39 



integuments; finally, the fronds of certain ferns have a green color, 

 even Avhen they grow in complete darkness. With regard to the seeds 

 of Acer, Asfraf/dliis, Celtic, and Raphawis, it has been shown by J. 

 B()hm that when they germinate in darkness they do not acquire any 

 green color; Flahault (1879) has obtained the same result for the 

 seeds of the Viola trk-oloi\i\\Q Acer pscudoplat(inui<.^-c\\\([ the Geranium 

 hicidum. 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 uj) in the seed certain reserve nutrition, which reserve, 

 originally formed under the action of light, can subsequently in the 

 act of germination temporarily replace the further direct action of 

 light. It would thus seem that in no case can dark heat truly replace 

 the action of sunlight. 



On the other hand, light 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 (185G), 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 (186()) on oats, shoAv 

 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 light which these plants receive. De Candolle, in his 

 botanical geography, says the effect of light 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 difference (235) corresponds to the fact that the 

 longest da}^ is 1] hours longer at Drontheim than in the Orkneys, 

 which increased sunlight enables the plant to comjilete 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 ()° 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 L3^nden (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. 



