INTRODUCTION. 



In the Desmidiea?, in like manner, a bag or cell forms be- 

 tween two individuals, and the entire contents of both these (or 

 indeed of four, if we regard the fronds as binate) pass out and 

 unite together to form one reproductive body, which becoming 

 detached, leaves the parent corpuscles altogether empty. Such 

 an occurrence is, I believe, not only unknown amongst ani- 

 mals, but is contrary to all our notions of animal propagation. 



Among the Conjugatae, Tyndaridea has an orbicular spo- 

 rangium, Staurocarpus a cruciform or quadrate one ; in 

 Zygnema it is formed within one of the coupling cells, and in 

 Mougeotia it is situated in the connecting tube. The Des- 

 midiese present us with corresponding variations. Their 

 sporangia are generally orbicular, but Staurastrum, Tetme- 

 morus, Closterium and Penium afford examples of cruciform 

 and quadrate ones : and although the reproductive body is 

 usually contained in the connecting tube, yet in Didy- 

 moprium Grevillii it is placed in one of the conjugating cells. 

 Lastly, we have the Didymoprium Borreri, in which the con- 

 jugated filaments form a kind of network in the same man- 

 ner as in Mougeotia. 



That the Desmidiea^ resist decomposition, exhale oxygen 

 on exposure to the sun, preserve the purity of the water 

 containing them, and when burnt do not emit the peculiar 

 odour usually so characteristic of animal combustion, are 

 other facts respecting this family, which taken singly might 

 have less value, but in their combination furnish a most im- 

 portant support to the arguments already adduced. 



On the foregoing statements I rest the claim of the Des- 

 midieee to be considered Algse ; and I confidently appeal to 

 any impartial person whether they do not at least require to 

 be answered before the conclusion which I have drawn can 

 be rejected. But on consulting the works of those who pro- 

 fess to prove the contrary, we find that the important facts 

 which I have here investigated are either altogether over- 

 looked, or passed by without an attempt to controvert them. 

 For my own part I believe them to be unanswerable. 



I have pointed out that the swarming of the granules 



D 2 



