ii DEVELOPMENT AND ENVIRONMENT 25 



seed formed a sufficient food supply, and aeration was 

 maintained by a stream of bubbles of oxygen through the 

 water. The rate of heating had to be reduced to about 

 1 C. in five minutes, in order to obtain accurate measure- 

 ments, but even so, the growth curve (Fig. 32) was 

 identical with that given by the fungus, showing the same 

 acceleration, and also stopping at about 37 '5 C. Slower 

 rates of heating were employed in the form of germination 

 trials in the incubator, when it became clear that some 

 similar " x-substance " was again operative. The germin- 

 ation of samples of seed stored at higher temperatures 

 is much quicker, though the ultimate germinating per- 

 centage is unaffected. Moreover, a 37 C. sample is 

 surpassed in four days by a 25 C. sample, not only in the 

 mean length of the radicles, but also in mean weight. 

 The sample at 37 C. is, in other words, stale, and needs a 

 prolonged exposure to lower temperatures if it is to be 

 restored to health. 



It should be noticed that a shade temperature of 40 C. 

 is frequently reached during the early summer, so that 

 some manifestation of this process, which the author has 

 denoted as " Thermotoxy," may be expected to occur 

 during the growth of the crop in the field. We shall see 

 later that such is actually the case. 



Meanwhile, one other point needs to be considered. 

 The heat- poisoned fungus is cured through removal of 

 " x," whether physically by washing, or chemically by 

 decomposition. The first method is impossible in the 

 higher plants, so that chemical decomposition must be 

 invoked ; the effect of overheating in one afternoon, 

 though plain during the following night, has disappeared 

 by the second night. 



Now it has long been a truism that every plant has an 

 " optimum temperature " for growth.* We have just seen, 



* See various writings by Mr. F. F. Blackman for critique of these 

 terms. 



