SOIL CONDITIONS AFFECTING PLANT GROWTH 53 



The results are illustrated in Fig. 9.^ 



Temperature. 



It is difficult to separate soil temperature effects from 

 temperature effects in general, and in practice the distinction 

 is unnecessary since the temperature of the air is largely de- 

 termined by that of the soil. 



The effect of temperature shows itself in three ways : — 



I. It profoundly affects the rate of growth of plants, and 

 thus determines what plants can be grown and what cannot 

 be grown in a given area. 



If the temperature is too low a yellow or purplish colour 

 may appear in the leaf, and the plant grows so slowly that 

 it is liable in its early stages to succumb to insect pests, such 

 as wireworms, and in its later stages to be cut down by 

 autumn frosts before it has time to ripen. If, on the other 

 hand, the temperature is too high, the plant becomes taller 

 than usual, less robust and, when much water is also supplied, 

 liable to fungoid pests that prove very troublesome in com- 

 mercial glasshouses. Only over a comparatively restricted 

 range of temperature is it possible to obtain the compact 

 sturdy habit aimed at by the grower. This favourable range 

 has not yet been correlated with other properties of the plant 

 and has to be discovered empirically; it is, on the whole, 

 lower for the seedling than for the growing plant, but it is 

 highest for the period of maturation. It varies for different 

 crops ; wheat requires a cool time for sowing but a hot time 

 for ripening, barley requires a cool, and oats a still cooler 



1 For other papers see C. A. Shull, Bot. Gaz., 1911, 52, 453-477, and W. A. 

 Cannon and E. E. Free, Science, 1917, 45, 178-180. 



