478 
GEOGRAPHY. 
BOOK V. 
The next, and by far the most important head under which 
the geographical distribution of plants is to be considered, is 
with reference to temperature and light. These depend, 
firstly, upon latitude ; and, secondly, upon elevation above 
the sea. 
As we proceed from the pole towards the equator, we find 
the temperature gradually increasing ; and, as we ascend from 
the surface of the ocean up into the atmosphere, we find the 
temperature gradually decreasing, until we reach a point at 
which perpetual frost holds his throne, and where vegetation 
ceases. 
In like manner we find, as we recede from the equator to 
the pole, we quit the country of palms and other arborescent 
monocotyledonous plants for the habitations of deciduous 
dicotyledonous trees, Coniferae, and cryptogamic plants ; and 
that as we rise into the atmosphere as considerable a change 
takes place. Thus, in Teneriffe, the foot of the mountain is 
occupied by Crithmum latifolium, succulent Euphorbias, Plo- 
cama pendula, and Prenanthes spinosa : to these succeed vines, 
corn, Canarina campanula, and Messerschmidia fruticosa : a 
third class, consisting of laurels. Ilex, Ardisias, heaths, and 
Viburnums, occupy the succeeding tract. These are sur- 
mounted by pines, Cytisus, and Spartium microphyllum ; and, 
finally, the scenery is closed by Spartium nubigenum, Juni- 
perus oxycedrus, Scrophularia, Viola, and Festuca. (See 
Humboldt's Travels.) 
Therefore, in considering the matter of the vegetation of a 
given climate, it is necessary to take into account the temper- 
ature peculiar to the latitude itself^ and the reduction caused 
hy elevation. 
The decrement of caloric, as we ascend into the air, will be 
understood by the following table, calculated by Daniell, 
from observations made by Mr. Green the aeronaut, in an 
aerial voyage performed in 1821. These are particularly 
instructive ; because they were all made within the space of 
half an hour, under circumstances which varied as little as 
possible. 
