> 
56 Vegetable Organography and Physiolegy.. 
can only glance briefly at the course of the sap, and weit the 
changes which take place in this fluid, in its circulation through 
the vegetable system. 
De Candolle discovered that the roots of exogenous plants were 
terminated by small organic bodies, composed of cellular tissue and 
woody fibre, enclosing in their centre a minute bundle of ducts. 
These bodies, which are termed spongioles, possess the power of 
absorbing fluids from the earth with wonderful rapidity. This 
power has been denominated endosmose, by Dutrochet, who dis- 
covered, by some ingenious experiments, that the accumulation of 
a fluid in organic cavities, like the cells of plants, imparted to 
those bodies a vital principle by which their cavities were alter- 
nately emptied and replenished. It is a law of this “ physico-or- 
ganic action” that the denser fluid always attracts the rarer ; and 
as the cells of plants are always filled with a matter of a denser 
consistence than water, this fluid, which always surrounds the 
roots of trees, is drawn up by the spongioles with great rapidity. 
This action possesses the property of rendering turgid the con- 
tents of these cavities, when once imbibed, so that a constant en- 
dosmose is kept up. Professor Daubeny, by some curious expe- 
riments, demonstrated that the roots of plants possess the power 
of selecting such materials as are required for their nourishment 
_and growth, and of rejecting matter whose introduction into theit 
tissues would prove deleterious to the plant. - In almost all plants, 
however, the ascending sap is found to be nearly uniform in its — 
composition ; consisting of water, holding in solution various 
mineral substances, with atmospheric air. Having once entered 
the spongioles, the sap is conveyed along in its vertical current 
rom one vesicle to another by this endosmometric property of the 
tissues of the plant, until it arrives at the leaves, where by its 
exposure to the atmospheric air it undergoes a chemical change; 
which prepares it to afford vitality and nourishment to the plant. 
This process is considered by physiologists as strictly analagous 
to the respiration of animals. 
The phenomena connected with the respiration of a 
_ the changes produced upon atmospheric air by vegetables, ate 
_ highly curious and interesting. It was ascertained many years 
0, by Bonnet, Priestley, and others, that healthy plants exposed 
to min light, were constantly evolving oxygen gas. But the 
: 
