sources. 



The cause of this rapid formation and multiplication 

 of absorbing points on many of the most vigorous peren- 

 nial plants is unknown, for they are evidently wholly 

 adventitious, not originating from buds like the natural 

 branches on the stems of plants ; and, furthermore, the 

 larger proportion of these minute roots are deciduous, 

 and only serve a temporary purpose, living but for a 

 year or two : a few of the stronger remaining perma- 



