254 SCIENCE AND FRUIT GROWING 



substance in the soil immediately after the heating, for such a 

 toxin would have had ample time to become decomposed into 

 food-material before growth was complete indeed, it was sug- 

 gestive that in the experiments with trees, those in the heated 

 soils were much slower in coming into growth than their fellows 

 in unheated soil. But, as the germination results taught, the 

 proportion of toxic matter is rapidly increased by raising the 

 temperature of the heating, and so by extending the observations 

 to soils which had been heated to as high as 150, sufficient 

 toxic matter was formed to outlive the time required for the 

 growth of the plants, and its effect on these then became evident. 

 Figs. 26 and 27 illustrate the results obtained in the case 

 of some of the plants examined. The seeds were sown in the 

 recently heated soils in April, the temperatures to which they 

 had been heated being marked on the pots (the " unheated " 

 soil is marked 30, though, perhaps, 25 would more nearly 

 represent the maximum temperature to which it may have been 

 exposed). In the case of the tobacco and tomatoes, photographs 

 taken on two different dates are shown ; on the first of these it 

 is evident that the toxic effect in the case of the highly heated 

 soils is still operative, the plants being very stunted, though the 

 presence of an ample supply of nitrogenous food, even at that 

 date, is evidenced by the size and appearance of the leaves. 

 As time went on this toxic effect was reduced, and the ultimate 

 growth of the plants, though still exhibiting some signs of 

 stunting, is much in excess of the growth in the unheated soil, 

 and in some cases in excess of that in soil heated to 100. The 

 average growth eventually obtained in the case of spinach, 

 tobacco, and tomatoes, on the one hand, and of three different 

 grasses, on the other, were 



Soil heated to Non-grasses. Grasses. 



30 ioo TOO 



60 193 243 



80 243 215 



100 248 329 

 125 192 421 l 



I50 120 371 1 



With the grasses, as will be seen, the effect of the toxin had 

 nearly entirely disappeared by the time growth was complete, and 

 the increase in growth due to the heating was very great ; this 



1 If the roots are included the maximum occurs at 150. 



