110 



BOTANY 



the spore develops without the fusion of the nuclei, which takes place only 

 just before germination begins. 



Germination. The fusion of the nuclei is followed by two successive nuclear 

 divisions, but of the four nuclei thus formed only two persist, and there are but 

 two new cells formed from the spore, instead of 4-8, as in Mesotsenium. The 

 young Desmids are somewhat simpler in structure than the mature forms, and 

 it is not until after the second division of the young Desmid that the complete 

 form is attained. The division of the spore-contents takes place after they are 

 set free from the thick membrane of the zygospore (Klebahn, 16). 



The Zygnemaceae 



The Zygneinaceae "Pond-scums," as they are sometimes called 

 are among the commonest of the fresh-water Algae. They are 

 evidently closely related to the simpler Desmids, from which they 

 have probably sprung, and from which they differ mainly in being 



FIG. 81. A-C, conjugation in Spirogyra sp. (X200). D, cell-division in S. crassa 

 (X 175). E, zygotes of S. communis, showing the fusion of the nuclei. (E, after 

 OVERTON.) 



united into long filaments. Their cell-structure corresponds closely 

 with that of certain Desmids. Thus Mesotsenium resembles almost 

 exactly a single cell of the filamentous genus Mesocarpns (Mougeo- 

 tia), while Spirogyra is represented among the Desmids by Spiro- 

 tsenia, and Zygnema by Cylindrocystis. 



In Spirogyra, the commonest genus, the thin-walled cylindrical cells show a 

 thin cytoplasmic layer lining the wall, and contain one or more ribbon-shaped 

 spiral chromatophores, in which are very conspicuous pyrenoids, about which 



