459 



POLYMORPHISM AND LIFE-HISTORY IN THE" 

 DESMIDlACEuE. 



By G. I. Playfair. 



(Plates xi.-xiv.) 



The present paper is to some extent a reply to certain criticisms 

 of my statements in previous papers, regarding the growth of 

 Desmids. Originally made to myself in some correspondence^ 

 with which I was favoured, these have recently appeared in print 

 in "The Algae of the Yan Yean Reservoir," by Dr. G. S. West, 

 F.L.S. (Journ. Linn. Soc. Bot., Vol. xxxix , 1909). On page 44, 

 Dr. West remarks : — "Judging by his continual references to- 

 ' immature forms,' Mr. Playfair seems to have rather curious^ 

 ideas on the growth of Desmids. He appears to imagine that a 

 Desmid may change its form, or develop spines or warts, at any 

 time during its existence, losing sight of the fact that, unless 

 dealing with monstrosities, at least one seinicell of any Desmid 

 must he mature. Cell-division, except under abnormal circum- 

 .stances, does not take place until the two halves of a Desmid are 

 equally developed, the newer half having arrived at maturity^ 

 Consequently, in any Desmid in which the two semicells are 

 exactly alike, growth has ceased and that individual is mature^ 

 Further alteration of form, excluding the possiVjility of changes 

 caused b}^ the attacks of parasites, does not take place after the 

 comi)letion of the development of the new half, and spines onc& 

 formed cannot become bifid or trifid, or in some other way 

 change their nature, as Mr. Playfair appears to imagine." [The 

 italics are Dr. West's].* 



* In a footnote Dr. West twits me with having mistaken a diatom for a 

 ClosUrium, quite unconscious apparently that in " Freshwater Algre of 

 the Third Tanganyika Expedition," (p. 140, PI. 5, f.lS) he has himself 

 described and tigured the same diatom, Xitzachia ?'ecersa, as Artkistrodesmus- 

 nitzchiokles, sp.n. 



