Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne (1867).djvu/74

56 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 centigrade, 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, Chaemarops 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 cultivated 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