350 Mr. J. Ralfs on the Mode of Growth in Calotbrix, ^^c. 



alteration of their direction. This is not the case in Scytonema 

 myochrouSf which acquires a very different habit owing to the vari- 

 ation in the direction of the dislocated extremities. In that plant 

 the new ends are curved towards the same side of the sheath ; 

 they do not pass each other, but issue from the side together and 

 at right angles to its axis. As both portions encounter equal 

 resistance they elongate equally, and consequently the branches 

 are said to be in pairs. Sometimes the dislocation does not take 

 place until after a loop has been formed by a lateral protru- 

 sion. Occasionally also the dislocation occurs without any cur- 

 vature of the newly formed ends, which then pass each other as 

 they do in Calothrix-, but this rarely happens except in the case 

 of lower or basal dislocations. The presence in the same speci- 

 men of both modes of branching proves that they depend on 

 modifications of the same law, notwithstanding their very dif- 

 ferent appearance. 



Calothrix mirahilis presents another variation in the direction 

 of the dislocated ends. At first sight the mode of branching 

 appears similar to that of Scytonema myochrous, and different 

 only in having more frequent divisions ; but closer examination 

 detects an essential difference. The filament indeed separates 

 as in Calothrix and the ends pass each other; but instead of 

 remaining within the same sheath, they immediately pass out 

 obliquely in opposite directions; consequently as both portions 

 are free and continue to elongate, they seem merely to anasto- 

 mose by cohesion at the convexities of their sheaths. As this 

 plant divides at short intervals, it has the appearance of intricate 

 network. 



In Rivularia also the branches are the result of dislocation, but 

 in that genus a globule is formed at the base of the branch at 

 the time of dislocation. 



A similar globule is present in the lower branches of some 

 species of Calothrix and Ccenocoleus ; in these however it is 

 usually developed only after the impaction of the branch, but 

 sometimes during the division of the filament. If the lower 

 portion of the filament elongates and passes the vesicle, its ap- 

 pearance does not differ from one formed after dislocation. If 

 the lower portion ceases to grow at the time of division, the plant 

 is like a simple filament here and there interrupted by a vesicle 

 or sporangium. 



In this group, however, the branching of the filament is not 

 invariably accompanied by dislocation : in Stigonema I believe 

 it never occurs, and even in Scytonema I have seen some species 

 allied to myochrous in which the branches were apparently pro- 

 duced in the usual manner by lateral protrusion without inter- 

 ruption of continuity. 



