384 
VEGETABLE PHYSIOLOGY. 
just as blood is rendered fit for that of animals. But, for this purpose, 
it is not sufficient that the external surface should be surrounded by 
the atmosphere; other aeriferous organs are provided, in the form of 
spiral vessels, which are placed internally, and convey air containing an 
unusual proportion of oxygen, which is obtained through the root, by 
their own vital force, from the earth and water. In a hundred parts 
of this air, twenty-seven to thirty parts are of oxygen, which is in part 
lost during the day by the surface of plants, under the direct influence 
of the solar rays.” 
Dr. L. adds—“ So that while true spiral vessels may be admitted as 
undoubted vehicles of air, ducts of all kinds, and especially dotted ducts, 
cannot be doubted to be the passages through which fluid is conveyed, 
when great rapidity is required. 
“ Of the Root .—It is the business of the root to absorb nutriment 
from the soil, and to transmit it upwards into the stem and leaves, and 
also to fix the plant firmly in the earth. Although moisture is, no 
doubt, absorbed by the leaves of all, and the stems of many, plants, 
yet it is certain that the greater part of the food of plants is taken up 
by the roots, which, hence, are not incorrectly considered vegetable 
mouths. 
u But it is not by the whole surface of the root that the absorption 
of nutriment takes place ; it is to the spongioles almost exclusively to 
which that office is confided; and hence their immense importance in 
vegetable economy, the absolute necessity of preserving them in trans¬ 
plantation, and the certain death that often follows their distraction.. . 
Forest trees, with very dense umbrageous heads, do not perish of 
drought in hot summers or dry situations, when the earth often becomes 
mere dust for a considerable distance from their trunk, in consequence 
of their foliage turning off the rain. The fact is, obviously, that the 
roots near the stem are inactive, and have little or nothing to do as pre¬ 
servers of life, except by acting as conduits while the functions of 
absorption go on through the spongioles, which, being at the extremities 
of the roots, are placed beyond the influence of the leafy canopy, receive 
the principal share of the rain shed off by the head ; and, as the roots 
are always spreading farther and farther from the main stem, they are 
continually entering new soil, the nutritious properties of which are 
unexhausted. 
“ It is generally believed that roots increase only by their extremi 
ties, and that, once formed, they never undergo any subsequent elonga¬ 
tion. It is probable that this peculiarity may be universal in exogenous 
plants; but it certainly is not constant in indigenous plants; and I 
doubt very much whether it be not confined to roots with a woody 
