Desmidiacece 143 



about a loss of the filamentous condition, accompanied by the develop- 

 ment of specialized morphological characters 1 , and this has gone on 

 hand in hand with the loss of sexual differentiation of the conjugating 

 cells. It has been previously mentioned that Desmidium cylin- 

 dricum is the only known Desmid in which the zygospore is formed 

 in one of the conjugating cells (presumably the female), and the 

 occasional reversion to this type of conjugation in Hyalotheca 

 dissiliens- goes far to prove that in all probability this was the 

 ancestral type of conjugation in the Desmidiacese. Moreover, it is 

 a type of conjugation which is represented at present by the 

 Zygnemea?, although amongst the Desmids its lingering remains 

 are only found in Desmidium cylindricum. A few years ago I 

 advocated a scheme of evolution of Desmids from ancestral fila- 

 mentous forms by descent through the genus Cylindrocystis 3 , and 

 the recent discovery of that extraordinary member of the Zygnemese, 

 Debarya Desmidioides W. & G. S. West 4 , is of surpassing interest. 

 This Conjugate fills up the link that was missing in the chain of 

 evidence which goes to show that Cylindrocystis and Mesotcenium, 

 and following on that nearly all the other genera of Desmids, were 

 most probably derived from filamentous ancestors. There is also a 

 great tendency towards the secondary assumption of the filamentous 

 condition. Not only has this resulted in the production of true fila- 

 mentous genera such as Spondylosiiim, Onychonema, Sphwrozosma, 

 Desmidium, etc., which had their origin from unicellular forms, but 

 this tendency reveals itself in certain species of genera which are 

 normally unicellular. Thus, filamentous forms are occasionally 

 met with of such species as Cosmarium obliqumn Nordst. 5 , C. 

 moniliforme (Turp.) Ralfs, C. Regnellii Wille 6 , Euastrmn binale 

 (Turp.) Ehrenb. 7 and Staurastrum inconspicuum Nordst. 8 , not to 

 mention certain of the tropical Pleurotcenia and Micrasterias 

 foliacea Bail., the latter being a true filamentous form of a typically 

 unicellular genus with complex cell-outlines. 



The Desmidiaceas is at the present day the family of Conjugates 



1 W. & G. S. West in Ann. Bot. xii, 1898, pp. 53, 55. 



2 Boldt in Bih. till Sv. Vet.-Akad. Handl. Bd xiii, no. 5, t. ii, f. 33 ; Joshua in 

 Jouru. Bot. xx, 1892. 



3 G. S. West in Journ. Linn. Soc. Bot. xxxiv, 1899, pp. 409 415. 



4 W. & G. S. West in Journ. Bot. 1903, p. 39, t. 446, f. 19. 



5 Nordstedt in Acta Univers. Lund, ix, 1873, p. 23, t. i, f. 8. 



6 W. & G. S. West in Trans. Linn. Soc. Bot. ser. 2, v, t. xv, f. 20 a'. 



7 W. & G. S. West in Ann. Bot. xii, 1898, p. 30, t. iv, f. 38. 



8 Borgeseu in Bot. of Faeroes, Part I, Copenhagen, 1901, p. 235, t. viii, f. 4. 



