NOETHUMBERLAJSri) AND DUKHAM. 55 



Influence of Temperature on Plant-distrihution. — 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 vegeta- 

 tion 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. Eecurring 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. 

 E^ovember 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. Februaiy 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 



