140 Chlorophycece 



very pale in colour, but the chloroplasts are quickly developed and 

 pyrenoids soon make their appearance. All Desmids which undergo 

 normal cell-division at the region of the isthmus consist of one of 

 the half-cells of the mother and a newly developed half, but in 

 certain species of Closterium and Penium the cell may consist of 

 portions of several generations. 



Asexual reproduction takes place very occasionally by the 

 formation of aplanospores. These have been seen in Spondylosium 

 nitens (Wall.) Arch. 1 and in Hyalotheca neglecta Racib. 2 



Sexual reproduction of a degenerate type takes place by the 

 conjugation of two individual cells and the formation of a zygospore. 

 The two conjugating cells, which in the vast majority of Desmids 

 are sexually indistinguishable, approximate and become enveloped 

 in mucus by a further secretion of the gelatinous pectose con- 

 stituents of the cell- wall. In many Desmids the semicells of each 

 individual come apart at the isthmus and the entire contents of 

 the cell issue as a gamete, the latter having the appearance of a 

 protoplasmic vesicle more or less filled with an irregular mass of 

 chlorophyll. In some species a protuberance of variable size arises 

 from the isthmus of each conjugating cell, and on the fusion of the 

 protuberances to form a conjugating- tube, the gametes issue into 

 it. The union of the gametes results in a zygospore, which 

 develops a cell-wall with three distinct layers. The inner layer is 

 thin and colourless, the middle layer is brown and firm, and the 

 outer layer either retains a smooth surface or becomes covered 

 with variously arranged warts or spines. Sometimes more than 

 two cells have participated in the formation of a zygospore, the 

 latter having been formed by the union of three 3 or even four 4 

 gametes. 



All the filamentous Desmids dissociate into their individual 

 cells prior to normal conjugation, with the exception of certain 

 species of the genus Desmidium, and the zygospores are formed 

 between the conjugating cells in all Desmids except Desmidium 

 cylindricum Grev. In this species the zygospore is formed within 

 the female cell as in Spirogyra and Zygnema. 



1 Wallich in Ann. Mag. Nat. Hist. ser. 3, vol. v, I860, t. vii, f. 10, 11 ; Turner 

 in Kongl. Sv. Vet. -Akad. Handl. Bd xxv, no. 5, t. xviii, f. 7, 8. 



' W. & G. S. West in Ann. Bot. xii, 1898, t. iv, f. 2H 27. 



:i West in Journ. Linn. Soc. Bot. xxix, t. xxiv, f. 5 ; W. & (Jr. S. West in 

 Journ. Koy. Micr. Soc. 1897, t. vi, f. 5. 



4 Turner, I.e. t. x, f. 16 e. 



