THE GREEN ALG^ 103 



of opinion as to the phylogenetic relation of the Zygnemoidese 

 and Desmidioideae, some looking upon the one and some upon 

 the other group as the more primitive (West and West ^j^ 

 Blackman and Tansley 6, etc.). This is a point however which, 

 with our present knowledge, it is impossible to determine, for 

 every argument can easily be inverted and employed to demon- 

 strate the reverse view. It is, therefore, unnecessary to discuss 

 these theories here, but a few words may be added on a very 

 ingenious interpretation of the Conjugatae recently put forward 

 by Oltmanns (53, p. 89). The latter removes three unicellular 

 forms (the genera Mesotccuiiun, Cylindwcystis, and Spirotcvuia) 

 from the Desmids and classes them separately as the Meso- 

 tseniaceae (fig. 4, g-k). These are distinguished by lacking the 

 bivalved wall and the mucilage-pores of the true Desmids, and 

 by the development of four new individuals from each zygospore 

 on germination. The three genera further show the three most 

 characteristic types of chloroplasts of the Conjugate group — 

 Mcsotcenium (fig. 4, h) having the flat plate of a Mougcotia, 

 Cylindrocystis (fig. 4, k) the two stellate chloroplasts of a Zygucma^ 

 and Spirotcenia (fig. 4, g) a spiral chloroplast like that of a Spiro- 

 gyra. In the majority of the Mesotaeniaceae the conjugation- 

 process is of a very simple type, the contents of the two 

 conjugating cells meeting and fusing in the much-enlarged 

 copulation-canal. Oltmanns regards the Mesotaeniaceae as 

 simple forms resembling the ancestors of the Conjugate group, 

 and derives from them on the one hand the Zygnemaceae and 

 on the other hand the Desmidiaceae. The former are charac- 

 terised by the formation of a single individual from the zygospore 

 on germination, while in the latter two individuals are always 

 formed. The zygospore of the Desmidiaceae, however, shows 

 two nuclear divisions on germination, but of the four nuclei 

 thus produced two regularly degenerate (Klebahn 36). This 

 Oltmanns regards as evidence of derivation from an ancestor 

 forming four individuals from each z3^gospore, i.e. from forms 

 like the Mesotaeniaceae. There can be no doubt that this opens 

 up a very suggestive point of view; on the evidence furnished 

 by the zygospores and other characters Oltmanns' Mesotaeniaceae 

 certainly seem to be simpler than the true Desmids, while they 

 are well suited to stand as a group resembling the ancestors of 

 the Zygnemaceae. Oltmanns' theory, moreover, dispenses with 

 the problematic relationship of Desmids and Zygnemaceae, and 



