§4] ACCLIMATIZATION TO EXTREME TEMPERATURES 257 



the continued activity of the contractile vacuole at a time when 

 no fluids are being taken into the protoplasmic body. 



From the foregoing considerations it appears probable that 

 one of the important characters of " spores " is the diminished 

 amount of free waiter held in the protoplasm; or, in other 

 words, its dryness. This dryness of the coagulable substance 

 would seem to be cause of its higher resistance. 



So far the evidence seems complete. Whether, however, loss 

 of water is the ultimate cause of the high resistance capacity 

 of hot-spring organisms or of those gradually acclimatized is 

 still uncertain. Analogy renders it highly probable that such 

 is the case. 



2. Acclimatization to Cold. — Just as organisms may become 

 acclimatized to high temperatures, so also may they live in very 

 cold regions. I cite a few examples : Several species of Pro- 

 tista are said to live in the Alps above the snow line, coloring 

 the snow red. (Shtjttleworth, '40.) A tardigrade is found 

 in the same locality. Certain insects live on or in the snow or 

 ice. Thus Desoria glacialis (or glacier flea) lives on the Swiss 

 glaciers, and on the snow live Podura hiemalis, Trichocera 

 brumalis (when the temperature is " below the freezing point," 

 Fitch, '46, p. 10), and other species of Trichocera and Podura. 

 Cf. also Boreus hiemalis and B. brumalis (Fitch). Although 

 swarm-spores are usually extremely sensitive to cold, Stras- 

 BUEGER ('78, p. 613) cites a case of a marine alga in which 

 they were being formed and thrown out when the temperature 

 of the water was between — 1.5° and — 1.8° C. 



Increased resistance to cold seems often the result of the 

 action of cold on the organism. Thus, while Schwarz ('84, 

 p. 69) found that Euglense gathered in the summer time were 

 not responsive below -f- 5° to 4- 6° C, Aderhold ('88, p. 320) 

 found that Euglense gathered in the winter would respond 

 even at 0°. We may say, the winter cold had in some way 

 lowered the heat attunement of these Protista. 



In seeking for an explanation of acclimatization to cold we 

 should recall that the cause of death from cold is chiefly the freez- 

 ing of water in the protoplasm, and the irritation of excessive 

 cold. Accustomed to great cold, protoplasm would doubtless 

 be no longer irritated by it ; whether under these circumstances 



