228 



ABSORBING SYSTEM 



abundantly branched filaments. The main branches are often five or 

 six times as thick as the ultimate ramifications; the former take the 



place of the roots of a Vas- 

 cular Plant, while the latter 

 are comparable to root-hairs. 

 There is thus a close physio- 

 logical resemblance between a 

 fully developed rhizoid and an 

 entire root-system (Fig. 86). 

 Except for the slender ultimate 

 branches, all the ramifications of 

 the rhizoidal system consist of 

 cellular filaments ; the indi- 

 vidual cells are usually long in 

 comparison with their breadth 

 and are separated from one 

 another by oblique transverse 

 walls. Both the thin-walled 

 character of the septa and their 

 oblique position which is an 

 embodiment of the principle of 

 maximum exposure of surface 

 facilitate diosmosis between 

 the successive cells, and thus 

 increase the conducting capa- 

 city of the whole filament. In 

 the fine ultimate branches, 

 which are responsible for a much 

 smaller amount of conduction, 

 ^ the septa are often of the 

 ordinary transverse type. The 

 lateral walls of Moss-rhizoids 

 are generally brown in colour, 

 in which case they exhibit an 

 evident differentiation into two 

 layers. The cell-contents com- 



Fully developed rhizoid of Tortula muralis. a, a young prise a peripheral protoplasmic 



lateral branch. , . , , . 



layer, which sometimes contains 

 leucoplasts or small pale-green chloroplasts, and a central cavity filled 

 with colourless sap. All the branches, as well as the main axis, display 

 very pronounced apical growth. 



The above general account may be supplemented by a detailed 

 description of the rhizoidal system of Tortula muralis, a plant which 



Fkj. S(i. 



