10 BRITISH DESMIDIACEjE. 



formed between the cells, arising by the coalescence of 

 two processes, one from each cell. This connecting- 

 tube may be conspicuous or exceedingly delicate, and 

 in many Desmids is only represented by the delicate 

 vesicle which surrounds the gametes. The gametes, 

 each of which consists of the entire contents of a cell, 

 issue into the connecting-tube (or into its repres,enta- 

 tive vesicle) and there fuse together forming a ::>/go.^n>rr 

 (or ~t/</ott'). This zygospore assumes a definite form, 

 depending upon the species of Desmid, and then 

 surrounds itself with a firm cell- wall, generally of a 

 brown, sometimes of a black colour, and consisting of 

 three layers. The zygospore may be spherical, ellip- 

 soidal, or of any degree of angularity. It may be quite 

 smooth, as in many species of Closterium and Cuxnin- 

 ritun ; it may be scrobiculate, as in Xanthidium arma- 

 ftiui; or it may be furnished with simple spines, 

 furcate spines, or branched processes, as in many 

 species of Cosma/rium and Staurastrum. 



We 'have occasionally seen zygospores formed by 

 the conjugation of three cells, and there is one record 

 of a zygospore produced by the union of the contents 

 of four cells. In Mesotsenium the conjugating tubes 

 may be put out from any part of the cell. Desmidium 

 cylindricum stands alone amongst Desmids in having 

 the zygospores produced in one of the conjugating 

 cells, as in the case of Spirogyra, thus exhibiting a 

 trace of sexual differentiation. Rarely the same phe- 

 nomenon has been observed in Hyalotheca dissiliens. 

 Lateral conjugation -or the conjugation of two 

 adjacent cells in a Desmid filament has been observed 

 in Spondylosium pulchrum var. planum. 



There is no doubt that conjugation frequently 

 takes place between two individuals which have just 

 separated by vegetative division, the two new semi- 

 cells being as yet only imperfectly developed. This is 

 frequently noticed in large species of the genus Glos- 

 terium, such as GL monUiferum and CL Ehrenbergii, 

 also in Micmsteriax deuticulata and in species of Cos- 



