296 , CULTIVATION OF PLANTS. 



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. The cotton plant 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 v allies, moisture of air, and 

 richness of soil. 



See Plate L 



Feet. 



The highest spot on which man ever trod, ID, 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, - 1 1, 125 



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



The highest limit of the Peruvian bark tree, - '.', .">(><) 



The lowest limit of pines under the equator, 5,685 



The highest limit of palms and bananas, 3,280 



LECTURE XLII. 



Plants as affected by cultivation, tyc. 



You may recollect that we have before remarked upon the 

 permanence of species, and have observed that although they 

 may in some respects be varied by cultivation, yet their dis- 

 tinctive characters will not be wholly lost. The differences 

 which exist in species are expressed by the terms races, vari- 

 eties, and variations. 



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

 kind, and continued from the parent plant to its offspring, by 

 being propagated by the seed. They are produced by strewing 

 pollen of one species upon the pistils of another ; the seed thus 

 formed will produce a plant resembling both. 



Varieties are a less important distinction than races ; they 

 are not continued by means of the seed, but produced by graft- 

 ing or continuation of the plant under some new circumstances. 



Variations denote the slightest kinds of difference ; they 

 are occasioned by peculiarities of climate, soil, moisture, dry- 



Elevation produces similar effects on vegetation, as distance from the equa- 

 tor Permanence of species Races Varieties Variations. 



