224 ABSORBING SYSTEM 



hair in penetrating the soil. This thickened apical membrane, in fact, 

 acts like a miniature root-cap. 



Owing to the thinness of their cell-walls, root-hairs are naturally 

 somewhat delicate and short-lived structures ; at a certain distance 

 from the root-tip they collapse, turn brown and die. The whole 

 absorbing tissue indeed likewise soon dies and disappears ; the root 

 is thenceforth covered by the suberised " exodermis," which represents 

 the outermost layer of the cortex, Bapid alternations of dryness and 

 humidity, transplantation and similar causes also usually lead to the 

 destruction of the root-hairs ; under these conditions death may be 

 preceded by pathological changes of form, such as irregular branching 

 (e.g. Brassica spp.), etc. 



According to the author's observations, two epiphytic Ferns which 

 are exceedingly common in Java and the rest of the Malay Archi- 

 pelago, namely, Drymoglossum nummularifolium and D. piloselloides, 

 are distinguished by the possession of exceptionally resistant and long- 

 lived root-hairs. Here, namely, when a root-hair dries up in conse- 

 quence of long-continued shortage of water, the cytoplasm and nucleus 

 withdraw to a basal portion of the hair which becomes marked off 

 from the rest by a more or less regular transverse constriction. At the 

 level of this constriction a cap-shaped septum is laid down, cutting off 

 the encysted protoplast from the withered distal region of the hair, 

 which is thereupon cast off. In this way a dormant piliferous cell is 

 produced, which only requires the access of water, in order to resume 

 its activity and give rise to a new root-hair. 



Hitherto discussion has been confined to the principal or nutritive 

 function of root-hairs. Some account must next be given of the 

 mechanical function which these structures perform in connection with 

 the fixation and distribution of the root-system in the soil. It is self- 

 evident that the above-described adhesion of the root-hairs to solid 

 particles must play an important part in anchoring the plant in the 

 ground. But this adhesion also produces a mechanical effect of a 

 different kind, in connection with the passage of the root through the 

 soil. By fixing the sub-apical region of the root firmly in the ground, 

 the root-hairs provide the resistance which that organ requires in order 

 to overcome the friction of the soil. Schwarz, however, has pointed 

 out, how essential it is that the actual growing apex should not be 

 deprived of its flexibility. As he says : " If the tip of the root were 

 fixed to the soil-particles by hairs, the stimuli produced by contact, 

 moisture and gravity would be largely ineffective, while the growth of 

 the root would also be retarded. For this reason the root-hairs almost 

 invariably first appear at some little distance from the apex. It is 

 interesting to note, that they approach more closely to the tip in roots 



