230 



ABSORBING SYSTEM 



The ecological importance of the dormant initials becomes even 

 more evident, when one considers that lateral branches never arise 

 adventitiously from other parts of the rhizoids and that, under 

 normal conditions, the green protonemal branches also originate from 

 the aforesaid initials. 



The resemblance of the members of such a rhizoidal system to 

 root-hairs is most obvious in the case of the thin-walled slender 



branches. In contact with soil- 

 particles, these ultimate branches 

 undergo the characteristic modifi- 

 cations of form which have already 

 been repeatedly mentioned ; they 

 flatten themselves, embrace the 

 particles and adhere to them pre- 

 cisely like root-hairs. Even the 

 main axis occasionally undergoes 

 similar changes of shape. One of 

 our figures (Fig. 87 c) depicts a 

 rhizoid which in its downward course 

 has evidently encountered a solid 

 object of considerable size. As a 

 result it has first flattened itself, 

 and has then on one side given rise 

 to a stout lateral branch which has 

 clung closely to the obstacle, while 

 on the other side it has continued 

 its growth in the original direction. 

 Like roots and root-hairs, rhi- 

 zoids serve as organs of attachment, 

 besides performing their principal 

 function of absorption. In those 

 Mosses which inhabit bark, and 

 which consequently derive their sus- 

 tenance almost entirely from atmospheric precipitation and from the 

 dust that accumulates among their tufted shoots, the mechanical 

 significance of the rhizoids comes especially into prominence. This 

 statement applies likewise to many of the species that grow on rocks. 

 Among such Mosses again as live in running water (Fontinalis, 

 Cinclidofus), the rhizoids are also purely mechanical in function, and 

 are, according to Paul, remarkable for the thickness of their walls. The 

 cable-like rhizoidal strands of certain Polytrich aceae, finally, which con- 

 sist of a number of slender rhizoids twisted together around a stouter 

 central one, are also probably constructed with a view to inextensibility. 



Fig. S7. 



Structure of the rhizoids of Tortilla muralis. 

 A, Small portion of the main axis of a rhizoid ; 

 i, initial cell of a lateral branch, x 450. B. 

 Portion of a rhizoid in a more advanced stage 

 of development ; an initial cell has given rise 

 first to a slender branch (s), and subsequently to 

 a stout one (a). C. Part of a rhizoid showing the 

 effect of contact with a large soil-particle upon 

 the main axis (see text). D. Coiled extremity 

 of a slender lateral branch. 



