THE REQUIREMENTS OF PLANTS 



27 



throughout. It varies even for different varieties of the same crop ; 

 plant breeders are continually trying to evolve strains suited to par- 

 ticular temperature ranges, e.g., wheats have been bred at Ottawa 

 to ripen in the northern parts of Canada. During the course of the 

 twenty-four hours the temperature may exceed the favourable limit for 

 growth, even in our own climate. F. Darwin {77) has obtained some 

 remarkable curves showing that the growth of vegetable marrows is 

 often inversely proportional to the temperature. In hot, dry climates 

 overheating is a very real danger, against which provision has to be 

 made. 



Water. — The relationship between the amount of growth and the 

 supply of water is shown by Hellriegel's experiments (128, Table V.) 

 with barley grown under favourable conditions in sand cultures. 



Table V. — Growth of Barley with Varying Supply op Water. 

 Hellriegel (128). 



100 represents the amount of water required to saturate the sand. 



The yield rises as the water increases up to a certain point and then it 

 falls off because the excess of water reduces the air supply for the 

 roots. In natural soils a further complication sets in when too much 

 water is present : certain reduction products are formed by bacteria in 

 absence of air and have a direct toxic action on the plant. 



Determinations have several times been made of the amount of 

 water transpired for every unit of dry matter formed in the plant under 

 particular conditions. It is not supposed that there is any direct 

 causal relationship between the two quantities, 'nor is there anyidefinite 

 ratio, the amount of water transpired increasing with the temperature 

 and to some extent with the water supply, but decreasing as the food 

 supply increases. The relationship between food supply and water 

 requirements is very interesting and is readily explained. The amount 

 of soluble nutrient salts a plant takes up, and presumably also the con- 

 centration of the cell sap, increases with the concentration of these salts 

 in the surrounding medium. But as the concentration of the cell 



