460 



EFFECT OF HEAT 



[Ch. XVIII 



These critical points for animals show the same variations with reference 

 to optima, minima, maxima, and range that those of plants do, and here 

 also these variations are clearly adjusted to the temperatures normally 

 experienced by the species. 



3. Some General Phenomena accompanying Heat Effects. — 



a. Latent Period. — We have seen that a change in the rate 

 of growth is produced by a change in temperature. If, how- 

 ever, the times of the two changes be carefully noted, it will 

 be found that a considerable interval elapses between them. 

 This interval, the latent period, varies with the temperature. 

 Thus AsKENAST ('90, p. 75) found that, in the case of maize 

 seedlings cooled to l°-5°, two or three hours elapsed before 

 decreased growth occurred ; whereas, when the seedling was 

 cooled to 0°, five hours elapsed. A similar effect follows a 

 change in the reverse direction. 



b. Sudden Change of Temperature. — If the radicle of a 

 seedling is suddenly transferred from water at or near 0° C. 

 to water at between 18° and 21°, two effects follow. The first 

 appears immediately after the transference, and consists in the 

 sudden elongation of the radicle. The second appears later, 

 and consists in a growth which is slower than that of the 

 normal radicle. These facts have been determined by Teue 

 ('95), who concludes that the first effect is of a physical nature, 

 and is due to the fact that, at the higher temperature, osmotic 

 pressure is greater ; hence the tension in the tissues is greater 

 and, consequently, they become stretched. The second effect 

 Teue regards as a response to the stimulus of the change, since 



