CLIMATIC RELATIONS 269 



the year, a gale being defined as a wind blowing at the 

 rate of 40 miles an hour. 



284. Sunlight. Knowing that sunlight is an essen- 

 tial factor in plant growth, it appears evident that any 

 considerable increase in the total amount of light which 

 the cereal crop receives, must exercise a great influence 

 in its development, whether such increase means more 

 hours of sunlight in the day or a greater number of clear 

 days in the season. It is known that clear weather 

 hastens maturity, and favors a greater production of 

 protein in the kernel, but temperature and dryness are 

 also concerned in protein production. To the northward 

 the period of growth of spring crops decreases as the 

 length of the summer days increases. If the mean daily 

 temperature be multiplied by the number of days in the 

 period of growth of the crop, the product, known as 

 " day-degrees," will increase from north to south. The 

 product of the mean daily temperature by the total 

 number of hours of sunshine, during the same period, 

 known as " sunshine-hour-degrees," increases in the 

 same direction. Therefore, as the total amount of sun- 

 shine itself increases in the opposite direction south 

 to north it would appear that there is a large effect of 

 the light rays proper, independent of heat rays, in assimi- 

 lation and growth. Sunlight also apparently increases 

 transpiration, greatly, independent of temperature. 

 Nearly all is yet to be learned as to the optimum light 

 conditions for the different cereals (see MacDougal, 

 1903). 



285. Phenology of the cereals. The study of the 

 different periodic phenomena which occur during the life 

 of a crop, such as heading and ripening, as influenced by 

 its environment, is known as phenology. As only cli- 



