THE REQUIREMENTS OF PLANTS 29 



Two deductions may be drawn : (i) water is economised by increasing 

 the food supply ; (2) the total amount of water required during the 

 growing season may be greater than is supplied by the rain, in which 

 case the balance must be otherwise provided, or the food cannot be 

 utilised. 



Over large areas of the world the rainfall is insufficient, and re- 

 course is had to irrigation. In endeavouring to ascertain the best 

 way of irrigating crops, two considerations have to be kept in view : 

 (i) excessive watering has secondary injurious effects on the soil, such 

 as the deterioration of the physical condition, the accumulation of 

 alkali salts, or the formation of toxic reduction products ; (2) the re- 

 quirements of the plant are not always the same, more water being 

 needed during the period of active growth than during germination or 

 ripening. Much more work is required from the physiological side 

 before definite rules could be laid down. Wheat would form a suit- 

 able plant for study, since it is the crop most commonly grown on 

 irrigated land. In the meantime, experiments like those conducted by 

 the Punjab Irrigation Department l have shown that the cultivator 

 everywhere tends to take too much water, with loss not only to others 

 on the same irrigation system, but also to himself. 



The amount of water in the soil has a marked effect on the char- 

 acter of the plant, the time of ripening, and the composition of the 

 grain. As the water supply increases, so the extent of the leaf surface 

 increases ; while a diminished water supply is met by a smaller leaf 

 surface, admitting of less transpiration. Thus on moist soils clays 

 and loams the plants usually have large wide leaves and grow to a 

 considerable size, whilst on the drier sands the vegetation is narrow 

 leaved and more stunted. A copious water supply leads to a more 

 protracted growth and to a retardation of the ripening processes ; in- 

 deed in very wet districts grain-crops are grown only with difficulty, if 

 at all, because ripening may be so long delayed that frosts intervene 

 and damage the crop. 



Water supply and temperature are the two chief factors determin- 

 ing the distribution of crops. In the warm dry eastern counties crops 

 are grown for seed ; great quantities of wheat and barley are grown in 

 Norfolk, Suffolk, and the Isle of Thanet ; mangold seed and turnip 

 seed is produced in East Kent. Wetter districts are more favourable 

 for swedes and oats ; very wet districts for grass. The warm, moist 



1 These and similar experiments are discussed by A. and G. L. C. Howard in Wheat 

 in India : Its Production, Varieties, and Improvement (Imperial Department of Agriculture, 

 India, 1909). 



