ON VEGETABLE PHYSIOLOGY. 21*7 



powerful, exosmose (flow outwards). The cause of their motion was 

 by Dutrocet referred to galvanism ; but it is now more generally be- 

 lieved to arise from " the attraction exerted between the particles of 

 the different fluids employed, as they meet in the porous membrane." 

 (Dr. Reid.) 



" Now the conditions requisite for this action, are two fluids of 

 different densities, separated by a septem or partition of a porous cha- 

 racter. This we find in the roots. The fluid in their interior is 

 rendered denser than the water around by an admixture of the de- 

 scending sap ; and the spongeole (or spongelet) supplies the place of 

 a partition. Thus then, as long as this difference of density is main- 

 tained, the absorption of fluid may continue. But if the rise of the 

 sap is due to the action of endosmose, there ought also to be an exos- 

 mose. This is found to take place ; for if a plant is grown with its 

 roots in water, the fluid surrounding them is soon found to contain 

 some of the peculiar substances they form, and which are contained in 

 the descending sap : thus a pea or bean would disengage a gummy 

 matter; a poppy would communicate to the water an opiate impreg- 

 nation, and a spurge would give it an acrid taste. 



" Thus we see how beautifully and how simply this action, extra- 

 ordinary as it seems, is accounted for, when its whole history is 

 known, on principles which operate in other departments of nature." 

 (Dr. Carpenter.) 



From this it must appear obvious to every one, that to keep plants 

 in a healthy state, the conditions of endosmose and exosmose must 

 be carefully maintained. Thus in the case of bulbs, maturing and at 

 rest, and of plants cut down in the autumn, such as Pelargoniums 

 and Fuchsias, the actions of the leaves being destroyed, the fluid, 

 rising by the force of endosmose, must gradually subside, and the 

 plants languish into a state of semi-vitality, till such time as genial 

 warmth shall expand the fluid within their latent buds, and cause them 

 to open and put forth new leaves. This is the reason why the appli- 

 cation of water to plants thus circumstanced should be carefully 

 avoided, excepting indeed a few special subjects, whose succulency is 

 not sufficient to keep them from being shrivelled up. 



