420 MR. SPENCER ON CIRCULATION AND 
in different parts of it; but it aids circulation much more by producing distention of the 
plant as a whole. In consequence of the average contrast in density between the water 
outside of the plant and the sap inside of it, the constant tendency is for the plant to 
absorb a quantity in excess of its capacity, and so to produce distention and erection of 
its tissues. It is because of this that the drooping plant raises itself when watered; for 
capillary action alone could only refill its tissues without changing their attitudes. And 
it is because of this that juiey plants with collapsible structures bleed so rapidly when 
eut, not only from the cut surface of the rooted part, but from the cut surface of the 
detached part—the elastie tissues tending to press out the liquid which distends them, 
And manifestly if osmose serves thus to maintain a state of distention throughout a 
plant, it indirectly furthers circulation, since immediately evaporation or growth at 
any part, by abstracting liquid from the neighbouring tissues, begins to diminish the 
liquid pressure within such tissues, the distended structures throughout the rest of the 
plant thrust their liquid contents towards the place of diminished pressure. This, indeed, 
may very possibly be the most efficient of the agencies at work. Remembering how 
great is the distention producible by osmotic absorption—great enough to burst a bladder 
—it is clear that the force with which the distended tissues of a plant urge forward the 
sap to places of consumption is probably very great. "We must therefore regard the aid 
which mechanical strains give as being one of several. Oscillations help directly to restore 
any disturbed liquid equilibrium ; and they also help indirectly, by facilitating the re 
distribution caused by capillary action and the process just described; but in the absence 
of oscillations the equilibrium may still be restored, though less rapidly and within 
narrower limits of distance. 
One half of the problem of the circulation, however, has been left out of sight. Thus 
far our inquiry has been, how the ascending current of sap is produced. There remains 
the rationale of the descending current. What forces cause it, and through what tissues 
it takes place, are questions to which no satisfactory answers have been given. That 
the descent is due to gravitation, as some allege, is difficult to conceive, since, # 
gravitation acts equally on all liquid columns contained in the stem, it is not easy to $è 
why it should produce downward movements in some while permitting upward — 
ments in others—unless, indeed, there existed descending tubes too wide to admit . 
. much capillary action, which there do not. Moreover gravitation is clearly inadequate 
to cause currents towards the roots out of branches that droop to the ground. Here the 
gravitation of its contained liquid columns must nearly balance that of the connected 
columns in the stem, leaving no appreciable force to cause motion. Nor does w 
seem much probability in the assumption that the route of the descending sap 18 through 
the cambium layer, since experiments on the absorption of dyes prove tht ot 
cellular tissue is a very bad conductor of liquids: their movement through it den 
take place with one-fiftieth of the rapidity with which it takes place through vessels”: 
ES cml no this occur in plants that have retrograded in the character of their tissues towards Be Aay 
abat RIR Fame ponam beret, such as those of Sempervivum, in which the cellular me s rd 
Bis peel nd Ae the oe Same, seem to have resumed to a considerable extent wha e lost much 
of vegetal cireulation—simple absorption from cell to cell. These, when they ut 
