INFLUENCE OF CLIMATE AND SEASON 85 



As a plant contains water as its largest constituent, 

 and as the whole of the plant food obtained from the 

 soil is taken up through the medium of water, while 

 the amount of water daily lost by the plant through 

 evaporation is very large, the necessity of a large 

 supply of water in the soil during the growing period 

 of a crop is very evident. The supply is often insuffi- 

 cient, as is shown by the much larger crops grown by 

 irrigation. On the other hand, an excess of water in 

 the soil prevents root development, and much percola- 

 tion causes a loss of nitrates and other soluble plant 

 foods in the drainage water. Deeply-rooted crops, as 

 wheat, red clover, lucerne, sainfoin, and mangel, are 

 those best fitted to resist drought ; while shallow- 

 rooted crops, as grass and turnips, are those which 

 suffer most from it. 



We have already seen that carbon, which forms 

 the largest ingredient of all vegetable substances, is 

 obtained by plants from the atmosphere under the 

 influence of light, and that a certain temperature is 

 necessary for this assimilation of carbon, and for the 

 other chemical processes which proceed in a growing 

 plant ; a sufficient supply of light and heat is therefore 

 required for the production of a crop. In a season of 

 deficient light and heat the harvest is always late, 

 growth having taken place more slowly than in an 

 average season. In the case of extremely cold and 

 cloudy summers the whole season may be too short 

 for maturing the crop, and the seed in consequence 

 may never be fully ripened. Early sowing is generally 

 advisable, as a longer period for growth is thus 

 afforded to the crop. 



As the character of the season determines the de^re^ 



