292 SCIENCE PROGRESS 



fully mature and therefore loses in market value. It is not 

 surprising, then, that the wheat regions of England are those of 

 low autumn and winter rainfall — the eastern counties ; here 

 wheat has always been grown, even before history began. 



The time when the wheat plant stands most in need of water 

 is after the seedling stage is well passed but before the ripening 

 processes begin. Thus in England a good rainfall coming in 

 late spring to replenish the store of water in the soil is a not 

 uncommon feature of a good wheat year. These water relation- 

 ships are, however, best brought out in countries of very low 

 rainfall, where the wheat is irrigated and where therefore the 

 water supply is under control. The Punjab Irrigation papers 

 contain a number of experiments showing that additions to the 

 amount of water lead to crop increases up to a certain point but 

 further water actually depresses the crop. It is upon this latter 

 point that most Irrigation Departments conduct demonstration 

 trials, since the cultivator almost invariably takes excess of 

 water to the detriment both of the Department and himself 



The extension of irrigation in India, Canada, the United 

 States, South Africa and other countries is giving to these water- 

 supply problems an economic importance they did not formerly 

 possess. In three directions particularly is further information 

 wanted from the plant physiologist. More detailed knowledge 

 is required of the periods in the life of the plant when the need 

 for water is greatest and of the effect produced on the plant 

 by giving greater or less quantities of water. The relationship 

 between water requirements and the supply of plant food 

 should also be further investigated : it is known in general 

 that manures by increasing the crop also increase the amount 

 of water needed but not to a proportional extent, since it is 

 found that manures economise the consumption of water by 

 plants. Thus in Leather's experiments unmanured wheat 

 transpired 850 lb. whilst manured wheat only transpired 550 lb. 

 of water for every i lb. of dry matter formed. Lastly the 

 effect on the grains of varying water supply requires to be 

 studied ; further reference to this will be made later on. 



Perhaps even more important than the actual amount of 

 water is the regularity of the supply, since plants will not 

 tolerate great or sudden variations. Water stored in the soil is 

 liable to loss by evaporation or by percolation, but both sources 

 of loss can be diminished by suitable cultivation. The methods 



