73 



/^ ^ or THE 



[ VJNIVERSITY 



OF 



in the open air and in the shade, but it consists still more in the dif- 

 ference between direct and ditlused light, between a clear sky and one 

 veiled with clouds, a ditl'erence which is still unappreciated, although 

 its efficaciousness may be proved by other phenomena, as, for exam- 

 ple, the union of a mixture of chlorine and hydrogen. 

 Humboldt adds : 



I have endeavored for a long time to call the attention of scientists 

 and physiologists to this difference; in other words, to the yet 

 unmeasured heat which direct light develops locally in the cell of the 

 living plant. (Cosmos, t. I, pp. 347-349.) 



TOTAL QUANTITY OF HEAT REaUIRED TO RIPEN GRAIN. 



Boussingault (1834), in his Rural Economy, computes the total 

 quantity of heat required to ripen grain by multiplying the mean 

 daily temperature of the air in the shade in centigrade degrees by the 

 duration, in daj^s, of the process of vegetation. This product is 

 known as the number of " day degrees " that the plant has experi- 

 enced or has required for the development from sowing to maturity. 

 (See Annual Report Chief Signal Officer for 1881, p. 1208.) Bous- 

 singault's results are given in the accompanying table : 



Day degrees required at differeiit latitudes. 



Plant and place. 



Autumn wheat: 



Alsace 



Alais 



Kingston . . . 

 Summer wheat: 



Alsace 



Kingston -.. 



Cincinnati . . 



Truxillo 



Quinchuqui 

 Winter barley: 



Alsace 



Alais 



Kingston ... 



Santa Fe.... 



Cumbal 



Latitudes 

 north. 



48 48 



44 7 



41 50 



48 48 



41 50 



9 00 



14 



44 7 



41 50 



4 35 



00 



Mean air Product of 

 i^i^-^ ^f temper- the days 

 ^i^°°l aturedur- bythi 

 mg cul- I tempera- 

 ture. ! ture. 



Dura- 



the cul 

 ture, 



Days. 

 137 

 146 

 122 



131 

 106 

 187^ 



loo" 



181 



122 

 137 



15.0 

 14.4 

 17.2 



15.8 

 2f).0 

 15.7 

 22.3 

 14.0 



14.0 

 13.1 

 19.0 

 14.7 

 10.7 



Day deg. 



2055 

 2 092 

 2 098 



2 069 

 2 120 

 2 151 

 2 208 

 2 230 



1 708 

 1 795 

 1 738 

 1 793 

 1 798 



The above table shows that the total quantity of heat required 

 increases as the latitude diminishes. 



THE STJNSHINE AND HEAT REQUIRED TO RIPEN GRAIN. 



Tisserand (1875) modifies Boussingault's hypothesis that growth 

 varies with heat and time, but adopts the rule that the work done by 

 a plant can be represented by the product of the mean temperature 



