RHIZOIDS OF FERNS AND LIVERWORTS 



227 



the influence of light, gravity and moisture, whereas genuine root-hairs 

 are quite free from these forms of irritability. 



The rhizoids of Liverworts and Fern-prothallia scarcely differ from 

 typical root-hairs, except in the presence of a basal wall. Each rhizoid 

 consists of a single thin-walled tubular cell, which, in contact with 

 solid particles, displays the same tendency towards expansion and 

 adhesion that has already been discussed in connection with root-hairs 

 (Fig. 85). Rhizoids further agree with root- 

 hairs in exhibiting very pronounced apical 

 growth. 



The members of the most specialised series 

 of Liverworts, the Marchantiaceae, possess 

 two sorts of rhizoids, namely the ordinary or 

 " simple " kind, and, in addition, -peculiar 

 " pegged " rhizoids, distinguished by the pre- 

 sence of numerous internal thickenings of the 

 cell-wall, in the shape of simple or branched 

 pegs, which project into the cavity of the 

 rhizoid on every side ; the pegs are closely 

 crowded and often distinctly arranged in spiral 

 series. The simple rhizoids are principally 

 attached to the midrib of the thallus, where 

 they extend close up to the apical region. The 

 pegged rhizoids, on the contrary, occur chiefly 

 on the wings of the thallus. It is these pegged 

 rhizoids which are mainly responsible for the 

 supply of water and mineral salts to the plant. 

 The author is therefore inclined to suspect that the pegs have no 

 special mechanical significance, but that they serve in the first instance 

 to augment the absorbing surface of the rhizoid by throwing the 

 ectoplast into folds. According to this view one is dealing with a case 

 of internal increase of surface ; and the pegged rhizoids are to be 

 regarded as better fitted for the task of absorption than their smooth- 

 walled prototypes. Kamerling has put forward a different theory 

 concerning the purpose of the pegs. He believes that they serve to 

 keep the gas-bubbles, which appear in the rhizoid when the influx of 

 water diminishes, " suspended in the centre of the cavity " and so to 

 enable water to pass between the bubbles and the wall. Further 

 investigation is certainly required, before one can come to a final 

 decision with regard to the physiological significance of pegged rhizoids. 



The rhizoids of Mosses are among the most remarkable of trichomes; 

 in every respect they display a very far-reaching adaptation to their 

 special functions. As a rule each rhizoid represents a system of 



Fio. S5. 



Distal ends of two rhizoids of 

 Mastigobryum trilobatum. A. With 

 numerous lobes. B. Flattened 

 into a disc-like structure. 



