Sunlight upon Aquatic Plants. 365 
first place, Messrs. West are able, by citing a fragmen- 
tary sentence from my paper on Assimilatory Inhibition, 
to quote on my apparent authority the statement that no 
fresh-water Algae are resistant to cold, and that all are 
killed by being frozen. It is perhaps hardly necessary to 
mention the fact that the statement made was expressly 
intended to refer only to those Algae actually examined. 
These were extremely few in number, for the above research 
was conducted solely in order to determine the influence 
of various normal and abnormal injurious agencies upon 
the function of C 0 2 -assimilation. Moreover, for this very 
reason, the plants employed were such as were in an actively 
vegetating condition, and which had been grown or had 
been kept for some time previously under optimal conditions 
of temperature, &c. It is well known that many plants 
can be gradually accustomed to low temperatures (or other 
injurious agencies) to which, when suddenly exposed, they 
inevitably succumb. Frequent reference is made to this 
long-known and frequently studied peculiarity throughout 
the work mentioned and in subsequent ones also. In the 
work first published, the only fresh-water Algae examined 
with regard to the effects of cold upon them were unde- 
termined species belonging to the genera Oedogonium and 
Spirogyra , and two to that of Cladophora . To have founded 
any general conclusions as to the resistant powers to cold 
of the great group of fresh-water Algae upon such isolated 
observations as these, would indeed have been somewhat 
premature. It is however certain that many, or perhaps 
very many or all, of the higher filamentous fresh-water Algae, 
and perhaps of other aquatic plants as well, are exceedingly 
sensitive, when in the actively vegetating condition, to 
temperatures approaching to or falling but little beneath 
the freezing-point. A few observations recently made may 
be of interest here, for they illustrate more precisely what 
the resistant powers to cold of such plants actually are. In 
all cases the plants employed were actively vegetating, and 
had been kept at a temperature of from 15 0 to 2o°C. for 
