RHIZOIDS OF MOSSES 229 



has furnished the author with material for the investigation of several 

 interesting ecological peculiarities of Moss-rhizoids. On examining the 

 growing main axis of a rhizoid of Tort a/a, one is at once struck by 

 the fact that all the transverse walls are laid down straightway in 

 the oblique position ; their obliquity is thus not brought about by 

 secondary displacement. The lateral branches make their first appear- 

 ance at a short distance from the growing apex, and invariably arise at 

 the acroscopic end (i.e. at the end nearest the apex of the branch) of a 

 cell. Opposite the oblique septum of such a cell the lateral wall 

 develops a papillate protuberance, the basal diameter of which is 

 roughly equal to the thickness of the main axis (Fig. 87 a). The 

 protuberance, which is filled with dense protoplasm, becomes cut off 

 from the mother-cell by a thin " watch-glass-shaped " wall, and now 

 represents the initial cell of a lateral branch. These initial cells do 

 not, however, necessarily at once develop into branches, but in a 

 certain number of cases though not in many pass first through a 

 period of rest, during which they may be quite fairly compared to the 

 dormant buds of a Dicotyledonous tree-trunk. The majority of initial 

 cells, however, develop without passing through this resting stage. If 

 they arise from young rhizoids, they grow out into thick lateral 

 branches, which turn downwards in response to the stimulus of gravity, 

 just like the main axis. Initial cells produced by older rhizoids, on 

 the contrary, give rise only to comparatively slender branches, which 

 slope downwards quite gently, or which may even follow a horizontal 

 course. These thin branches are cut off by means of transverse septa 

 from the initial cells, which maintain their individuality unchanged 

 and, as their subsequent behaviour shows, become for the time being 

 converted into dormant initials. 



When the main axis has ceased to grow, and the principal 

 branches have reached a certain age, some of the dormant initials give 

 rise to a new series of thick lateral branches. If such an initial had 

 at a previous stage produced a slender branch, the new member 

 invariably arises on the physically lower side of the old one (Fig. 87 B). 

 Those initials, however, which previously at once entered the resting 

 state, on resuming their activity as a rule straightway produce thick 

 branches ; in exceptional cases they first produce thin branches, and 

 give rise to thicker members later on. Every thick lateral branch 

 behaves just like the main axis in respect of its method of 

 ramification. The above-described arrangements evidently provide a 

 very effective means of regulating the output of absorbing organs. 

 The plant is thus enabled to take advantage of favourable conditions 

 without loss of time, while with the aid of its resting initials it can 

 survive unfavourable periods with comparatively slight risk of damage. 



