GEOGRAPHICAL DISTRIBUTION OF PLANTS. 
29 
Trethum necterus is a fine species, with dark purple flowers, 
and well worth growing. 
sfsclepias tuberosa is a very scarce and splendid plant. A col¬ 
lection of it makes a beautiful bed, so that it could be grown by 
every amateur. 
ON THE 'ADVANTAGE OF GARDENERS UNDERSTANDING 
THE GEOGRAPHICAL DISTRIBUTION OF PLANTS. 
SUBSTANCE OF A PAPER READ TO THE WEST LONDON GARDENERS’ ASSOCIATION, 
BY MR. WILLIAM KEANE. 
In the study of any trade or profession which an individual 
may be inclined to pursue, the great advantage is to know the 
particular subjects most necessary to be acquired ; and as I, in 
my humble opinion, consider the geographical distribution of 
plants very necessary to be studied by gardeners, I will, without 
further preliminaries, enter into the investigation and elucidation 
of the subject. 
As we advance from the pole to the equator, we observe the 
temperature gradually increasing; and as we ascend from the 
surface of the sea into the atmosphere, we find the temperature 
gradually decreasing until we reach a boundary where our further 
inquiries are stopped by the deep snow, and where vegetation 
ceases. In taking into calculation the vegetation of a given 
climate, it is necessary to investigate the temperature peculiar to 
the latitude itself, and the reduction caused by elevation. 
The effect of elevation is not the same in Europe with all 
plants: there are many that are found to grow indifferently upon 
the valleys and upon mountains, as high as perpetual snow; but, 
on the other hand, there are many whose boundaries are circum¬ 
scribed, either by elevation or equivalent temperature. Many 
plants indigenous on the plains in the north of Europe, occupy 
the mountains of the south. The causes of the influence of 
elevation upon plants are ascribed to reduced temperature, to a 
greater intensity of solar light, and to a decrease in humidity. 
The rarefaction of the air as we ascend, produces a correspond¬ 
ing increase in the intensity of light. In taking a view of the 
influence caused by latitude, we find that, in the countries situated 
near the equator, the vegetation consists of palms and thick 
