Zygnernacece 129 



(fig. 46 B). Palla 1 placed this species under a new genus of the Conjugate 

 owing to the absence of pyrenoids in the plants he observed. More recently, 

 however, precisely the same species has been discovered with pyrenoids, and 

 as the latter are subject to much variability their presence or absence is of no 

 generic value 2 . The most remarkable species of the genus is D. Desmidwides 

 W. & G. S. West 3 , which, up to the present time, has only been observed 

 from Cornwall. The cells are short, only 2^ 6^ times longer than the 

 diameter and 7 '1 8'6 \t in thickness (fig. 46 F I). There is a most evident 

 constriction between all the cells of the filaments, and the latter dissociate 

 with the greatest ease into their individual cells, conjugation only occurring 

 between the free, dissociated cells. This plant throws much light on the 

 origin of the Desmidiacese. 



Genus Zygnema Ag., 1824. [Zygogonium Ktitz. 1843.] The 

 filaments are simple, consisting of a single series of cylindrical 

 cells placed end to end, and sometimes exhibiting a slight con- 

 striction at the points of junction. Each filament possesses an 

 external mucous coat which is sometimes remarkable for its size 

 and strength. There are two star-shaped chloroplasts suspended 

 in the median line of each cell, each one containing a single 

 large pyrenoid. Sometimes the chloroplasts are very indefinite, 

 their form and disposition being difficult to make out. This is 

 particularly the case in Z. ericetorum (Kiitz.) Hansg., some forms 

 of which greatly resemble species of Microspora or of Rhizoclonium. 

 The coalescence of the gametes takes place either in one of the 

 gametangia (the female) or in the conjugating-tube. Species in 

 which the latter occurs were referred by Kiitzing to a separate 

 genus Zygogonium. The zygospores are globose or ellipsoid. 



There are about a dozen British species of this genus, of which Z. ericetorum 

 (Kiitz.) Hansg. (fig. 47 C) is the most widely distributed. This species lives 

 equally well in water or on damp heaths or peaty moors, and it frequently 

 assumes a purple or violet colour owing to the formation of phycoporphyrin 

 in the cell-sap. It is an Alga which fulfils an important function on some of 

 the heaths and moors. In the drier and hotter periods of the year, thickly- 

 matted sheets of Z. ericetorum, often many square feet in extent, are found 

 covering wide patches of almost bare sand or peat, round such plants as 

 Drosera, Carices, etc. These mats of Zygnema have great absorptive capacity, 

 greedily taking up water, and in this way they regulate the moisture of the 

 surface soil, the thriving of some of the smaller Phanerogams depending to a 



1 E. Palla, ' Ueber erne neue, pyrenoidlose Art und Gattung der Conjugaten,' 

 Berichte Deutsch. Bot. Gesellsch. xii, 1894, Heft 8, pp. 228236, t. xviii. 



2 W. & G. S. West in Ann. Bot. xii, 1898, p. 49; in Journ. Bot. Aug. 1900, 

 p. 289. 



3 W. & G. S. West in Journ. Bot. 1903, p. 7 (Sep.), t. 446, f. 19. 



W. A. 9 



