[ 195 ] 
A STUDY OF SOME OF THE FACTORS 
CONTROLLING THE PERIODICITY OF 
FRESHWATER ALGA3 IN NATURE 
By WILLIAM J. HODGETTS, M.Sc. 
(With 6 figures in the text) 
VII. SPIROGYRA 
o less than 14 species of Spirogyra (namely S. injiata, protecta 
IN (= calospora), colligata 1 , longata, nitida, catenceformis, Grevil- 
leana, condensata, maxima, bellis, varians, Jurgensii, mirabilis, Weberi) 
have been observed in the pond during the period of observation, but 
although a few are hardy forms the majority are “spring annuals” 
(to use Transeau’s term), and occur only during April—June, and 
are very rare or absent at other times. 
As already stated (Section VI), Spirogyra predominates in the 
shallow-water marginal province, and rarely occurred in quantity in 
the central part of the pond during spring (February—May), 
culminating in a maximum generally in May, and rapidly declining 
in June, at the end of which month all species disappear, or become 
very rare, after producing zygospores. In the autumn (October— 
November) Spirogyra may again develop and become locally common 
in the pond, but is never so abundant as in the spring. The vernal 
phase was present in each of the four years, and was especially well 
developed in May 1919, when twelve species were observed, while an 
autumnal phase was present in 1918 and 1920, but was absent in 
1919 (see below). The species which have appeared and conjugated in 
the autumnal phase are S. inflata, protecta, colligata and varians 2 . 
The pronounced development of Spirogyra during the spring (March 
—May) has been noted by many observers (14, .16,17, is, 30, 31, etc.), 
and the less-marked autumnal phase has been recorded by Fritsch 
and Rich (14,16,17), Delf (12), as well as others. 
The frequency-curves of S. inflata, S. protecta and 5. colligata are 
given in Fig. 4. Of these three species the first two are the hardiest 
1 Hawkesley Hall pond is the original locality where this interesting species 
(see Hodgetts, 20) was found. 
2 Fritsch and Rich (17) give S. varians as an example of a species which 
“appears to be quite confined to the vernal phase.” Conjugating specimens of 
S. varians, however, have been collected during the autumn months from 
several other ponds near Birmingham. 
5-6 
