no THE ALGAE 



colourless and rhizoidal in character. Filaments of two different 

 species have been known to fuse, the form of the hybrid zygospore 

 being determined by the characters of the female thread. Azygo- 

 spores, which have arisen parthenogenetically, and akinetes also 

 form other means of reproduction. 



Zygnemaceae : Zygogonium (zygo, yoked; gonium, angle). Fig. 6i 



The commonest species of this genus, which is sometimes re- 

 garded as a subsection of the genus Zygnema, is the terrestrial Z. 

 ericetorum. The cells of this species each contain a single axile 

 chloroplast, whilst in Zygnema, of course, there is a pair of very 

 characteristic stellate chloroplasts (Fig. 6i, a). At low temperatures 

 the walls develop a very thick cellulose layer, whilst the sap is 

 coloured violet by a pigment (which is probably an iron-tannin), 

 especially when tiie threads are subjected to strong hght. Sexual 

 reproduction is rare but when it does occur the gametes are formed 

 from only a part of the protoplasm. In an Indian species azygo- 

 spores are apparently the only means of reproduction and even 

 these are scarce. Aplanospores and akinetes are commonly formed, 

 and there is one abnormal form growing on Hindhead heath (Eng- 

 land) which only exists in the akinete stage. 



Zygnemaceae: Mougeotia (after J. B. Mougeot, a French botanist). 

 Fig. 62 

 The filaments of the different species are conmionly unbranched, 

 although they may occasionally possess short laterals. The chloro- 

 plast is a flat axile plate lying in the centre of the cell and orientated 

 according to the Hght 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 

 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 Mougeotia tenuis. Reproduction by 

 means of thick-walled akinetes and parthenospores occurs com- 

 monly, at least five species having only the latter mode of propaga- 

 tion. 



