370 Ewart . — The Action of Cold and of 
be collected and concentrated upon the plant by the concave 
walls of the chamber in which it lies, and thus may keep 
the temperature of the plant from falling below its own 
special freezing-point during the daytime. The chlorophyll, 
along with the water, &c., which the plant contains, forms 
a very efficient warmth absorbing agency. It is probably 
by means of the heat thus directly absorbed that the ice 
immediately surrounding a frozen plant may be melted, 
even when exposed only to the relatively feeble rays of the 
wintry sun. I have frequently found the vegetative shoots 
of Elodea enclosed in ice in this manner, and have seen in 
some cases that the stem and most of the leaves were in 
part or entirely living, although the vegetative shoots of this 
plant are always infallibly killed when properly frozen. In 
plants covered by not more than two or three centimetres 
of snow, radiation from above cannot supply any appreciable 
amount of heat to the plant, owing to the relatively 
high opacity of loose or freshly fallen snow when in mass 
both to radiant light and heat. Nor can any noticeable 
amount of heat be derived by the plant under these 
circumstances by radiation or conduction from beneath. 
The special efficiency of snow as a protective agency seems 
to be simply due to the fact that the mixture of air and 
ice of which it is composed forms an admirable insulating 
medium so long as the temperature remains low, while at 
the same time the loss of heat by transpiration or by radiation 
is reduced to nil or to a minimum. 
The case mentioned by Messrs. West, in which filaments 
of Spirogyra were found, when thawed out from the ice in 
which they had been frozen for some time, to be in conjuga- 
tion, and that nevertheless the vitality of the filaments was 
unimpaired, is hardly applicable to the point at issue, and 
moreover, involves two contradictory statements. Surely in 
the process of conjugation the vitality of the filament is 
always impaired, for with the completion of the process the 
further existence of the filament ceases. Each zygospore 
is a new organism, and possesses very different resistant 
