f 



214 PLANTS AFFECTED BY CUl.TIVATION. 



The variety of plants in the torrid zone is very great. Trees are 

 more numerous, in proportion to other plaats,than in the temperate 

 zones ; the same tribes which are there slender and humble plants, 

 here spread into lofty trees, many of which are adorned with large 

 and l^eautiful flowers. The richest fruits and spices, and the most 

 valuable medicinal plants, are found here. In ascending the moun- 

 tains of the torrid zone, as the temperature varies, each section has 

 its own distinct plants ; and we find in succession the production of 

 every region from the equator to the poles. 



As the mountains of the torrid zone afford every variety of cli- 

 mate between their base and their summit, so they are capable of 

 producing all the vegetables of every climate; but, as latitude in- 

 creases, temperature diminishes, so, generally speaking, the produc- 

 tions, as we proceed from the tropic northward or southward, cor- 

 respond with the elevation at which the same plants will grow upon 

 a mountain within the tropics. Every plant requires, other circum- 

 stances being the same, the same mean annual tepiperature ;* for 

 example: the plantain-tree and sugar-cane require a mean annual 

 heat of from 82 to 83 degrees; but ~70 degrees of mean annual heat 

 is not found beyond the 27th degree of latitude ; consequently, the 

 plantain and sugar-cane M'ill not ripen in the open air in a higher 

 latitude ; and this Baron Humboldt has found to correspond with 

 the height of 3000 feet under the equator. Cotton will not flourish 

 without 68 degrees of heat, which is not found beyond 34 degrees of 

 latitude, which corresponds with about 3600 feet of elevation at the 

 equator. The same reasoning applies to all other plants, with the 

 exceptions arising from warm valleys, moisture of air, and richness 

 of soil. 



Fret above lii.' level or the sea. 



The highest spot on which man ever trod 19,400. 



The highest limit of the lichen plant 18,225. 



The lowest limit of perpetual snow under the equator 15,730. 



The highest limit of pines under the equator 12,801. 



The highest limit of trees under the equator 11,125. 



The highest limit of oaks under the equator 10,500. 



The highest limit of the Peruvian bark-tree 9,500. 



The lowest limit of pines under the equator 5,085. 



The highest limit of palms and bananas 3,280 



LECTURE XLI. 



PLANTS AS AFFECTED BY CULTIVATION — CH.\NGE OP THE ORGANS — DISEASES — 

 ECONOMICAL USES. 



We have before remarked upon the permanence of species, and 

 that though they may in some respects be varied by cultivation, yet 

 their distinctive characters will not be wholly lost. The differences 

 which exist in species are expressed by the terms races, varieties, 

 and variations. 



Races are those differences in a species which are of a striking kind, 

 and continued from the parent to its offspring, by being propagated 

 by the seed. They are produced by strewing pollen of one species 

 * For explanation of mean annual temperature, see note, page 149. 



Torrid zone— Production of every region found in ascending mountains of the tor 

 rid zone— Elevation prodiices similar clllcts on vegetation, as distance from the equa^ 

 vor- -Permanence of species— Races. 



