VEGETABLE PHYSIOLOGY. 
These facts show us that it is priacipally by means of mechanical strength, or contrivances for 
lessening the specific gravity of the entire structm-e, that plants resist the atti-action ch-awing them 
toward the siu'faee of the earth ; but when we come to consider the condition of the fluids, the juices 
of vegetables, we meet with certain other forces acting in opposition to the mere weight, and perform- 
ing a very important part in the distribution of the saps through the various parts of the plant. The 
two most remarkable of these are capillary attraction and endosmose. 
Capillary .attraction. — Everyone must have remarked, that when a piece of sugar or of sponge 
is laid upon a wet sm-faoe, the liquid rises up and wets these substances throughout ; and we fmd the same 
thing to occm- in aU porous bodies, when placed in contact with a hquid capable of wetting them. The 
phenomenon is most distinctly exhibited by dipping the end of a fme glass tube into water, when we see 
the liquid rise to a level inside the tube, higher than that of the sm-face in the vessel holding the water. 
Very thin tubes, like those used in such expeiiments, are termed capillary, or haii'-like, from capilla, a 
hair ; and hence this power which they possess of drawing up, as it were, the liquid within them, is 
called capUlary attraction. Examples of this attraction are fui'nished by a great mmiber of objects in 
daily use : thus, in a lamp, the oil or spu-it is di-awn up in the wick by the atU'actioa exerted in the little 
interspaces between the fibres of the cotton, which act as capillary tubes ; the interspaces of the porous 
sugar- and sponge, before alluded to, act in the same manner ; and porous earthy substances, such_ as 
coarse unglazed pottery, or pmnice stone, afford other examples. Vegetable structures, wliieh are so 
very porous and spongy, may naturally be expected to exhibit capillary attraction in a striking man- 
ner, and it does undoubtedly play a considerable part in the tx-ansmission of fluids through them. Its 
action in such structures may be well demonstrated by dii^puig one end of a piece of cane into tiu-pen- 
tine, and, shortly after, applying a Hght to the other end, the tm-pentine wiU rise in the capillary tubes 
which run thi-ough the cane, and take fire and bm-u at the top. We learn another fact, too, from this 
experiment, namely, that the rise of fluids in the interior of plants may be the result of a physical 
force, and not immediately of the vital power of the plant. Capillary attraction is most influential in 
the higher plants — in those which have stems built up of delicate woody tabes ; and there is no doubt 
that it is a most active agent in effecting the rise of the sap in ti-ees. It is not necessary for us here 
to enter into the philosophical theories by which this attraction is accounted for. They involve some 
rather abstract considerations, which would be out of place here, and it is sufficient for the subject we 
have in hand to know that it is an universal phenomenon. 
Endosmose. — The other force which has been spoken of is, probably, dependent fnndamentallj' upon 
somewhat analogous causes ; but here again I shall confine myself to a statement of the general and 
weU-ascertained facts. The capillaiy attraction we found to be exerted by bodies capable of bein(/ 
wetted by the liquid ; for, where this is not the case, as -with a glass tube when dipped into quicksilver, 
no attraction, but actually a repulsion and depression of the level is produced. This capability of 
being icetted is evidently dependant on a peculiar attraction, similar to that which causes certain 
hquids to mix intimately with each other, like spirit and water ; while others, such as water and oil, 
repel each other, and remain separate. 
When we place a thin plate of any porous substance between two Kquids which wiU readily mix 
with each other, but of which one will loet the porous substance through more readily than the other, 
we find that the former passes thi-ough more quicldy than the latter. For example, a piece of bladder 
is readily wetted through by water, but not so by sph'it ; if we tie a bladder over the end of a piece 
of tube, or the glass chimney of a lamp, pour spuit into this so as to fill it up to a certain mai-k, 
and then imm erse the lower end in water, the water wiU penetrate the bladder, which it readily wets, 
and then meeting with the spiiit, mixing vrith tliis, will cause the level inside the tube to rise till the 
mixtm-e of the spirit and water flows over the top. The same experiment may be made in substituting 
syrup, or a solution of gum, for the spu-it, with the same effect ; but in proportion as the fluid in the 
interior is capable of wetting the bladder, there will be an opposite cm-rent fi-om within, outwards. 
Thus, when there is a solution of gum in the interior, and water outside, the water, which readily wets 
the bladder, passes in freely and mixes readily with the gum, the gum passes out but slowly, and thus 
the level of the inside fluid rises. The inward cm-rent is called endosmose ; the outward, exosmose ; 
and the force or rapidity of the current of endosmose is generally proportionate to the difference of 
density m the two fluids : thus, in the case of a solution of gum, the thicker it is, the greater will be 
the force of the cm-rent of water into the cavity containing it. Endosmose is undoubtedly the chief 
agent in the motion of the nuti-ient fluids of plants. The fluids within the roots are more dense than 
the water in the moist earth sun-ounding them ; and the action of evaporation from the upper parts of 
plants is continually at work, dm-ing active Tegetation, to thicken the fluids -ndthin the tissues ; and 
thus a cm-rent of endosmose is maintained from cell to cell in those parts whore no open passages exist 
