142 Chlorophycew 



the genera Sphcerozosma and Spondylosium, but species of these 

 genera conjugate normal-ly only after dissociation of the filaments. 



The four Desmids Closterium lineatum Ehrenb., Cl. Ralfsii 

 Br^b. var. hybridum Rabenh., Gylindrocystis diplospora Lund, and 

 Penium didymocarpum Lund, normally produce double zygospores, 

 and I have reason to believe in the last-named species that con- 

 jugation occurs between four cells produced by two consecutive 

 vegetative divisions. After carefully considering the conjugating 

 examples of this species which I have been fortunate enough to 

 obtain from several localities, I am forced to the conclusion that 

 there are two zygospores in close approximation, each one having 

 been formed by the union of a distinct pair of gametes (vide 

 fig. 52 D and E). In the conjugation of the two Glosteria mentioned 

 above, each half-cell produces a gamete, and here again there are 

 two zygospores each produced by the union of a pair of gametes, 

 one from a semicell of each plant. 



The zygospore rests for a considerable time before germination. 

 The actual process of germination was first described by De Bary 1 

 and has rarely been observed or followed out. The two outer 

 coats burst and the protoplasmic contents issue in a thin vesicle 

 composed of an extension of the innermost coat. The contents of 

 this vesicle divide into two, four, or eight cells, each of which 

 becomes invested with a new cell-wall and gradually assumes the 

 form of the adult. If the species is one with characteristic 

 markings the first-formed cells are devoid of them, but on the first 

 vegetative division the new semicells acquire the distinguishing 

 ornamentation of the species. The newly formed individuals 

 divide repeatedly, the first few generations showing a slight in- 

 crease in size. 



Only one true case of hybridization has been observed amongst 

 the Desmids and in that case the development of the zygospore 

 was not followed out 2 . All other recorded cases of hybrids are 

 conjectural and most of them are obviously forms produced by 

 ordinary vegetative division. 



There are many strong reasons for regarding the Desmidiaceae 

 as a degenerate family of Conjugate which has originated by retro- 

 gression from filamentous ancestors. The degeneration has brought 



1 De Bary, 'Untersuchungen iiber die Fam. der Conj.,' Leipzig, 1858. 



- Archer in Quart. Journ. Micr. Sci. 1875, pp. 414, 415. This was a zygospore 

 produced by the conjugation of two cells, one of which was Euastrum Didelta (Turp.) 

 Ealfs and the other Euastrum humerosum Ralfs. 



