74 
Cause of vital Action in Animals and Plants. 
The proper juice, or sap, after it lias been changed by the leaves into a nutritious 
fluid, according to M. Dutrochet, descends both by the bark and the allmrnum or 
soft wood, through tubular ohlong cells. These oblong cells give out the nutritive 
juice contained in them, through their sides ; and in spring, when the sap ascends, 
it takes up a portion of this juice for the developement of the leaves and the 
growth of the plant. The pith has neither the power of conducting the ascending, 
nor the descending sap. 51 . Dutrochet agrees with Linnteus, Dr. Darwin, and others, 
considering it to be, to the vegetable, what the brain and spinal marrow are to the 
animal. Dr. A. T. Thomson ( Lectures on Botano. vol. i. p. 379.) conjectures 
that the pith is intended chiefly to give bulk and stability to the young shoot ; be- 
cause, whenever this becomes ligneous and able to support itself, the pith dries up 
and diminishes in volume rather than increases. 
Besides the vessels for the ascent of the sap, and those for the descent of the pro- 
per juice, there are a third description of vessels, which radiate from the centre of 
stems to their circumference, and are commonly called medullary rays. These 
rays are composed of oblong tubes or cells, extending from the centre of the wood 
to the bark, where they are met by similar tubes, in apparent but not real conti- 
nuity in the bark. The radiated trachese of the wood give out the ascending sap 
from the lymphatic tubes, and the radiating vessels of the bark give out the pre- 
pared sap, or proper juice, from the descending trachea* , or oblong cells, or the 
bark. The juice and the sap, thus poured out between the wood and the bark, 
unite in forming a nutritive fluid which, consolidating, adds to the bulk of the 
plant, in the form of alburnum or soft wood, and liber or inner bark. This, it 
must be acknowledged; is a very simple and beautiful theory, and altogether con- 
sistent with matter of fact. 
The next point which M. Dutrocliet proceeds to determine is, the cause of the 
progression of the sap in the lymphatic tubes, oblong cells, and radiating tracheae. 
It is unnecessary to trace the proofs, that there is no actual circulation of the sap 
in plants but merely an ascending and descending current, and a lateral diffusion 
and union. The condition of a plant requisite to admit of the exercise of these 
functions is, the susceptibility of becoming turgid by the ‘application of water; in 
other words, that which distinguishes a dead plant from a living one is, the tur- 
gidity of its cellular parts. A dead plant may have its tubes, cells, and trachea 
filled with fluid, but these will never become turgid : a living plant, on the con- 
trary, which has been apparently dead, when one extremity is placed in fluid, 
becomes filled with it throughout to an excess productive of turgidity. In a sepa- 
rated part of a plant, the ascension of the sap depends on the susceptibility of tur- 
gidity of all the parts of the section. In a plant growing in the soil, the cause of 
turgidity, or of the rise of the sap, is to be found in the minute conical bodies 
which terminate each radicle. M. Dutrochet, by careful examination with a 
microscope, found that the minute conical termination of the radicle was furnished 
with other projecting bodies, like sponges, which perform the office of the piston 
of a syringe, and have the power of introducing into their cavity, and through their 
sides, the water which comes in contact with their exterior surface, and which, at 
the same time, opposes the exit of any fluid these spongioles imbibe To this 
power 51. Dutrochet has applied the term endosmose (endou, inward, osmos , im- 
pulse) ; and he has proved its existence, on a larger scale, in the coecum or blind 
gut of a young chicken, which lie filled half full of milk, firmly closed at the 
open extremity, and then immersed in water. At the end of twenty-four hours 
the coecum had imbibed seventy. three grains, and at the end of thirty. six hours, 
one hundred and seventeen grains of water, and become very turgid. From this 
time its weight diminished ; and, at the end of thirtv-six hours, it had lost fifty- 
four grains of the water which it had absorbed, and 'the milk had become putrid. 
This experiment 51. Dutrochet considers as demonstrating, that the absorption of 
the water depends on the fluid in the cavity being denser than that which surrounds 
the organ ; and that, as long as this dense fluid remains undecomposed, the endos- 
mose, or absorption continues ; while, as soon as it becomes putrid, the endosmose 
ceases, and the water passes out of the organic cavity as rapidlv as it had entered it. 
Farther experiment proved to 51. Dutrochet, that when the ccecum was filled with 
a thinner fluid than that in which it was immersed, this thin fluid nassed o„t nf it 
into the other. This action he cail exosmose (e*, out, osmos. impulse). He 
farther proved that fluids ol a less density than water, when the solution contain- 
ed m the coecum is alkaline, produce endosmose ; and, when it is acid, exosmose- 
