Periodicity of Freshwater Algce in Nature 199 
centration is distinctly unfavourable towards at any rate the more 
sensitive species of Spirogyra. 
Fritsch and Rich ( 16 , 17 ) were led to suggest that the normally 
recurring vernal phase of Spirogyra was dependent among other 
conditions upon the realisation of a certain degree of concentration 
of the water, and although they reached this conclusion mainly from 
a consideration of the rainfall-data alone nevertheless their conclusion 
has been amply verified in the case of Hawkesley Hall pond. 
It is obvious that the different species of Spirogyra are not all 
alike in the way they react towards external conditions. In the case 
of the present pond the hardiest species observed is S. inflata , which 
tended to be almost perennial, and, as already stated, is more tolerant 
in the matter of concentration, temperature and bright sunshine than 
all the others noted; while 5 . protecta is also hardy but rather less 
so than S. inflata. These two species are also those which appeared 
first and developed best in the autumnal phase of Spirogyra (when 
this was present) and which often persisted in some quantity through¬ 
out the winter. A rather less tolerant species is 5 . colligata , which 
appeared in the autumnal phase of 1920, while S. variant (present 
and conjugated in September 1920, as well as in May 1919, and the 
spring of 1912) probably comes near S. colligata as regards its degree 
of sensitiveness. The position of S. nitida (present in autumn of 1918, 
but not conjugating) is rather doubtful, but it does not appear to 
be a very sensitive species. 
Very sensitive species which developed only in the vernal phases 
(March—May) are S. catenceformis, Grevilleana, condensata, Jurgensii, 
mirabilis, longata, maxima and Weberi 1 . These prefer a mean monthly 
temperature between 6° and 12 0 C., abundant bright sunshine and 
a moderately low concentration (between 12 and 18 degrees in the 
present pond) of the water. S. bellis developed rather exceptionally 
in the pond, but was confined to one small area, during June—August 
1919, when the mean temperature was about I4°-I7° C., and the 
concentration rising between 20 and 26 degrees, conditions which 
would undoubtedly preclude the growth of the majority of the 
species of the genus. 
Pevalek ( 24 ) in his recent paper on the Algae of N. Croatia found 
that the species of Spirogyra which appeared first in the annual cycle 
were the narrower ones with a single chloroplast and replicate end- 
walls, while in summer the wider species with several chloroplasts 
1 Doubtless these species might occur and even conjugate in the autumn if 
conditions were very favourable; Fritsch and Rich (17) noted S. Weberi in 
Abbot's pond in September 1905. 
