88 
REVIEW. 
the under surface apj3lied to the water. If the leaf retained its verdure the longest with the 
upper surface on the water, the absorbing power of the upper surface was to be regarded 
as the greatest ; but if it retained its verdure the longest with the under surface on the 
water, then the absorbing power of the under surface was to be regarded as the greatest. 
Some leaves were found to retain their verdure the longest when moistened by the upper 
surface, and some when moistened by the under surface ; and some were altogether indifferent 
to the mode in which they were applied to the water. But the inference deducible from the 
whole, and deduced accordingly by Bonnet, was, that the leaves of herbs absorb moisture 
chiefly by the upper surface, and the leaves of trees chiefly by the under surface. What is 
the cause of this singular disparity between the absorbing surfaces of the leaf of the herb, 
and of the tree ? The physical cause might be the existence of a greater or of a smaller 
number of pores found in the leaves of the herb and tree respectively. The chemical cause 
would be the peculiar degree of affinity existing between the absorbing organs and the fluid 
absorbed. Duhamel seems to have been content to look to the physical cause merely, 
regarding the lower surface of the leaf of the tree as being endowed with the greater capacity 
of absorbing moisture, chiefly for the purpose of catching the ascending exhalations which 
must necessarily come in contact with it as they rise, but which might possibly have escaped 
it if absorbable only by the upper surface, owing to the increased rapidity of their ascent at 
an increased elevation ; and regarding the upper surface of the leaf of the herb as being 
endowed with the greater absorbing power, owing to its low stature, and to the slow ascent 
c-f exhalations near the earth. This did not throw much light upon the subject, and the 
experiments were still deemed insuflEicient, as not representing to us the actual phenomena 
of vege tation, though the fact of the absorption of moisture by the surface of the leaf is fully 
confirmed by such phenomena. 
" If, after a long drought, a fog happens to succeed before rain falls, so as to moisten 
the surface of the leaves, plants begin to revive, and resume their verdure long before any 
moisture can have penetrated to the roots. Hence it follows incontestably, either that 
moisture has been absorbed by the leaf, or that transpiration had been suddenly stopped 
by the closing up of the pores, now stomata, of the leaf, or both. The efficacy of rains 
themselves, and of artificial waterings, may be accounted for partly on the same principle, 
for they have not always penetrated to the root when they are found to have given fresh- 
ness to the plant. The moisture then, that enters the plant as an aliment, is taken up by 
means of the pores or stomata, or in default of visible pores or stomata, merely by means of 
the absorbent power of the epidermis, not only of the root and leaf, but often, as it is to be 
believed, b}'' the other parts of the plant also, at least when they are soft and succulent. 
" By what means do the gases enter the plant ? If water or other non-elastic fluids are 
capable of penetrating the outer bark of plants, whether furnished with visible pores or not, 
gases may well be supposed to be endowed with a similar capability. It might be asked, 
however, whether the water and the gases enter by the same pores where pores are found 
to exist. But though there appears to be nothing absurd in the assertion of the affirmative, 
yet it seems probable that each has its ow^n peculiar powers or stomata. At least, it is 
known that some surfaces which repel moisture exhibit no evidence leading us to suppose 
that they repel the common air. This is well exemplified in the case of cabbage-leaves, in 
the time of rains and dews, when the drops roll along the upper surface of the leaf without 
wetting it, or lodge in its folds, like globules of quicksilver. So also in the case of fruits 
covered with bloom. It is probable, therefore, that all such leaves and vegetable surfaces 
as repel moisture are fitted rather for the inhalation of air, which they have long been 
regarded as capable of effecting; and in times in which it was fashionable to look for 
analogies between the plant and animal, in everything whatever, leaves were even regarded 
