DESMIDIUM. 341 



growth of cells is different, and approaches rather more 

 closely to a true reproduction, which, however, it is not. 

 Each. of these Desmidia consists of two portions, or cells, fre- 

 quently contracted at their points of junction. These seg- 

 ments, when they have arrived at maturity, separate from 

 each other, and from the open extremity of each a little 

 mucous pouch extends : this imperceptibly increases in size, 

 and finally assumes the form and characters of the originally 

 formed segments. This mode of formation of cells is highly 

 curious, and it is one which may be accounted for by refer- 

 ence to the form and constitution of two portions of those 

 DesmidecE which present it. 



Another fact, not as yet alluded to, has been advanced in 

 support of the animality of the Closteria by Mr. Dalrymple : 

 this is the presence in each extremity of Closterium of a 

 distinct organ or vesicle which contains a number of active 

 and revolving molecules. But moving and revolving par- 

 ticles are met with in many undoubted Algce, and, therefore, 

 . their presence in Closterium cannot be considered as decisive : 

 moreover, no such organ presents itself in any other genus 

 of Desmidece, the proofs of the vegetability of which may 

 be regarded as decisive, and to which the Closteria are too 

 evidently related for the idea to be entertained that the one 

 are animal and the other vegetable productions. 



The Desmidece are found in old boggy pools, the waters 

 of which are not periodically dried up, but which are per- 

 manent for several years. They do not usually float on the 

 surface, but are found in cloud-like masses near the bottom. 

 They are best removed on pieces of linen, on which, if 

 moistened regularly, they may be preserved unaltered for 

 several days. 



Section i. 

 Frond filamentous. 



51. DESMIDIUM Ag. 



Char. Frond filamentous, simple, spirally twisted, fragile, 

 articulated and angular ; mature cells bipartite ; angles 



