Sunlight upon Aquatic Plants. 377 
resistant power to freezing and dessication to be developed, 
so also may the accommodation to a marine habitat induce 
a correspondingly greater resistant power in plants which 
have been gradually transferred from the one medium to the 
other. Kjellmann has indeed observed that arctic marine 
Algae may continue to grow and even flourish at or even 
below zero centigrade, a power which no fresh-water Alga is 
known to possess. We have apparently here to do with an 
adaptive modification of similar character, but taking place 
in the reverse direction to that occurring in those Oscillatorias 
which can grow and flourish in hot springs. 
Similarly with regard to those plants which constitute the 
Flora of the regions of perpetual snow, we find that these are 
plants which exhibit special adaptive modifications suiting 
them to their peculiar habitat. It must be remembered that 
in such regions the snow is commonly extremely hard and 
compact, and that the surface is usually more or less dirty 
and covered with dust fallen from the air, along with numerous 
micro-organisms which find here conditions suitable for their 
development. When exposed to direct sunlight, the insolation- 
temperature will be relatively extremely high for all absorbent 
bodies which are not cooled by radiation, conduction, or 
evaporation, whereas at night the temperature falls many 
degrees below zero. Since the snow reflects so much light 
and absorbs so little, it follows that coloured microscopic 
organisms may be at a temperature many degrees above 
zero when exposed to sunlight, although the snow on which 
they lie shows no immediate signs of melting. Even in 
gullies or hollows, sheltered from the sun, the amount of 
radiated heat which reaches the organisms may suffice to 
keep them unfrozen during the day, although the snow always 
remains perfectly hard. When the sun is shining upon them” 
alpine plants are exposed to extremely intense illumination, 
such as many plants are unable to withstand. Hence a pro- 
tective red pigment is often found in the microscopical snow- 
organisms which contain chlorophyll. This pigment absorbs 
as much as possible of those rays which exercise an injurious 
