253 



rejecting all mean daily temperatures below 6° C, and thus finds 

 1,496° as the mean value of the sum of temperatures from the date of 

 sowing to that of flowering. The similar sum from the date of head- 

 ing to flowering is 8(50°, or 1,490° less 689°. 



The fourth epoch, or the ripening of the wheat, occurs when the 

 sum total of the mean dail}^ shade temperatures since the date of 

 flowering, rejecting all below 6° C., amounts to 815° C., and in the 

 climate of Paris this occurs about fortj^-five or forty-six days after 

 the date of flowering. The range of uncertainty in this last interval 

 is only four or five days, owing largely to the uniformity of the 

 climate at this season. It is the best defined of all the periods and 

 so well ascertained that, knowing any actual date of floAvering we 

 can safely predict the date of ripening. In proportion as we 

 approach the latter date the process of ripening seems to concentrate 

 itself more and more within the wheat ; water and sunlight become 

 less and less important ; rain becomes a source of uneasiness as to 

 the harvest, and the intensity of sunshine has only an indirect 

 influence on the quality and quantity of the grain. The influence 

 of sunlight during the first phase or germination is negligible and 

 probably nothing; it is a maximum at the beginning of the fourth 

 phase, but diminishes rapidly as the fourth phase progresses and in 

 proportion as the wheat becomes more yellow. We shall therefore 

 consider the amount of sunshine, or more properly the total radiation 

 from sun and sky, during the first thirty days after flowering and 

 neglect its amount during the remainder of the period up to maturity. 



The following table shows the amount of radiation, as expressed 

 by Marie-Davy in actinometric degrees or percentages and com- 

 puted from actual observations of his actinometer at Montsouris for 

 the various stages of growth, viz, the second or heading stage from 

 germination to heading, the third or flowering stage from heading 

 to flowering, and fourth for the first thirty days of the fourth or 

 ripening stage immediately following the flowering: 



