26 
William J. Hodgetts 
XX. GENERAL CONCLUSIONS REGARDING THE 
CONCENTRATION OF THE WATER 
Up to the present very little exact work has been done by 
algologists on the relation between the growth of the various algal 
species in nature and the concentration of the water, although several 
workers on algal periodicity have dealt with the influence of this 
factor in a more or less speculative manner. Fritsch and Rich ( 14 ,16) 
were perhaps the first to point out that the concentration of the 
water was an important factor concerned in the growth of freshwater 
algae in nature. In the present study, as was indicated in the 
Introduction, special attention was directed towards concentration- 
conditions, and although the methods used in this connection (see 
Section IV) are not altogether above criticism from the chemical 
standpoint, yet the results obtained show conclusively that with 
many algae the concentration of the water is a very important factor 
controlling their occurrence in nature. On the one extreme we get 
forms like Volvox aureus (see Section XVII) adapted to the very 
lowest concentrations observed in the pond, and on the other there 
are several species (Tribonema affine, Chlamydomonas Reinhardi, 
Oscillatoria spp., Phormidium spp.) the occurrence of which to a 
great extent is determined by very high concentrations, while each 
concentration observed during the period of observation has at least 
one (usually several) algal species, the development of which it 
tends to favour, although of course it is always possible for other 
factors to step in and influence growth in some way. 
The origin of the high concentrations observed at certain times 
in Hawkesley Hall pond has been discussed in Section IV, where it 
was shown that the marked fluctuation noted in the concentration 
was due not only to the effects of evaporation, and dilution by rains, 
but also to the addition of soluble organic matter to the water by 
the decay during certain months of the year of large quantities of 
the floating leaves of Potamogeton natans, as well as masses of various 
filamentous algae. It became apparent during the course of the work 
that a correct idea of the concentration could not be obtained from 
a record of the water-level of the pond, and still less from an ex¬ 
amination of the rainfall-data. It was also shown that soluble 
organic matter made up the greater part (o-6 to o-8) of the total 
soluble matter in the water, so that there is reason to believe that 
those species which are favoured by high concentrations are probably 
more or less mixotrophic in their mode of nutrition in nature. 
