368 Ewart . — The Action of Cold and of 
although no formation of zygospores was observed in them, 
no precise conclusions can be drawn. It is sufficient to 
recall how frequently the existence of special reproductive 
organs in fresh-water and marine Algae has been overlooked 
by a series of investigators to make certain how unreliable 
any such negative observations are, however carefully con- 
ducted they may be. The most painstaking and conscientious 
investigator would shrink from the task of subjecting to 
thorough and systematic microscopical examination the 
entire contents of even a small pond filled with vegetable 
life, and yet, as is well known, the existence of only a few 
zygospores might cause, under renewed favourable conditions, 
the original vegetation to be restored in a comparatively 
short period of time. Moreover, a formation of asexual 
aplanospores or of resting-cells might escape observation 
with even greater readiness. That, however, the zygospores, 
aplanospores, and resting-cells may remain dormant for 
prolonged periods of time is well known, and that they 
can resist considerable extremes of temperature, such as 
would be immediately fatal to the plants when in the 
actively vegetating condition, is extremely probable, and 
indeed almost certain from the observations which Messrs. 
West record. It must, however, always be remembered that 
at the bottom of the pond, in the mud, &c., in which the 
spores lie, the temperature is not necessarily and indeed is 
rarely the same as it is on the frozen surface. 
Nor does it follow that plants imbedded in ice or covered 
by snow are frozen. Both snow and ice are very bad 
conductors of heat, and therefore, as is well known, a layer 
of snow acts as a protective mantle to the plants which it 
covers. Living plants have a power of producing heat which 
may be so marked, when no transpiration is going on, as 
to raise their temperature several degrees above that of the 
surrounding medium. Beneath the snow, however, this 
factor hardly comes into play, since at low temperatures 
the vital activities of even the most resistant plants are 
reduced to a minimum or to nil, and it is upon the con- 
