1 46 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



same way by division of the endochrome, and the formation 

 of a new membrane round each division. It may be true, that 

 in some instances the sac may be absorbed, and the discs set 

 free, but that is a totally different question. The real dis- 

 tinctive character resides rather in their narrowness, in con- 

 sequence of which they appear annular. This character, 

 however, loses its preciseness, in some cases, either by the 

 narrowness of the endochromes in true Oonfervce, or their 

 elongation in Oscillatorice. 



119. Generic characters are afforded by the different con- 

 ditions of the two principal elements of whicb they are com- 

 posed, viz., the external common tube, and the endochromes. 

 The two are seldom luxuriant at the same time, and the differ- 

 ent conditions exhibited by both are often very curious. The 

 simplest case, perhaps, is that of Oscillatoria, which consists 

 of straight or sKghtly curved chains of endochromes, contained 

 in a common tube. When disposed to divide, two of the en- 

 dochromes, or the two parts arising from one original endo- 

 chrome, recede from each other ; the outer tube contracts 

 at the point of division, separates there, and thus gives rise 

 to two distinct organisms. In some cases, however, the outer 

 tube is of a less yielding substance, and, in consequence, the 

 divided parts retain their place in the tube, which dilates 

 in proportion as these new threads are again divided ; and 

 thus we have a common tube containing a number of 

 threads, or, in other words, we have the genus Microcoleus. 

 The threads remain in this case perfectly distinct, but fre- 

 quently the divided ends slip or grow over each other, 

 sometimes becoming attenuated, and thus a sort of spurious 

 ramification takes place, and finally, in some cases, union be- 

 tween the two portions, as in Ccenocoleus. Sometimes, again, 

 it happens that a connecting cell is formed, as in the Nostochine 

 genus Sphcerozyffa. Now if the thread is disunited below this 

 cell, the next endochrome becomes elongated, and the original 

 thread is thus continued beyond the fractured portion, being 

 more or less diverted from its course, and we have thus the 

 peculiar structure of Calothrix. In some cases, the terminal 

 globose or elliptic ceU is formed after division, and sometimes 



