FLUID PARTS OF VEGETABLES. 
105 
a stream. Tt has been thought that the circulation of sap was wholly susi)en Jea 
during winter ; this, however, seems not to be the case ; for we may ooserve 
during this season a gradual development of some parts of the plant ; we see 
many plants preserving the freshness and verdure of their foliage, and mosses put- 
ting forth their flowers. We must, then, believe that the sap is in perpetual mo- 
tion, susceptible of being accelerated or retarded by changes of temperature, and 
humidity or dryness of the earth. The development of buds must be attributed to 
the ascension and redundancy of the sap, which dilates and nourishes their parts. 
In spring, when the ascent of the sap is accelerated, the buds enlarge rapidly, and 
their complete development is soon perfected. 
120. The theory of the movements which take place between 
fluids of diiferent densities, called Endosmosis and Exosmosis^ 
01 imlihition and permeahility^ explains many of the phenom- 
ena in vegetable physiology. It is found that if two fluids oi 
different density be separated by a membrane without visible 
pores, the lighter fluid is imbibed by the membrane ; the denser 
fluid also passes outward ; thus the two fluids undergo the same 
process of transudation : tlie name of Endosinose^ to pass wirh- 
in, and Exosmose^ to pass without, has been given to these 
changes. This process may account for the fact, that about the 
roots of plants their peculiar qualities may be detected in the 
soil; and the same is observed of water in which a plant has 
been made to vegetate. The cell containing a fluid thicker than 
water, imhihes water by endosmosis^ and yields by exosmosis a 
portion of its liquid contents to a contiguous cell containing 
a still denser fluid. When the leaf-buds become stimulated 
by an increase of sap, the Endosmotio process commences in 
their cells, and gradually extends to the roots, which are ab- 
sorbing new supplies of fluid from the soil. The juices which 
had been accumulated during vv^inter pass through changes, cer- 
tain substances are dissolved, and the sap becomes thickened ; 
thus the endosmotio process is greatly increased, and an active 
circulation goes on through the whole system of the plant. 
Toward the latter part of the season light and heat are less 
powerful, the leaves perform their office more languidly, and 
there is more equilibrium in the density of fluids, until circula- 
tion ceases. 
121. The vascular texture appears by its tubes and channels to afford great fa- 
cilities for the ascension of the sap. In imperfect plants, such as mushrooms and 
lichens, which are wholly composed of cellular texture, it is not known that there 
is any ascent of sap, but they seem to be nourished by fluids absorbed from the air 
The question naturally arises. By what force is the sap made to ascend, contrary to 
the laws of gravitation ? Some have asserted, that this phenomenon was owing to 
the contraction and dilatation of the air, and of the juices of the plant ; others have 
referred it to the action of heat ; these two propositions, however, amount to the 
same thing, since heat is the cause of the contraction and dilatation referred to. 
Some ascribe the ascent of the sap to the irritability of the vessels, and the energy 
f>i vital power. The latter is but a vague and unsatisfactory explanation, since w« 
Perpetual motion of sap — Cause of development of buds. — 120. Endosmosis and Exosmosis.— 
"21. Yascular texture unlike the cellular in affording facilities for the aicensiou of sap— Explaaatiiin* 
of the causes of the ascent of the sap. 
