42 



INTRODUCTION. 



VIII GEOGRAPHICAL DISTRIBUTION OF PLANTS. 



Tropical VfgHalloii. 



1 . Diffusion of Vegetation. The geographical distribution of vegetable forms is chiefly 

 affected by climate, including under this head not only temperature and moisture, but de» 

 gree of light, and density or rarity of the air, and by soil. We find, indeed, traces of vege- 

 tation in the depths of the ocean, in dark caverns, in the hot waters of thermal springs, on the 

 loftiest mountains, and even in the bosom of perpetual snow ; but these are comparatively 

 rare and feeble, and the full vigor and glory of vegetable life appear only where it is unfolded 

 under favorable influences. In regard to temperature it will be found, that the distribution of 

 heat, rather than its degree, has the greatest influence upon vegetation ; the growth of plants 

 which require a long and moderate heat, depends on the average summer temperature ; that 

 of those which require a short but great heat, on the temperature of the warmest month, and 

 that of those which are unable to resist a considerable cold, on the temperature of the coldest 

 month. Thus many of the fruits of southern Europe cannot be raised to advantage in some 

 parts of the United States, because the plants are killed by the frosts, although the length and 

 warmth of the summer are sufficient to ripen their fruits. So maize or Indian corn and tobac- 

 co are produced in some parts of America whose winters are excessively severe, because the 

 genial heats of a short summer are more favorable to their maturity than the longer and milder 

 summers of Europe. Some plants require much moisture, but also need the strong light and 

 heat of a clear sun, and will not thrive in the cloudy climates, where a serene sky is almosi 

 unknown. Sandy or siliceous soils are necessary for some vegetables, and calcareous ingre- 

 dients are equally essential to others. 



2. J^umber and Distribution of Species. Each plant has generally a determinate climate to 

 which it is best adapted ; there are other climates, however, in which it can be raised, though 

 less advantageously, but beyond certain limits it ceases to grow altogether. The whole number 

 of species at present known amounts to 44,000, but it is estimated that the total number of 

 existing species is about 80,000. 



The most simply organized plants, such as mosses, lichens, grasses, &c, which form the 

 lowest order of the vegetable creation are the most widely diffused ; the more perfect tribes 

 are in general Hmited to particular regions, and, in some cases, as for example, the cedar of 

 Lebanon, to a particular mountain or district. 



3. Vegetation of the Frigid Zones. There are properly no plants which are peculiar to 

 the frigid zone, because the mountains of the torrid zone, embracing every variety of climate 

 between their base and summit, are capable of producing all the vegetables of the temperate 

 and frigid regions. The number of vegetable species in the frigid zone is small ; the trees arc 



