Mesocarpeas 



335 



Conjugates. This basis of distinction also has its weaknesses as well as its 

 merits. It must be realized that without their chloroplasts the genera Spiro- 

 gyra and Zygnenm are absolutely indistinguishable, and that in the absence of 

 the chloroplast-characters the distinction between the two genera could not be 

 maintained. This fact is significant when considered alongside the conditions 

 prevalent among many of the Desmidiaceae (the only other family of Conju- 

 gates), in which axile and parietal chloroplasts of diverse character often 

 occur in closely allied species of the same genus and sometimes in different 

 individuals of the same species. 



Since all the evidence indicates 

 that the ZygnemaceaB should remain 

 intact as a family, it is here suggested 

 that it be subdivided into the three 

 sub-families Mesocarpeae, Zygnemeae and 

 Spirogyreas, this subdivision being based 

 upon chloroplast-characters and not on 

 the method of conjugation. Of these the 

 Mesocarpese is the lowest group and the 

 Spirogyreas the most advanced. 



Sub-family MESOCARPETE. The Algae 

 of this sub-family are the narrowest 

 and most delicate of the filamentous 

 Zygnemaceae. The thickness (3 41 yu,) of 

 the filaments and relative length (2 35 

 times the diameter) of the cells vary 

 between wide limits, and the cell-wall is 

 relatively thin. Each cell contains a 

 single chloroplast, usually in the form of 

 an axile plate (fig. 210 A and ), which 

 may extend from end to end of the cell 

 or only occupy the median portion 

 (fig. 209 D). 



The chloroplast contains from 2 to 14 

 conspicuous pyrenoids, generally arranged 

 in a linear series. It is in some species 

 of Debarya (D. desmidioides, fig. 213 

 G K ; D. cruciata) that there are only 

 two pyrenoids, and in some of the thicker 

 species of Mougeotia the pyrenoids are 

 more or less scattered (fig. 210 A). The 

 chloroplasts of adjacent cells usually lie 

 in the same plane, so that a whole filament of cells may present the full 



Fig. 210. A and B, Motif/eotia sp., x500; 

 A, cell showing the full face of the 

 chloroplast ; B, cell showing edge of 

 chloroplast. C and D, germination of 

 the zygospore of Mougeotia parvula Hass., 

 x 390. E, germination of the zygospore 

 of M. laetevirens (A. Br.) Wittr., x 190. 

 n, nucleus ; p, pyrenoid. (C E, after 

 De Bary.) 



