Influence of Temperature on Plant-distribution.—The influence of temperature on the distribution of plants is a complicated matter to understand and explain. We must remember, in the first place, that in some plants the root, and in shrubby and woody plants the stem also, lasts for many years, and bears many successive crops of flowers and seeds; and that where there is only one crop produced, the plant has sometimes to live through the winter and sometimes not. A species can only grow where it gets heat enough to perfect its seeds; and if too much heat or too much cold comes whilst it is growing it will wither and die. When the thermometer sinks down to freezing-point all vegetation is suspended, but the degree of heat at which different plants begin to grow is very various. This is a matter of the highest importance, and one that should be understood clearly, that all degrees of temperature below a special point, a point which is high up in the scale for some species, low down for others, do not help a plant to grow, to elaborate its sap, to develop its leaves and flowers, to perfect its seeds. Each species, it has been said, is a thermometer of which the zero is the minimum of temperature at which vegetation is possible for it. Let us take an instance and illustrate this by figures, for although from the complicated elements that have to be taken into account such figures cannot possibly be exact, yet we can show in that way the most clearly what we mean. It is estimated that 43° Fahrenheit is the zero of the common barley. Suppose a crop of barley to be planted in October. Recurring to our tables, we find the average highest daily temperature of the month is 53° in the shade, 59° in the sun. The seeds germinate, and the little shoots show themselves above the surface, bright and green. November comes, when the shade temperature falls to 45°, that in the sun to 49°. In December the figures are 42° plus 5°, in January 40° plus 3°, that is, in the full sun, in the warmest part of the day, the heat does not reach high enough to influence the barley at all. February is January over again. In March the temperature in the shade is not much higher, but the influence of the sun is greater. In April it is 9° above 43° in the shade, and 17° more in the sun, and the young plants waken from their
