THE PLANT 25 



shoot. The rate of growth of the root is usually con- 

 trolled by the temperature of the soil, provided that the 

 soil is sufficiently moist, and that the stem is sending 

 enough food to it from the leaves. In the early spring 

 the soil is too cold for the root to function freely, and the 

 stem suffers in consequence, though the air may be warm 

 enough. Soil temperatures at a few inches below the 

 surface scarcely vary their annual change from one year 

 to the next, so that the date of sowing depends on deep 

 soil temperature in the first instance, which is the same 

 on the same day in the same field each year. 



Further, to sow excessively late will result in failure 



of the seedlings through overheating. This phenomenon 



recurs at later stages of the plant's history, 



Poisoning an( j mer ^ s some attention. Seed germinated 

 by Heat. 



in incubators at various temperatures will 



provide simple illustrations. At 15 C. the germination 

 is slow, while at higher temperatures the rate of growth 

 increases ; between 20 C. and 30 C. the rate of growth 

 is doubled, and if an incubator is adjusted to 36J C., it 

 is possible with Egyptian cotton to prepare a report on 

 the germination capacity of a sample within twenty-four 

 hours from receiving it. If, however, a sample thus 

 incubated is left for two days, it will be surpassed by 

 those samples kept at lower temperatures, and in three 

 days will be almost irreparably injured. This injury is 

 due to accumulation of poisonous excreta in the tissues, 

 these being thrown off in the chemical processes of growth 

 more rapidly at high temperatures than the rate at which 

 the plant can dispose of them. 



