177 



in the following table, where the figures represent the sums of sun- 

 shine temperatures necessary to complete the growth from germi- 

 nation to harvest. 



Marie-Davy (1881), in his chapter on the influence of heat on the 

 time required for vegetation, adopts the principle enunciated by 

 Boussingault, of the equality of the sum total of the temperatures, 

 but thinks that the temperature required to bring a plant to the 

 flowering stage is the sum of the mean daily temperatures in the full 

 sunshine, and not the temperature of the air in the shade. According 

 to his view, the heat is needed in the soil in the early part of the 

 growth of the plant; but after the flower is formed, or during the 

 process of perfecting the fruit, sunlight is needed, and during this 

 stage he uses the actinometric degrees of the Arago-Davy actinometer 

 as an index of the progress of the plant. I have, therefore, in the fol- 

 lowing table collated the figures given by him for wheat. The third 

 column gives the sum total of the mean daily shade temperatures, 

 counted from February 1 of each year up to the date at which the 

 total amounts to 1,264° C, or within half a day thereof, that being 

 the adopted shade constant for the flowering of wheat that was sown 

 on or about the 21st of March. The fourth and fifth columns give 

 the dates and sum totals of temperatures observed with a naked-bulb 

 thermometer on the grass in the full sunshine, assuming 1,569° C. 

 jis the thermal constant for this thermometer. The sixth column 

 gives the observed dates of flowering. As these dates agree with those 

 in the fourth column better than with those in the second column, 

 Marie-Davy considers them as confirming him in the use of tlie 

 unprotected solar thermometer. In order to bring out the total effect 

 of sunlight and sun heat Marie-Davy has comi)uted the sum total of 

 actinometric degi-ees from February 1 up to the dates given in column 

 2 and in column 4, respectively. These results are given in columns 

 7 and 8, which show that 1878 was a very precocious year, as com- 

 pared with the others, in that the date of flowering was very early, 

 but the sum total of its actinometric degrees was very small and its 

 crops were very poor. 1879 and 1877 show larger actinometric sums, 

 but the largest sums are given by the years 1873, 1874, 1875, and 1876, 

 which were also very excellent crop years. 



2667—05 M 12 



