Spirogyreae 



347 



The genus Pleurodiscus is peculiar in the possession of parietal discoidal 

 chloroplasts, two of which are present in each cell. There is only one known 

 species PI. purpureus (Wolle) 

 Lagerh. in which the cell-sap is 

 coloured purple with phycoporphy- 

 rin. Conjugation takes place as 

 in Zygnema and the zygospore is 

 formed in the female gametangium. 

 Pyxispora, a genus known only 

 from West Africa (vide W. & 

 G. S. W., '97), bears the same rela- 

 tionship to Zygnema that Mowjeotia 

 does to Debarya, the gametes being 

 formed from only part of the proto- 

 plast of the gametangium (fig. 216 

 A and B}. In this genus the outer 

 wall of the ripe zygospore splits 



equatorial crack (fig. 



along an 

 216 C). 



The genera are : Zygnema Agardh, 

 1824 [inclus. Zygogonium Kiitzing, 1843 ; 

 Zygogonium De Bary, 1858 ; Wille, '97 ; 

 '09] ; Pleurodiscus Lagerheim, 1895 ; 

 Pyxispora W. & G. S. West, 1897. 



The doubtful genus Mesogerron Brand 

 ('99) does not appear to belong to the 

 Conjugatse but rather to some group of 

 the Ulotrichales. 



-py 



Fig. 216. AC, Pyxispora miraMUs W. & G. S. 

 West ; A and B, conjugated specimens, x 520 ; 

 C, zygospore, x 1000. D, vegetative filament 

 of Pleurodiscus purpureus (Wolle) Lagerh., 

 x 600 (after Lagerheim) ; E, single vegetative 

 cell of Zygnema sp., stained and mounted in 

 Venetian turpentine, x 500. ch, chloroplast ; 

 py, pyrenoid ; n, nucleus; t, tannin globules. 



Sub-family SPIROGYRE.E. This 

 sub-family includes the largest of 

 the Zygnemacese, some species of 

 Spirogyra attaining a diameter 

 of over 160 ft. The distinguishing 

 character of the group lies in the 



band-like parietal chloroplasts, which are for the most part spirally twisted. 

 Spirogyra is the best known and most conspicuous of the Conjugatse, its cells 

 exhibiting great variability both in their diameter and relative length. The 

 cell-wall is very firm and the mucous coat, which is well developed, especially 

 in the larger species, renders the plants very slimy to the touch. 



The chloroplasts of Spirogyra are disposed in the lining layer of cytoplasm 

 in the form of spiral bands, and they vary in number from one to twelve or 

 fourteen in each cell. In some species they are coiled into very close spirals, 

 but in others they are practically straight and longitudinal ; in some, as in 



