VEGETABLE PHYSIOLOGY. 
385 
structure. Occasionally roots appear destined to act as reservoirs of 
nutriment, on which those of the succeeding year may feed when first 
developed, as is the case in the Orchis, the Dahlia, and others : but it 
must be remarked that the popular notion extends this circumstance 
far beyond its real limits, by including, among roots, bulbs, tubers, and 
other forms of stems. 
“ By some botanists, and among them M. Decandolle, it has been 
thought that roots are developed from special organs, which are to them 
what leaf-buds are to branches ; and this function has been assigned to 
those little glandular swellings so common of the willow, called lenti¬ 
cular glands by Guettard, and lenticelles by Decandolle. 
“ Of the Sap .—For the sustenance of plants a fluid is necessary, 
which is absorbed by the i>ots from the earth, then sent upwards into 
the stem, afterwards impelled into the leaves, whence it descends 
through the liber, transferring itself to the inmost parts of the wood. 
This fluid, which constitutes the blood of plants, is called the sap. 
When first introduced into the system, and even when altered in some 
degree by having dissolved the various substances it encounters in its 
passage, it is true sap: afterwards, when its nature has been more 
changed by elaboration in the leaves, it becomes what is called the 
proper juice . 
“ If the sap be examined in its most simple state, it will be found to 
consist of water, mucilage, and sugar. As the two last can scarcely have 
been absorbed directly from the earth, it is inferred that, as soon as 
the fluids taken up by the roots enter the system, they suffer some 
chemical decomposition, the result of which is the production of muci¬ 
lage and sugar. In addition to the supply of sap which is obtained 
by the roots, a certain quantity is, no doubt, also absorbed from the 
atmosphere by the leaves, as is evident from succulent plants, which 
will continue to grow and acquire weight long after their roots are 
severed from the earth. This absorption, on the part of the leaves, 
chiefly takes place during the night, or in cloudy weather; while 
perspiration, on the other hand, goes on in the day-time in bright 
weather. 
et With regard to the chemical nature and changes of the sap, I can¬ 
not do better than give the statement of Link, with some necessary 
alterations. ‘ The food of plants must he composed of oxygen, hy¬ 
drogen, carbon, and azote. Water consisting of oxygen and hydrogen 
alone is not sufficient. Many experiments, indeed, have been insti¬ 
tuted to prove that pure water is a sufficient food, especially by Van 
Helmont, Eller, Bonnet, Du Hamel, and others; but it is probable, as 
Walerius has inferred, that the water out of which plants are formed 
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VOL. v. —NO. LX!V. 
