ALG^ 187 



single unbranched or branched filament of cells, the only known modes 

 of multiplication being the conjugation of zoogametes and the direct ger- 

 mination of larger zoospores. In the lowest two classes, the Chroolepidece 

 and Ulotrichacece, embracing a very small number of genera, the filament 

 is usually unbranched ; in the two higher, the ConfervacecB and Pitho- 

 fhoracecB, further vegetative activity is displayed in the copious branching ; 

 and in the former we have an indication of affinity with the Multi- 

 nucleatse in an occasional plurality of nuclei. 



The exact course of evolution from the isogamous Confervoideae is 

 obscure, but it would appear to have taken place in three distinct lines. 

 The first of these, which evidently came to an abrupt conclusion, is the 

 CoNjUGATiE, consisting of the Zygnemacem, MesocarpecE, and Desmidiea, 

 a well-marked and sharply differentiated group with no near affinities. 

 The first two orders, consisting of unbranched filamentous forms, are 

 probably derived directly from the Confervoideae, although the change 

 in the mode of reproduction is very abrupt. The production of zoospores 

 is entirely suppressed, and they are reproduced solely by the conjuga- 

 tion of cells belonging to the same or to different individuals. The 

 Desmidiese must then be regarded as a group adapted, by a certain 

 amount of retrogression in both vegetative and reproductive characters, 

 to life in shallow water ; and derived, through such filamentous genera 

 as Desmidium and Hyalotheca, from Zygnemaceae with lateral con- 

 jugation. By some writers the Diatomaceae are associated with the 

 Desmidieae ; our reasons for placing them among the Protophyta will 

 be given hereafter. The mode of reproduction by conjugation attains 

 its climax in the Mesocarpeae. 



The second line of descent is that of the brown seaweeds. In the 

 Pn^ospOREiE we have every shade of transition in the mode of repro- 

 duction from isogamous to heterogamous. The typical Phaeosporeae, 

 such as Punctaria and Ectocarpus, are characterised by the possession of 

 two kinds of zoosporange, unilocular and multilocular. The zoospores 

 produced in these two kinds of zoosporange present no difference in size 

 or form ; but those from the unilocular sporanges appear in all cases to 

 germinate directly, while those produced in the multilocular sporanges 

 are sometimes zoogametes with sexual functions. In some families one 

 or the other kind of zoosporange is suppressed. In the EctocarpacecR and 

 some other genera we have a mode of reproduction closely resembling 

 that in the isogamous Confervoideae, except in the greater differentiation 

 of the cells which become zoosporanges, a conjugation of zoogametes 

 which are to all appearance exactly alike, though a slight differentiation 

 is exhibited in the fact of one of them coming to rest and partially losing 

 its cilia before conjugation takes place. In the Cutleriacecz the differ- 



