8 INTRODUCTION. 



distinguish these nearly-aUied groups. He observes of Hor- 

 mospora mutabilis, Breb.*, " lis sont le plus souvent gemines, 

 se multipliant par une division spontanee (deduplication) 

 transversale, comme cela arrive dans quelques autres Pleuro- 

 coccoidees. Une division analogue a lieu dans les Desmi- 

 diees, auxquelles on serait d'abord tente de rapporter les 

 Hormospora; mais les demi-corpuscules (hemisomates) 

 des Desmidiees developpent a leur point de separation 

 une nouvelle portion semblable a la premiere, tandis que, 

 dans I'accroissement des Nostocinees, les corpuscules sont 

 divises en deux par un etranglement transversal, sans qu'il 

 s'ensuive une reproduction sur chacun des points de rup- 

 ture. II y a dans ce cas, comme je I'ai dit ailleurs, dedupli- 

 cation simple. Dans les Desmidiees, il y a deduplication et 

 reduplication," 



It is with unfeigned diffidence that I venture to dissent 

 from the opinion of one possessing so profound a knowledge 

 of these tribes, and I do so only from conviction, the result 

 of close and repeated investigations. 



I have stated my belief that the same changes occur in 

 both the Desmidiese and the Nostochinese. A cell in Micras- 

 terias has two hemispheres, just as a joint in Anabaina has ; 

 in both these separate, and in both each hemisphere becomes 

 again a perfect sphere ; and if in Micrasterias the two hemi- 

 spheres were united by their whole bases, there would not 

 remain even an apparent difference between them. 



The form of the cylindrical cells no longer helps us in 

 tracing the method of division. In Penium as in the Conju- 

 gatse, they seem merely to elongate and then divide. As I for- 

 merly suggested, in a paper read before the Botanical Society 

 of Edinburgh, I consider it extremely probable that in all 

 the simple Algae the cell or joint consists of two valves, and 

 that additions occur at their junction, the original parts 

 remaining unaffected : but this it may never be possible to 



* Annales clcs Sciences Natiirellcs, Jan. 1844. 



