134 



ChlorophycecK 



There are about 24 British species of the genus, S. gracilis (Hass.) Kiitz. 

 being the smallest, with a minimum thickness of about 10 /i, and S. crassa 

 Kiitz. the largest, with a maximum thickness of about 150 /j. Several species, 

 such as S. tenuissima (Hass.) Kiitz. (fig. 48 C), S. gracilis (Hass.) Kiitz., 

 S. communis (Hass.) Kiitz., S. varians (Hass.) Kiitz. and S. nitida (Dillw. ! 

 Link (fig. 49 A), are abundant in every part of the British Islands except in 

 the most mountainous districts. S. velata Nordst. and S. calospora Cleve are 

 notable for their beautifully marked zygospores. 



Plants of this genus often cause trouble in cress-beds, the 

 matted masses of Spirogyra preventing the growth of the cress 

 plants. 



Genus Choaspis S. F. Gray, 1821. [Sirogonium Kiitz., 1843.] 

 It is very doubtful if this genus should be separated from Spirogyra. 



The filaments are similar 

 to those of many species 

 of the latter genus and 

 the chloroplasts, although 

 more or less straight 

 and longitudinal, are not 

 straighter than those of 

 Spirogyra majuscula or 

 S. pellucida. There are, 

 however, certain constant 

 distinctions. There is a 

 remarkable absence of 

 the external mucous coat, 

 in consequence of which 

 this is the only Conjugate 

 which is not slimy or 

 slippery. The process of 

 conjugation is peculiar. The gametangia, more especially the female 

 one, become swollen and sometimes genuflexed. The conjugating- 

 tubes are not distinct, but the walls of the gametangia come into 

 apposition and a perforation is formed at the point of contact. 

 The chloroplasts also completely disintegrate, even before the 

 gametes have receded from the walls of the gametangia. The 

 coalescence of the gametes takes place in the female gametangium. 

 Conjugation only appears to affect indiscriminately a few of the 

 cells of the filament, and as these cells are usually shorter than the 

 ordinary vegetative cells, they may perhaps be specially set apart 

 as sexual cells, almost comparable to those which are specially cut 



Fig. 50. A C, Choaspi* xtictica (Eng. Bot.) 

 O.K., from Ingleton, W. Yorks. (x 120). 



