56 A NEW FLORA OF 



winter's sleep, roused like the sleeping princess in the story, into 

 new life and vitality. The zero of a plant is generally the grand 

 crowning fact upon which its range depends. It is evident, that 

 within considerable limits, the time over which the heat it needs 

 is spread is not material. If we take a handful of seeds and sow 

 half of them under the shadow of a tree, and the other half in a 

 sunny bed, the plants in the exposed place will flower and seed 

 the earliest. The difference in the time of ripening the seed, in 

 some experiments which De Candolle performed at Geneva, was 

 eight days for the common cress, sixteen days for flax, twenty- 

 nine days for candytuft. And he even, in his great handbook of 

 Geographical Botany,* attempts to express in figures the amount 

 of heat which different species need to bring them to perfection, 

 estimating, for instance, that the zero of the beech is 5 centi- 

 grade, and that it requires 2500°; that the holly needs 2200°, 

 parting from 7°; Dianthus Carthusianorum 2500°, parting from 

 6°; or to take a more southern plant, Chmmarops humilis, the 

 only wild European palm, 2700°, parting from 19°. It seems 

 clear that' chickweed, groundsel, and a number of northern and 

 alpine plants, have a zero not much above freezing point, many 

 of our wild British species probably from 40° to 45°, but that, in 

 many tropical species, it goes up to 60° Fahrenheit or more. 



The temperatures, then, which exercise a paramount influence 

 on plant -distribution, are the sums of summer heat over and 

 above various points. It will be clear, from what has been 

 explained already, that there is no essential connection between 

 these and the annual means, and that the relations of one to the 

 other are excessively variable. We cannot illustrate better how 

 this state of things operates than by again recurring to the cul- 

 tivated cereals. In the Andes, where the temperature is nearly 

 the same all the year round, they cannot grow grain much above 

 7000 feet above sea-level, where the annual mean is 55°. In 

 Britain we have to stop at about 44°, and in Switzerland they 

 stop at 40° ; but in Norway wheat goes up to the 64th, oats to 



* Geog-raphie Botaniqne raisonnee, ou exposition des faits principaux et des lois concer- 

 iiant la distribution geographique des plantes de Vepoque actuelle, par M. Alph. De Candollo 

 2 Vols. Paris: Masson, 185.5. 



