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 strugele 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, and 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 “arpet 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. Kyery zone is endowed with 
peculiar charms-—the tropical in the variety 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 yon 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 distinet zones 
of plant life exhibits features which are absent or at most more or less 
feebly represented in the other zones, 
