564 Comparative Physiology. 



upper part of the stem ceasing the moment it was cut by the 

 ground ; while the portion of the root in the ground still con- 

 tinued to pour forth sap, though successively cut after clearing 

 away the soil, till the terminal parts of the root were reached, 

 thus showing the seat of the constant absorption of the sap, 

 necessitating the ascent, to be there. The ascent by the 

 central vessels of the fibre was shown by a cut radicle placed in 

 liquid, with the conical j)oint immersed, being found, by the aid 

 of a lens, to have its cut surface covered with moisture, which 

 issued first from the central part. Agardh attributes the ascent 

 of the sap to a polarising action of the roots and leaves, by 

 virtue of which roots attract and leaves exhale, like the oppo- 

 site poles of a magnet." To give the propelling power, how- 

 ever, the name of a vis a tergo is no definition. If the polarity 

 produced by currents of electricity were generally admitted, or 

 had rested on jDroof by experiment, not on assumption, then we 

 would have had a force from behind capable of definition. The 

 endosmose power is at present received as the main agent in 

 absorption. Coupled with tliis, we have the opinions of De- 

 Candolle, Knight, and others, who supposed the propellent power 

 principally a vital action produced by the contractility of the 

 tissue. On these principles, the phenomenon of endosmose 

 should be greatly assisted by the exhalation from the leaves, as 

 it was found by the experiments of Hales to be ; and, though in 

 the above experiments of Dutrochet the vine was found to pour 

 forth sap for a time after being cut, it could only arise from the 

 peculiar force of the circulation in that plant (which is described 

 in the Library of Useful Knowledge as having been sometimes 

 found to support a column of mercury 38 in. high), and could 

 only continue so long as the descending sap had power to thicken 

 the ascending fluid sufficiently to produce endosmose. Had the 

 leaves been on the stem, the quantity absorbed, and, of course, 

 the propellent force, would have been found greater. Had the 

 lower end of the cut stem been immersed in fluid, it would 

 have continued for a time to issue at the upper end as before ; 

 and, had the spongioles been removed from the water of the 

 soil, the phenomenon of circulation would probably have ceased 

 altogether, showing the absence of any vis a tergo. 



In very soft herbaceous plants, the cut ends of stems and 

 under surfaces of leaves are found to carry on growth for a 

 considerable time, and some succulent plants, as i^cus elastica, 

 which do not require much water, are found to thrive nearly 

 as well with the roots out of the soil as in it, provided the 

 atmosphere is moist ; all these point out the general spread of the 

 function of absorption, produced wherever the tissue is soft and 

 circumstances render it necessary. For plants in general, how- 

 ever, wherever a special apparatus of roots is provided, their 



