M. G. Thuret on the Reproduction of certain Nostochinese. 5 



portions, which proceed with their development independently. 

 Hence it is that we not unfrequently observe under the micro- 

 scope, young examples of Nostoc attached together by an inter- 

 posed heterocyst. 



II. 



The other Nostochinea of which I have to speak belong to 

 the genus Cylindrospermum, Ralfs (Kiitz. pro parte). This 

 genus comprehends a portion of the species which were formerly 

 united under the name of Anabaina, Bory, and which consist of 

 moniliform filaments, analogous to the chaplets of Nostoc, but 

 forming an indeterminate gelatinous stratum. Certain joints of 

 the filaments are transformed into heterocysts ; others acquire 

 an elliptical form, more considerable dimensions, and become 

 the sporanges. The different positions which may be occupied 

 in the filament by the sporanges and heterocysts have served to 

 break up Anabaina into several genera. Those proposed by 

 M. Kutzing are too vaguely defined to be adopted without re- 

 striction. Mr. Ralfs has proposed divisions founded upon more 

 precise conditions, which seem to me admissible*. I shall 

 merely remark, that it is at least superfluous to separate generi- 

 cally plants united by such close affinities, and that it would be 

 better to restrict the divisions established by Mr. Ralfs to the 

 rank of subgenera, preserving the name of Anabaina for the 

 whole group of species. As to the latter name, it should be 

 maintained in any case ; and Mr. Ralfs is wrong in proposing 

 to replace it by Trichormus, Allm., supposing that the priority 

 belongs to the genus Anabcena, established by M. Ad. de Jussieu 

 in the family of Euphorbiacese. The latter was only published 

 in 1824 f, while Bory St. Vincent's genus dates from 1822 J. 



In the genus Cylindrospermum, as defined by Mr. Ralfs, the 

 heterocyst forms the last joint of the filament, and the sporange 

 occupies the next joint. The filaments are endowed with a very 

 slow but appreciable creeping motion. The joints are cylin- 

 drical j they contain a substance of a bluish-green colour, a little 

 granular, and they multiply like those of Nostoc ; that is to say, 

 after having elongated in the direction of the length of the fila- 

 ment, they become cut in two by a transverse division. The 

 last joint, before changing into a heterocyst, is like the rest, but 

 the granules it contains disappear by degrees ; the joint acquires 

 a yellowish tinge, becomes larger, and assumes a more or less 

 elongated ovoid form (fig. 8). At this epoch it is almost always 



* Ann. Nat. Hist. 2nd ser. v. p. 321. pi. 8 & 9 (1860). 

 t De Euphorbiacearum generibus medicisque earum viribus tentamen, 

 p. 46. 



.t Bictionnairc classique d'Histoire Naturclle, i. p. 307. 



