GEOGRAPHICAL DISTRIBUTION OF PLANTS. 521 
from passing its limits towards the north; the heat exercises the 
same influence towards the south. All laity have their polar 
and their tropical limits. 
Humidity of,the atmosphere and the solar influence have, on 
the other hand, a notable influence on the geographical distribution 
of plants. It is still more necessary to consider the influence of 
elevation. In proportion as we rise in the atmosphere the tem- 
perature decreases, and this lowering of the temperature is so 
sudden, that in ascending a mountain we pass through many 
degrees of decreasing temperature in the course of a few hours. 
From this it follows that a high mountain under the equator may 
be clothed at its base in the richest vegetation, while its summit 
is covered with eternal snow, and the space. between is clothed 
with all the diversity of vegetation (on a limited scale) which the 
traveller meets with in his journey from the equator to the pole. 
With these general remarks upon the principal causes which 
influence the distribution of plants, we shall proceed to consider the 
botanical circumstances, namely, the zones of vegetation which 
result from them. 
For botanical purposes we may divide the surface of the globe 
into three great zones:—1. The Torrid zone, which comprises the 
tropics between 24° north and south latitude. 2. The Temperate 
zone, which in each hemisphere extends from the tropics to the 
polar circle. 8. The Polar zone, which in the northern hemi- 
sphere includes all beyond the arctic, and in the southern all 
~ beyond the antarctic circle. 
The Tropical zone, which receives the direct rays of the sun all 
but perpendicularly, is almost entirely exempt from winter. It 
includes the warmest regions of the globe. The year is there 
divided into two seasons—the one dry and burning, during which 
vegetation is sensibly suspended ; the other the rainy season, 
during which vegetation revives. This large zone, which erhbraces 
in its circle continents, seas, and islands of all sizes, and which 
bristles with immense chains of the loftiest mountains in the 
world, presents climatal changes equally diversified, yielding 
productions which are very far from resembling each other. To 
render the subject clearer, it will be necessary to heap this zone 
into middle, — and tropical zones. 
