Desmidiacece 



139 



each division occupying about a day in the smaller species and 

 several days in the larger species. The first step in this cell-fission 

 is an elongation of the isthmus, causing a slight separation of the 

 two semicells. The elongated isthmus generally becomes swollen 

 and soon shows signs of a constriction midway between the two 

 semicells. By this time the nucleus has completely divided and 



Fig. 51. A, Closterium Ehreribergii Menegh., from Glyder Fawr, N. Wales ( x 184). 

 B, Cl. Leibleinii Ktitz., from near March, Cambs. (x334). C, Micrasterias 

 oscitans Balfs, var. mucronata (Dixon) Wille, from Kerry, Ireland ( x 184). 

 D, Penium cucurbitinum Biss., from near St Just, Cornwall- ( x 435). E, Staura- 

 strum Kjelmanni Wille, from 2600 ft. on Glyder Fawr, N. Wales ( x 435). 

 F, Cosmarium Klebsii Gutw. (a form), from near Ely, Cambs. ( x 435). 



shortly afterwards the constriction deepens, cutting the median 

 portion into two young semicells, which usually remain in contact 

 by their apices until they are practically full-grown. The orna- 

 mentation of the cell-wall gradually makes its appearance on the 

 young semicells as they assume the normal size. At first they are 



