MesocarpesB 339 



formed by the concentration of most of the protoplast in the median part of 

 the cell, after which two somewhat oblique walls appear cutting off this 

 central portion from the remaining distal parts. 



Twin parthenos pores have been seen by Wittrock ('72) in Mougeotia genuflexa, and 

 irregularities are sometimes met with in the conjugation of various species of Mougeotia. 

 Cases have been observed in which the terminal cell of a filament has entered into conju- 

 gation with another cell through its free end, no conjugation-tube being developed, and 

 rare instances occur in which three cells, each belonging to distinct filaments, have entered 

 into conjugation (W. & G. S. W., '98). Equally rare are the hybrid examples in which 

 conjugation has occurred between species of Mougeotia of different thickness. 



Twin zygospores have been observed in Mougeotia capucina (Bory) Ag., this phenomenon 

 being strictly comparable with that prevailing in certain species of Desmids, such as 

 Closterium lineatum and Penium didymocarpum (W. & G. S. W., '98). 



On the germination of the zygospore the outer coats are burst through by 

 an outgrowth clothed in the inner membrane (fig. 210 C). This outgrowth 

 becomes cylindrical, rapidly elongates and ultimately forms the new filament 

 by repeated cell-division. In the Craterosper mum-section of Mougeotia the 

 outgrowth makes its exit from the spore by the removal of a lid (fig. 210 E) 

 and it attains a considerable length before any transverse wall appears in it, 

 such walls not making their appearance until there have been formed four 

 chloroplasts and four nuclei. 



The genus Gonatonema was established by Wittrock (78) to include two 

 Alga3 with the vegetative characters of Mougeotia but in which only aplano- 

 spores(?) were formed. Some four or five other species of this kind have 

 since been found and aplanospores have also been seen in certain true species 

 of Mougeotia (W. & G. S. W., '07 ; consult fig. 211 H and /). Spore-formation 

 begins in Gonatonema by a distention of the cell in the middle. This dis- 

 tention, however, is not equal all round the cell, but is stronger on one side, 

 the cell bending at the same time like a knee. The formation of spores 

 almost always takes place simultaneously in all the cells of a filament, the 

 cells bending alternately to the right and to the left, so that the filament 

 assumes a zig-zag line (fig. 211 A, E and F). Wittrock stated that in 

 Gonatonema ventricosum the spores were formed without any preceding act of 

 conjugation and he also gave cogent reasons for regarding them as neutral 

 and not as parthenospores. As yet the detailed cytology of the spore-formation 

 in Gonatonema has not been worked out, but as described originally by 

 Wittrock (78) in G. ventricosum, and as subsequently observed both in that 

 species and in G. Boodlei (W. & G. S. W., '97 A), there is in the early stages 

 of spore-formatign a more or less complete division of the protoplast into two 

 parts which subsequently fuse together (fig. 211 G). The behaviour of the 

 nucleus during this process may prove to be of great interest and would 

 probably decide whether the spores should really be regarded as aplanospores 

 or not. It is possible that some are zygospores and others parthenospores, 



222 



