28 PLANT LIFE OF ALABAMA. 



to their surroundings with equal facility, there would be no limit to 

 their dispersion over every part of the globe where life exists, only 

 with the reservation that in the struggle for the possession of the 

 ground those of highest vitality, endowed with greatest power of re- 

 production and possessed of special facilities for dissemination, would 

 overcome the less favored and eventually establish themselves, to the 

 exclusion of others, upon the available space. It is by the diversity of 

 their climatic requirements, their varying degrees of dependence upon 

 the universal environmental factors air, light, heat, and moisture (the 

 last two being the principal ones) that plants are restricted within 

 specific limits. 



The dependence of plant distribution upon heat is demonstrated by 

 the different character of the vegetation under different parallels from 

 the poles to the equator, arid vertically in the ascent from the shore of 

 the sea to the heights of the mountains. As Alexander von Humboldt 

 graphically and eloquently expresses it: "Unlike in design and weave 

 is the carpet which the plant world in the abundance of its flowers has 

 spread over the naked crust of the earth, more densely woven where 

 the sun ascends higher on the cloudless sky, looser toward the slug- 

 gish poles, where the early returning frost nips the undeveloped bud 

 and snatches the barely matured fruit. Every zone is endowed with 

 peculiar charms the tropical in the variet}^ and grand development of 

 its production, the northern in its fresh meadows and in the periodical 

 revival of nature and the influences of the first breezes of the spring. 

 Besides having its own special advantages, every zone is marked by a 

 peculiar character." It is a fact well established by observation that 

 the same or more or less closely related forms will often appear under 

 similar climatic conditions in parts of the globe widely separated by 

 oceans or deserts. This applies, if not to predominating specific and 

 generic types, at least to representative orders. On the summits of 

 mountains, covered for the greater part of the year with snow and ice, 

 plants are found which are at home in the Boreal Zone; again, the flora 

 of the equatorial zone bears the same general features around the globe. 

 Plants with the same climatic requirements, calling therefore for the 

 same physiological functions, necessarily show similarities in their 

 morphological development, and thus we find that in different parts 

 of the globe the plants exhibit a stronger or feebler resemblance 

 morphologically. 



PLANT ZONES OF HUMBOLDT BASED ON ISOTHERMAL LINES. 



Alexander von Humboldt was the first to divide upon these princi- 

 ples the surface of the globe into botanical zones and to lay the foun- 

 dation of the science of plant geography. Each of these distinct zones 

 of plant life exhibits features which are absent or at most more or less 

 feebly represented in the other zones. 



