PHYSIOLOGY OF PLANTS. 
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of leaves; and those of the same author, of Duhamel, and 
Bonnet, demonstrate, that absorption, on the contrary, is 
much greater at the inferior surface than at the superior. To 
prove this, the superior surface of one leaf, and the inferior 
surface of another, were covered with varnish, and the conse- 
quence was, that the former, in a given time, suffered little 
diminution of weight, but the latter became much lighter. 
A train, similar leaves were laid upon a surface of water, and 
it followed, that those which had their superior surface 
inverted, gained little weight, and for the most part died 
in a few days; while such as had their inferior surface ap- 
plied to the water, became much heavier, and flourished 
many months. These facts make it evident, that the perspi- 
ration and absorption are not performed by the same vessels, 
but that each has its peculiar organs. 
It has been commonly supposed that perspiration takes 
place chiefly when the air is warm ; and absorption, on the 
other hand, when it is cold and moist. But, unless the ves- 
sels peculiar to absorption, which are placed in the under sur- 
face of the leaves, were kept constantly in action, they would 
necessarily collapse or decay. All absorbing organs have a 
peculiar structure, and an action depending on life, that such 
an organization is present in the leaves of plants, it is reason- 
able to conclude, because dried leaves do not absorb. The 
same reasoning is applicable to the absorption performed by 
the roots : for when a small portion of the root of a hyacinth 
growing in water is cut off, the whole root dies, and new 
roots are shot out, having their extremities peculiarly 
adapted to the absorption of nourishment. 
The noxious matter carried oft’ by perspiration, requires 
large dilution to prevent its hurting the delicate structure of 
the leaves, and in this state accordingly it is thrown out on 
their surface. Here the noxious part is excreted, but part of 
the diluting fluid is re-absorbed, to serve the purpose of secre- 
tion, which could not be performed, unless the common 
juice, or sap, were previously prepared. In the same manner, 
in the animal’s body, saline and putrid matter, carried off 
by the urine, must be liberally diluted, to prevent it from 
injuring the tender structure of the kidneys; yet, when it is 
safely lodged in the bladder, a part is re-absorbed, and the 
grosser excrementitious matter is alone thrown out. Some- 
thintr of the same kind happens in the perspiration of 
animals. They certainly take in something useful from 
the surface of their bodies ; and this is probably performed 
by vessels opening outwards, different from the common 
exhalents. The great quantity of water, absorbed during the 
