440 
NATURE 
7 
| March 8, 188 3 
the shape and size of leaves : first, the nature and amount 
of the supply of carbonic acid; and second, the nature 
and amount of the supply of sunshine. But as leaves 
also aid and supplement the roots as absorbers of water, 
or even under certain circumstances perform that func- 
tion almost entirely alone, a third and subordinate element 
also comes into play in many cases, namely, the nature 
and amount of the supply of watery vapour in the air. 
Fic. 1. 
This last element, however, we may leave out of con- 
sideration for the present, confining our attention at the 
outset to the first two. 
Carbonic acid is the true fool of plants: water, one 
may say, is only their drink. The roots can almost 
always obtain a sufficient amount of moisture ; and 
though no doubt there is sometimes a fierce struggle for 
Fic, 2. 
this material between young plants, yet its effects are not 
usually so obvious or so lasting on the shape of the parts 
concerned. 
must be built up there exists a competition between plants 
as great and as evident as the competition between car- 
nivores for the prey they pursue, or between herbivores 
for the grasses and fruits on which they subsist. The 
plant endeavours to get for itself as much as it can of 
But for the carbon of which their tissues | 
this fundamental food-stuff; and all its neighbours en- 
deavour to frustrate and to forestall it in the struggle for 
aérial nutriment. Again, the carbon is of no use without — 
a supply of sunlight in the right place to deoxidise it and | 
render it available for the use of the plant. Hence these 
two points between them mainly govern the shapes of — 
leaves. Natural selection insures in the long run the — 
survival of those types of foliage which are best fitted © 
Fice 2. 
for the performance of their functions as mouths and — 
stomachs in the particular environments that each species 
affects. Accordingly, in the final result each plant tends 
to have its chlorophyll disposed in the most economical 
position for catching such sunlight as it can secure ; and 
it tends to have its whole absorbent surface disposed in 
the most advantageous position for drinking in such par- 
ticles of carbonic acid as may pass its way. The import- 
ance of the first element has always been fully recog- 
| nised by botanists; but the importance of the second 
appears hitherto to have been too frequently overlooked. 
At the same time, the shape of the leaf in each species 
is not entirely determined by abstract considerations of 
fitness to the function to be performed: as elsewhere 
in the organic world, evolution is largely bound by heredi- 
tary forms and ancestral tendencies. Each plant inherits 
a certain general type of foliage from its ancestors ; and 
