GEOGRAPHICAL DISTRIBUTION OF PLANTS. 



157 



lias been made a road of communication, over 

 which the plants of Europe may pass to America, 

 and those of America be in turn transported to 

 the fields of Europe ; by means of which New 

 Holland may send her snowy and fantastic forms 

 to adom the lawns and conservatories of Britain, 

 receiving in exchange the not less valuable pro- 

 ductions of her farms and culinary gardens. 



But it is not sufficient for vegetation that 

 the seeds and roots of plants be merely trans- 

 ported from place to place by the agencies al- 

 ready mentioned. Unless earned to a congenial 

 climate and soil, they sooner or later perish, and 

 again disappear from a country unadapted to 

 their nature. Year after year living seeds are 

 carried from the shores of tropical America, and 

 deposited, by the gulf stream, on the coasts of 

 Europe, without securing to themselves any per- 

 manent existence in its flora ; and of our culti- 

 vated exotics, how few have become even im- 

 perfectly naturalized 1 What then, it may be 

 inquired, are the conditions necessary to the 

 successful development of vegetation, and its un- 

 aided continuance by descent? Undoubtedly 

 they are various, both in kind and degree, each 

 particular species, perhaps, requiring some mo- 

 dification of the general conditions. In Britain 

 how often do we see a sharp frost of spring, or 

 early summer, lay prostrate the gayest beauties 

 of the garden, yet spare the humbler flowers that 

 adom unbidden our fields and groves. Continued 

 drought at times converts the fresh verdure of 

 an English landscape into brown aridity. And 

 while the sheltered valley may be adorned with 

 lofty trees, on the exposed hiUs that bound it, 

 these forest monarchs, crouching before the blasts 

 of heaven, are scarcely able to raise their dis- 

 torted and ungraceful boughs a few feet above 

 the surface of the ground. Again, the clear 

 stream and stagnant morass, the porous gravel 

 and the adhesive clay, the saline soil of the coast, 

 and the vegetable earth of the peat bog, are each 

 distinguished by some peculiarities in the plants 

 they produce ; and when by any chance the spe- 

 cies flourishing on one of these soils are trans- 

 ferred to another, their feeble growth and altered 

 habit frequently prove sure evidences how little 

 their new situation is congenial to them. 



Certain conditions, then, of the atmosphere, as 

 regards temperature and moisture ; of the soil, as 

 regards qualities and composition ; and of their 

 situation, as regards altitude, exposure, and shel- 

 ter — all influence the distribution and localization 

 of plants. 



Temperature. Geographers have divided the 

 globe into zones, corresponding to the modifica- 

 tions of heat on.its surface. Generally speaking, 

 the temperature diminishes from the equator, 

 where it is greatest, to the poles, where the mean 

 heat of the sun is least. Vegetation also follows 

 this course with regard to particular kinds of 



vegetation. The torrid zone is the region of 

 palms; the temperate zones of oak and other 

 magnificent trees of the forest. As we reach the 

 extremities of the temperate zone, and under 

 those of the frigid, pines, birches, and other hardy 

 trees only thrive. At last we come to a region 

 of heaths and lichens ; these, too, begin to dis- 

 appear on the verge of the snow line ; and at 

 last extreme cold shuts out vegetation altogether. 

 The plant which is found to approach nearest to 

 the pole, and which there is good reason to be- 

 lieve even reaches it, is the palmella nivalis, or 



red snow, a minute cryptogam ic plant, which is 

 found incmsting the surface of the snow like 

 drops of blood. 



In considering the influence of temperature, 

 however, it must be remarked that the degree of 

 heat does not regulai'ly coincide with the latitude 

 or distance from the equator. Various causes tend 

 to modify the heat both of the earth and atmos- 

 phere ; such as long ranges of continent or of 

 ocean, and locality as regards the eastern or 

 western sides of islands or continents. 



Altitude has also an effect on temperature, and 

 on the localization of plants. As elevated situ- 

 ations are colder than others on a level with the 

 ocean, the higher we ascend mountains the lower 

 the temperature becomes, tiU at last we reach 

 their summits tipt with snow ; and thus we ex- 

 perience a change of climate corresponding to 

 that which takes place between the equator and 

 the poles. A similar change of vegetation is 

 also observable. Thus, in ascending the Alps 

 or Pyrenees, we find the oaks and vines charac- 

 teristic of a temperate climate around their base. 

 A little higher these have disappeared; but pines, 

 birches, and alders, stiU remain. StiU higher, 

 the absence of trees, while there yet appear small 

 willows and heaths, with many mosses and saxi- 

 frages, recalls the treeless flora of the polar re- 

 gions. Many of the plants found high on the 

 mountains of South Europe, are indeed specifi- 

 cally the same as those of Spitzbergen and Green- 

 land. Below them we have Lapland species ; 

 lower still those of Britain. Nearly one half of 

 the plants of Spitzbergen are found on the hills 

 of Scotland ; those of England, lower in height, 

 have only one-fourth. The altitude at which per- 

 petual snow lies on the mountains of the equator 

 is about 16,000 feet, becoming lower as we advance 

 to the poles, and resting on the sea level in 70° or 

 80° north latitude ; but the height of this snow 

 line varies greatly from local circumstances. As 



