Zygnemacese 331 



another family, the Temnogametacese, to include the genus Temnogametum, but neither of 

 these group-names can be upheld. 



Blackman & Tansley ('02) put forward a classification largely based upon the sug- 

 gestions of Palla. The Conjugatoe were divided into two sections, the Desmidioidese and 

 the Zygnemoidese, the latter containing Palla's three families in the order Spirogyracese, 

 Zygnemacese and Mougeotiaceee. 



Schmitz ('03) reverted to the three families Desinidiacese, Mesocarpacese and Zygne- 

 macese originally suggested by de Bary. His Mesocarpaceie is, however, a most unnatural 

 group since it includes both Pyxispora and 'Zi/gogonium.' Moreover, he revives the name 

 'Mesocarpus' in a sense quite different from its original application. 



West (G. S. W., '99), from general morphological considerations, and Lutkeniiiller ('02), 

 from a detailed study of the structure of the cell- wall, were agreed upon the classification 

 of Desmids, and Liitkenu'iller instituted the two sub-families Saccodermae and Placodermse. 

 (Consult G. S. W., '04). 



Oltmanns ('04) not only included both the Conjugatse and the Bacillariese in the 

 Akontse, bxit he also divided the Conjugate into the Mesota3niacefe, Zygnemaceae and 

 Desmidiacese. The suggested association of the Diatoms with the Conjugate has already 

 been discussed (vide p. 119) and it need only be repeated here that there is no evidence to 

 show that the two groups are in any way nearly related. The three families of the Conju- 

 gatae proposed by Oltmanns were defined as follows : 



Mesot&niacea?. Unicellular; cell-wall in one entire piece; conjugation taking place 

 without the dislocation of the wall into two equal pieces ; zygote on germination produces 

 four embryos. 



Zygnemacea>. Filamentous ; zygote on germination produces only one embryo, which 

 has a rudimentary rhizoid. 



Desmidiacese. Unicellular ; cell-wall in two pieces ; cells more or less constricted in 

 the middle ; zygote on germination produces two embryos. 



The present author is unable to accept Oltmanns' proposed families ; neither could the 

 late Dr Liitkemiiller (consult G. S. W., '15). The ' Mesotteniaceae ' is equivalent to the 

 tribe Spirottenisu of the sub-family Saccodermse clearly defined by Liitkemuller, and the 

 complete removal of these Algee from the rest of the Desmidiaceos does not appear to be in 

 keeping with their true affinities. In this work the Desmidiacese, .for reasons set out at 

 length later on, are regarded as highly specialized and not primitive. 



Wille ('09) introduced into the Zygnemacese a new sub-family the Zygogoniea) to 

 include the genus Zygogonium, but a careful survey of the morphological characters and 

 reproduction of ^ Zygogonium ericetorum' lends no support to the maintenance of such a 

 sub-family or even of the genus ' Zygogonium.' (Consult p. 346 ; also West & Starkey, 

 '14; '15.) 



In the present volume only two families of the Conjugate are recognized. 

 Farn. Zygnemacese. Filamentous types with cylindrical cells. 

 Fam. Desmidiacete. Unicellular and mostly greatly specialized types. 

 Filamentous condition secondarily acquired in a few genera. 



Family Zygnemacese. 



This family of Conjugates has a world-wide distribution and includes 

 some of the most abundant of freshwater Alga?. The thallus is an unbranched 

 filament consisting of a single series of cylindrical cells. Rare instances of 



