39 
integuments; finally, the fronds of certain ferns have a green color, 
even when they grow in complete darkness. With regard to the seeds 
of Acer, Astragalus, Celtis, and Raphanus, it has been shown by J. 
B6hm that when they germinate in darkness they do not acquire any 
ereen color; Flahault (1879) has obtained the same result for the 
seeds of the Viola tricolor, the Acer pseudoplatanus.and the Geranium 
lucidum. Similarly as to the other seeds above enumerated the stud- 
ies of Sachs and Flahault render it probable that in most cases there 
was stored up in the seed certain reserve nutrition, which reserve, 
originally formed under the action of hght, can subsequently in the 
act of germination temporarily replace the further direct action of 
heght. It would thus seem that in no case can dark heat truly replace 
the action of sunlight. 
On the other hand, hght can replace heat in the process of vegeta- 
tion. This was first shown by De Candolle, and a striking illustra- 
tion is quoted by Moleschott (1856), who shows that by the influence 
of light during the resplendent nights of the polar regions the har- 
vests ripen in a short time, while many days of our autumn heats 
in lower latitudes scarcely suffice. It is the quantity of light and 
the quality of the radiations that these plants receive that enable 
certain cereals, such as barley and oats, to be cultivated as far north 
as 70° of latitude. The observations of Schleiden on the potato, of 
De Candolle on the radiola, and of Haberlandt (1866) on oats, show 
that there exist decided differences in the quantities of heat neces- 
sary to the development of different species of vegetables under differ- 
ent latitudes, and that the most important cause of these differences is 
the quantity of hght which these plants receive. De Candolle, in his 
botanical geography, says the effect of ight is shown in the northern 
limits of certain species; thus the radiola is perfected by a total sup- 
ply of heat represented by 2,225 day-degrees in the Orkneys at 59° 
north, but by a total of 1,990 day-degrees at Drontheim, latitude 
north 63° 25’; the difcrence (235) corresponds to the fact that the 
longest day is 14 hours longer at Drontheim than in the Orkneys, 
which increased sunlight enables the plant to complete its growth 
better under the same temperature. 
Wheat furnishes a still more striking example. It begins to vege- 
tate when the temperature in the shade is about 6° C., and observation 
has shown that it requires the following day-degrees to ripen: At 
Paris in 138 days, total shade temperature 1,970° C.; at Orange, 117 
days, total shade temperature 1,601° C.; at Upsala, 122 days, total 
shade temperature 1,546° C.; at Lynden (North Cape), 72 days, 
total shade temperature 675° C. Or, if we use, not the shade temper- 
atures, but those of a thermometer exposed to the full sunshine, as 
is done by Gasparin, then the above figures become at Orange, 2.468 
day-degrees; Paris, 2,433 day-degrees; Lynden, 1,582 day-degrees. 
