HEAT 



103 



to cold, perhaps combined with maturation 

 phenomena, is shown by the fact that simi- 

 lar gemmules hatched after three days in 

 similarly favorable temperatures in the 

 spring. After extended hibernation, develop- 

 ment of this sponge takes place at tempera- 

 tures as low as 2^^ to 5° C. 



The accelerating effect of exposure to low 

 temperatures, freezing included, is also 

 shown, among others, by grasshopper eggs 

 in the middle latitudes. These develop more 

 rapidly if placed at low, even freezing, 

 temperature and later transferred to a 

 liigher one, and the process appears to be 

 related, superficially at least, to the so- 

 called vernalization of plants. Seeds of win- 

 ter cereals can be "vernahzed" by adding 

 water until the seeds barely sprout and then 

 chilling them to 3° or 5° C. or even freez- 

 ing for an average length of thirty-five to 

 forty-five days or over winter. When 

 planted in the spring, such seeds develop as 

 though they belonged to spring varieties 

 (Miller, 19cj8). From naturafistic evidence, 

 Darling (1937, 1938) has suggested that 

 cold, as such, may act on the gonads of 

 birds and mammals as a stimulating agent 

 alternative to the well-authenticated stimu- 

 lation produced by light (p. 121), and 

 that low temperatures may be responsible 

 for the marked development of the gonads 

 of hibernating mammals before they emerge 

 from hibernation. C. R. Moore and his co- 

 workers (1934) had already tested this 

 point with the ground squirrel, Citellus. The 

 reproductive system of this rodent under- 

 goes a marked regression after the annual 

 spring breeding season. Experiments with 

 diet, darkness, constant warm or cool tem- 

 peratures, brief transfer from hibernation to 

 warm surroundings, and similar manipula- 

 tions have yielded essentially negative 

 results so far as the induction of develop- 

 ment of the male reproductive system is 

 concerned during seasons when such 

 development is unusual in the ground squir- 

 rel. In the female, however, with constant 

 cold and darkness, periods of oestrum have 

 been induced and maintained for many 

 months at times in the year when the ani- 

 mals are ordinarily sexually inactive. 



In medial latitudes, cold-hardy animals 

 that emerge in early spring frequently have 

 northern afiBnities or a northern origin, 

 while forms that appear in late summer 

 tend to have a southern origin or southern 

 afiBnities. 



HEAT HARDINESS 



Hot springs furnish the warmest environ- 

 ments known to be inhabited by active 

 organisms. The blue-green algae Fhormid- 

 ium bijahense and Oscillaria filiformis ap- 

 parently hold the record for multicellular 

 plants. They five in the thermal waters of 

 Yellowstone National Park at a temperature 

 of 85.2° C. Living bacteria have been found 

 in even hotter water at 88" C. (Copeland, 

 1936). 



Brues (1939) discounts, pending more 

 evidence, certain reports that larvae of 

 brine flies (Ephydridae) Live at tempera- 

 tures of 55° and even at 65° C. and that 

 rhizopod protozoans have been taken from 

 water at 58° C. He regards 50° to 52° C. 

 as the highest temperature compatible with 

 the fife of plants other than those just men- 

 tioned, and of active animals. Encysted 

 animals and plant seeds resist much higher 

 temperatures. An examination of the tem- 

 peratures at which animals have been taken 

 in thermal waters indicates that the major- 

 ity of these heat-tolerant animals live in 

 water below 40° C. As the temperature de- 

 parts more and more from the usual opti- 

 mum, the number of species that can 

 tolerate such a temperature becomes re- 

 duced. This is a phase of a much more 

 general principle that can be stated as fol- 

 lows: Wherever and whenever conditions 

 approach a pessimimi (see p. 213), the 

 biota becomes impoverished, the more so, 

 the closer the approach to the Hmits of tol- 

 eration. In East Indian thermal springs, 

 Brues records fifty-seven species from 36 

 to 40°; twelve, from 41 to 45°; four, from 

 46 to 50°; and four from above 50° C. 



Deaths from heat may occur at much 

 lower temperatures. Some one-celled, snow- 

 dwelling algae cannot resist temperatures 

 higher than 4° C. (Luyet and Gehenio, 

 1938). The alga Phacocystis poucheti dies 

 at 11.6° C, and, despite fittle exact work 

 on the subject, it is known that many sorts 

 of animals are killed by heat before the 

 temperature reaches 20° C. In general, 

 fishes and marine invertebrates are less re- 

 sistant to heat than are terrestrial insects 

 or mammals, and animals from streams are 

 less resistant than are related animals from 

 small ponds. Such differential resistance is 

 probably a result of natural selection. One 

 reason for the lack of more data concerning 

 the point at which heat deaths occur is 



