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William J. Hodgetts 
to be perennial, and the frequency-curve of which is given in Fig. 5. 
Five “maxima” of varying intensity are shown (three autumnal and 
two vernal), namely in August and November 1918, May 1919 and 
1920, and September 1920, the species in all cases growing mainly 
in shallow water towards the margin of the pond. 
Delf (12) observed that a wide Mougeotia (probably M. scalaris), in 
ponds on Hampstead Heath, showed an autumnal as well as a vernal 
phase; while the sterile species noted by Fritsch and Rich(i7) in 
Abbot’s pool also exhibited an autumnal development in certain 
years, but tended to have its maxima during December—February. 
M. scalaris in Hawkesley Hall pond is a hardy form as regards 
its ability to withstand extremes of temperature, the five maxima 
occurring respectively at the following mean temperatures: 16-5°, 
5*5°, 13*5°, 12 0 and 13*5° C. (monthly mean temperatures of the air). 
A comparison with the data given in Fig. 2 shows that the species 
does not require any considerable amount of bright sunshine for 
good vegetative growth, but that abundant bright sunshine and a 
moderately 'high temperature (monthly mean between n° and 
13-5° C.) are requisite for zygospore-formation. In May of both 1919 
and 1920 there were slight maxima, and the species conjugated, and 
this month in both years was very sunny (Fig. 2). Zygospores were 
very scarce in the feeble maximum of September 1920 (a dull 
month); and the larger maxima of August and November 1918 (the 
former a rather sunny and the latter a dull month) were purely 
vegetative, the sunshine of August 1918 apparently not being 
sufficient to induce conjugation. 
There is distinctive evidence, therefore, that the conditions 
favouring vegetative growth, and those best for the conjugation of 
M. scalaris are not identical; and further instances of the same 
phenomenon will be noted in the case of the Desmidiaceae (Section 
IX). Fritsch and Rich (18) have pointed out the connection between 
bright sunshine and the fruiting of Mougeotia parvula , as well as 
other Algae, thus confirming Klebs’ conclusion by direct observation 
in nature. It is certain, however, that many Desmids (see Section 
IX), as well as some species of Spirogyra, can conjugate without the 
stimulus of abundant bright sunshine. 
With regard to the influence of the concentration of the water 
on the growth of M. scalaris, the evidence does not seem to point 
to any particular degree of concentration being essential for growth, 
in fact the species is a very hardy one in the matter of concentration 
of the water. Thus between November 1919 and April 1920, when 
the concentration fell steadily from 30 to 8 degrees (see Fig. 3), the 
