73 
in the open air and in the shade, but it consists still more in the dif- 
ference between direct and diffused light, between a clear sky and one 
veiled with clouds, a difference 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 REQUIRED 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 days, 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 different latitudes. 
| 
Dura- Mean air Product of 
Plant and place. Tatitades woe Cr ceredun| ee anys 
. Peay || als cul- | tempera- 
ture. ture. 
Autumn wheat: St Days. | a: | Day deg. 
PAUIBACOpS naa teen es ee oe ei ec i ce an es 48 48 137 15.0 2 055 
INAV G ap be ee sas gos BOSSE Se EEO SS Seana 44% 146 14.4 2 092 
SINS GON eres a ae ene a esa eee 41 50 122 17.2 2 098 
Summer wheat: 
PAISA COM a8 22! so 2¢b ce. cette ete hi icts tess ee cs 48 48 131 15.8 2 069 
Kain STON Wa = sos sree sae ees ae esse ee = st oe SEs = 41 50 106 20.0 2 120 
(Oyie(G hikakeh Sle So Sae = es ee ee ees 39 «6 137 aay 2 151 
Abgrb.gl keys Sa ee ee Pee Opes Se See a ae See a ee 9 00 100 22.3 2 208 
@uinchuquies: 3-28 2 o-c ese 8ae- here poem aces ste cs O 14 181 14.0 2 230 
Winter barley: r—ih 
PNISHCOR ROM (scree tee os ee eee ee eee ee 48 48 122 14.0 1 708 
ING WT fo iy DSRS Pee ee eee eee Cee eee aes 447 137 13.1 1 795 
REIN PSCOM Pena ans et see wa tee tates Sess has 41 50 92 19.0 1 738 
Sani tanh Omer ores. cus yt a cease aes ee aoe 2 4 35 122 14.7 - 1 793 
(Chrba a 67 |S Se eee ee eee 0 00 168 | 10.7 1 798 
The above table shows that the total quantity of heat required 
increases as the latitude diminishes. 
THE SUNSHINE 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 
