102 CHLOROPHYCEAE 



papillae are crossed. In some monoecious species, however, 

 lateral conjugation occurs, the processes being put out from ad- 

 joining cells on the same filament. 



The last phase to be described is that of zygotic contraction which 

 is brought on by further action of the contractile vacuoles, after 

 which a thick three-layered wall develops around the zygote, the 

 middle layer or mesospore frequently being highly sculptured. 

 The zygospore occasionally germinates almost at once, thus pro- 

 ducing plants that account for the autumn maximum, but it is 

 usually dormant until the following spring. Meiosis takes place 

 when the zygote germinates and four nuclei are formed of which 

 three abort, the plants thus only exhibiting the haploid generation. 

 A two-celled germling is formed, the lower cell being relatively 

 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. Azy go- 

 spores, which have arisen parthenogenetically, and akinetes also 

 form other means of reproduction. 



Zygnemaceae : Zygogonium {zygo, yoked ; gonium, angle). Fig. 69. 



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. 69 A). At low temperatures 

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

 coloured violet by phycoporphyrin, especially when the threads are 

 subjected to strong light. Sexual reproduction is rare but when it 

 does occur the gametes are formed from only a part of the proto- 

 plasm. In an Indian species azygospores 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 which only exists in the akinete stage. 



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

 Fig. 70. 



The filaments of the different species are commonly 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 



