INTRODUCTION. 11 



mor'uini and Eunxtrnin. For there to be an 



tl 



remains of sexuality under these circumstances, differ- 

 entiation of sex could only occur immediately prior to 

 conjugation. 



The development of the zygospore has been worked 

 out by De Bary, and there is no alternation of genera- 

 tions. He found that the gametes completely left the 

 gametangia and united in from fifteen to forty minutes, 

 but we have often known this process to take many 

 hours. The time occupied in the escape and fusion 

 of the gametes depends largely on the particular 

 species under consideration. The development of the 

 external ornamentation of a zygospore may take many 

 days. When germination takes place the outer wall 

 of the zygospore splits and the entire contents escape, 

 surrounded by the colourless inner cell-wall. This 

 escaped cell enlarges and soon becomes constricted, 

 another constriction arising 'at right angles to the first 

 one. Two or four new plants are thus formed which 

 very soon appear of typical form and size. If the 



*J t' JL 



species bears external ornamentation, these first- 

 formed cells are devoid of it, but on the first vegetative 

 division the new semicells acquire the characteristic 

 markings of the species. Hoffmeister, from observa- 

 tions on another species, states that the contents of 

 the escaped cell divide repeatedly, forming eight or 

 sixteen cells which have the same form, but not the 

 same size as the parent cells, and that these cells then 

 escape from the zygospore. Zygospores always rest 

 some months before germination. 



Double zygospores are found in some species, but 

 little is known concerning their formation and nothing 



O O 



of their germination; they occur in Olosterium I in en - 

 turn., CL Ralfsii var. hybridum, ( 'ylindrocystis diplospora, 

 and Penium didymocarpum. 



PHYLOGENETIC RELATIONSHIPS OF THE DESMIDIACE.E.- 

 The Desmidiaceas must be regarded as a degenerate 

 family of unicellular Algge evolved by retrogression 

 from sexually differentiated, filamentous Conjugates. 



