Zygnemaeece 



131 



Z. pectinalum (Vauch.) Ag., Z. cruciatum (Vauch.) Ag. and Z. insigne 

 (Hass.) Kiitz. (fig. 47 E) are the three largest British species, usually occurring 

 in ponds or in road-side ditches. The smallest form of the genus is Z. Vauch- 

 erii Ag. var. stagnate (Hass.) Kirchn. (fig. 47 B), the filaments of which reach 

 a maximum thickness of 13 /x. Z. anomalum (Hass.) Cooke is a species 

 largely confined to upland bogs and is remarkable for its wide mucous in- 

 vestment. 



Genus Spirogyra Link, 1820. This is the best known and 

 most abundant genus of Conjugate, and it has a distinct preference 

 for low-lying, quiet waters, 

 such as those of ponds 

 and ditches. The fila- 

 ments are quite simple 

 and occur in bright green 

 flocculent masses, often 

 several feet in diameter. 

 The cells are cylindrical 

 and they exhibit great 

 variability both with re- 

 gard to their diameter 

 and their relative length. 

 The cell- wall is firm, with 

 an outer mucous coat 

 which renders the plants 

 very slimy. In most 

 species the transverse 

 cell-walls are quite plane, 

 but in some of the smaller 

 species there is a curious 

 annular ingrowth of cel- 

 lulose at the ends of each 

 cell (vide fig. 48 C). When 

 this is present the cells are 



Fig. 48. A, Spirogyra majuscula Kiitz., from 

 Coates, Gloucestershire, single cell showing chloro- 

 plasts and nucleus (x 300). B, Spirogyra sp., 

 from Coates, Gloucestershire ( x 370). C, S. tenuis- 

 sima (Hass. )Kutz., from Mitcham Common, Surrey 

 ( x 390). n, nucleus. 



said to possess " replicate ends." It is a character which is constant 

 for the species in which it is found, although the ingrowths are not 



net over the soil, intergrown with small herbaceous plants and shrubs. The mat- 

 like sheets of the Alga eagerly absorb the atmospheric moisture during dewy nights, 

 affording by this means a refreshing protection to the roots of many other and 

 larger plants during the glowing heat of the following day. Welwitsch states that 

 the growth and thriving of the numerous small phanerogamous plants in these 

 places is conditional on the co-presence of the Alga. Cfr W. & G. S. West in 

 Journ. Bot. 1897, p. 303. 



92 



