STUDIES OF BRITISH FRESHWATER ALG.E. 
47 
rivularis, cf. also S. jugalis) never form zygospores.^ This is only 
capable of explanation if we assume that the conjugation-process and 
zygospore-formation depend on the occurrence of certain combinations 
of external conditions, which are different for the different species. 
Table of normal occurrence and reproduction of Spirogyra 
IN THE Fish Pond, Abbot’s Leigh. 
S. affinis (Hass.), Petit.... 
Present during the greater part of the year, 
but generally absent in August. Re- 
productive : April to July, 1906; and 
May, June, October, 1907. 
S. jugalis (Dillw.), Kiitz, 
Present during the greater part of the year, 
but generally absent in midwinter and 
during July or August. Never re- 
productive, although commencement of 
conjugation-process observed in April, 
1905. 
8. longaia (Vauch.), Kiitz. 
Forming zygospores in May, 1907. 
8. neglecta (Hass.), Kiitz. 
Normally present from April to August, 
but absent during the remainder of the 
year (possibly present in October, 1907). 
Reproductive : in 1905 and 1906 during 
the greater part of its period of occurrence. 
8. nitida (Dillw.), Link..,. 
Present from January to June of 1905, and 
from April to J uly, 1 906, Reproductive : 
May, 1905. 
8. rivtdaris, Rabh. (?) ... 
Present during the greater part of the year, 
but absent during some of the summer 
months. Never reproductive. 
8. varians (Hass.), Kiitz. 
Present from about March to May in each 
normal year, and quite absent during the 
remaining nine months. Reproductive : 
during the greater part of its period of 
occurrence. 
8, JVeheri^ Kiitz.... 
Present during the first half of the year in 
1905, 1906, and 1907, also in September, 
1905. Reproductive : April to May, 
1906, and June, 1907. 
^ It is interesting to compare these species with the species ^ of (Edogonium ; 
like the latter, S. rivtilaris is present all the year round (though never in very 
large amount), and it may be that the uniformity of conditions which leads to 
this constancy is responsible for the non-occurrence of conjugation. 
