204 William J. Hodgetts 
few (e.g. Closterium acerosum) prefer the shallower water at the sides 
of the pond. 
As regards their occurrence in the pond these Desmids may, for 
convenience, be divided broadly into two groups, the majority, 
however, falling in the second group. 
Group i contains those Desmids which attain their maximum 
either in the spring or autumn (or have two maxima, one in spring 
and another in autumn). To this group belong Hyalotheca dissiliens, 
Sphcerozosma granulatum and Closterium acerosum (Fig. 6). 
Group 2 includes those Desmids which attain their maximum 
development at the end of summer or in early autumn (June— 
September), and includes all those of the list, with the exception of 
the three placed in the first group. Closterium striolatum, however, 
rather tends to form a connecting-link between these two groups. 
From a consideration of the meteorological and other data it 
seems certain that the most important factors concerned in the 
development of all the Desmids of the pond are temperature and 
the concentration of the water. It is found by a comparison of the 
various curves that pronounced development of any particular 
Desmid depends on the conjunction of certain average temperature- 
conditions with a more or less definite concentration of the water, 
and that if only one of these conditions is favourable then the 
unfavourable one plays the part of a limiting factor and prevents 
the increased growth of the species. 
Hyalotheca dissiliens (Fig. 6) is the hardiest of all the Desmids 
present, and a perennial constituent of the algal flora of the pond. 
It tolerates considerable extremes of concentrations (as in latter half 
of 1919, and first half of 1920), but nevertheless became more or 
less abundant only when the concentrations were moderately low 
(between 10 and 18 degrees—as during October—November 1918, 
April—May 1920, August—September 1920). 
Again this species can withstand considerable variations in the 
temperature-conditions although distinctly preferring the moderate 
temperatures of spring and autumn; while the most unfavourable 
time for this Desmid, during the period of observation, was the hot 
summer of 1918. The lower temperatures of winter affect this Desmid 
in a much less marked degree than do the higher summer-tempera¬ 
tures. No relation to bright sunshine is apparent. The absence of a 
distinct vernal phase in 1920 was probably due to the marked 
dilution which the pond underwent during February—April of this 
year, favourable concentration-conditions apparently not arising 
until the higher unfavourable temperatures had. set in—in short, the 
