CONJUGALES 



103 



according to the light intensity, whilst the nucleus is to be found in 

 the centre of the cell on one side of the chloroplast. Fragmentation 

 takes place by method (a) as described for Spirogyra (cf. above), 

 and knee joints or geniculations are also common. At conjugation 

 the gametes are formed from only part of the cell protoplast as in 



Fig. 69. Zygogonium ertcetorum. A, Zygnema stellinum, cell and nucleus before 

 division ( x 500). B, the same, after division ( x 500). C, Zygogonium, stages in 

 conjugation. m = male nucleus, /) = conjugation process. D, terrestrial form 

 ( X 1065). E, aplanospores formed from drying up of filament ( x 542). (A, B, 

 after Cholnoky; C-E, after Fritsch.) 



Zygogonium^ fusion taking place either by way of papillae or 

 through a geniculation. The zygote is cut off by new walls and so 

 becomes surrounded by two or four sterile cells depending on 

 where the zygospore has been formed. Most of the species are 

 isogamous but anisogamy is known in Moiigeotia tenuis. Repro- 

 duction by means of thick-walled akinetes and parthenospores 

 occurs commonly, at least five species having only the latter mode 

 of propagation. 



