214 
PLANTS AFFECTED BY CULTIVATION. 
The variety of plants in the torrid zone is very great. Trees are 
more numerous, in proportion to other plants, 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 beautiful 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 temperature;* 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 will 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^iohiiess 
of soil. 
Feet ubove the level of the sea. 
The highest spot on which man ever trod • • 19,400. 
The highest Hmit of the hchen plant 18,225. 
The lowest hmit of perpetual snow under the equator •• 15,730, 
The highest hmit of pines under the equator 12,801. 
The highest limit of trees under the equator 11,125. 
The highest Hmit of oaks under the equator 10,500. 
The highest hmit of the Peruvian bark-tree 9,500. 
The lowest limit of pines under the equator 5,685. 
The highest limit of palms and bananas 3,280 
LECTURE XLI. 
PLANTS AS AFFECTED BY CULTIVATION — CHANGE OF 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, pag e 149. ^ 
Torrid zone— Production of every region found in ascending mountains of the tor 
rid zone— Elevation produces similar effects on vegetation, as distance irom tiie equa- 
w- -Permanence of species— Races. 
