38 GEOGRAPHICAL DISTRIBUTION OF PLANTS. 



which varies with the latitude and the elevation in which plants grow, and 

 considerably also with the nature of the soil, its condition with respect to 

 water, and its exposure and shelter. The degree of temperature required by 

 different plants varies exceedingly from that of the cold regions of the 

 frigid zone, through the temperate regions of both hemispheres, to the torrid 

 zone. For the culture of the first description of plants, a shady situation, 

 and a soil kept constantly moist, in order that it may be kept continually 

 cool by evaporation, constitutes the artificial or garden station ; while to pro- 

 duce a garden station for plants of the warmer regions the various kinds of 

 artificial climates produced in plant houses are necessary. Hence the great 

 importance to cultivators of a knowledge of the natural stations of the plants 

 they cultivate, as well as of the structure and functions of plants generally. 

 It will, therefore, be useful to notice briefly the external circumstances 

 which influence the natural distribution of plants ; and these may be reduced 

 to temperature, light, water, soil, and the atmosphere. 



135. Temperature has by far the most important influence on the distri- 

 bution of plants ; because it would appear, that each species is so constituted 

 as to thrive only within certain limits of heat and cold, and that any excess 

 beyond these limits is injurious to it. Hence the geographical boundary of 

 any species is restricted by the extremes of temperature which the plant 

 will bear, and yet bring its seeds to maturity. 



136. The temperature of any place depends principally upon its latitude, 

 and its elevation above the sea. From the poles to the equator, the temper- 

 ature gradually increases ; and measuring from the level of the sea into the air, 

 the heat gradually decreases, till we arrive at a point, which is to be found 

 on the mountains of all countries, where water exists only in a state of ice 

 or snow. Hence, in forming an estimate of the temperature of any place, 

 the latitude of that place, and its elevation above the sea, are to be jointly 

 considered. From actual experiment, in the neighbourhood of London, by 

 Green the aeronaut, it has been found that when the air was 74 at the 

 surface of the earth, at an elevation of about 3000 feet, it was 70 ; at 

 10,000 feet, 69; and at 11,293 feet, 38. The difference in time between 

 making the first observation and the last was about 27'. According to De 

 Candolle, heat decreases in France at the rate of one degree of latitude for 

 every 540 feet of altitude ; so that the temperature of a place 3240 feet 

 above the level of the sea in 45 N. lat. equals that of a place in about 51 ft 

 N. lat. on a level with the sea. In the middle of the temperate zone, 

 Humboldt found that the mean heat of the year diminished at the rate of 

 2 N. lat. for every 600 feet of altitude. From the powerful influence on 

 temperature produced by elevation, arises the great variety of plants which 

 are found between the base of a mountain and its summit ; though there are 

 a vast number of plants in all countries that will grow indifferently on 

 plains and on mountains as high up as plants will vegetate. There are 

 a few plants, however, that have their range of elevation and of latitude 

 comparatively limited ; as, for example, the Sweet Chestnut, the Olive, the 

 Mulberry, and the Fig. 



137. According to Humboldt, the geographical parallels of latitude do not 

 indicate corresponding degrees of heat either in the old and new world, or in 

 the northern and southern hemispheres. In the former, heat diminishes 

 more rapidly as we recede from the equator ; and in the latter beyond the 

 parallel of 34, corresponding latitudes indicate a greater degree of cold in 



