THE FORMATION OF WOOD IN PLANTS. 419 
then, will cause an entrance of air into the tubes: the columns of liquid they contain 
will be interrupted by bubbles. It seems, indeed, not improbable that this entrance of 
air may take place even when there is a good supply of liquid, if the mechanical strains 
are so violent and the exudation so rapid that the currents cannot refill the half-emptied 
vessels with suflicient rapidity. And in this case the intruding air may possibly play 
the same part as that contained in the air-chamber of a force-pump—tending, by mode- 
rating the violence of the jets, and by equalizing the strains, to prevent rupture of the 
apparatus. Of course when the supply of liquid becomes adequate, and the strains not 
too violent, these bubbles will be expelled as readily as they entered. 
Here, as before, let me add the conclusive proof furnished by a direct experiment. To 
ascertain the amount of this propulsive action, I took from the same tree, a Laurel, 
two equal shoots, and, placing them in the same dye, subjected them to conditions 
that were alike in all respects save that of motion: while one remained at rest, the 
other was bent backwards and forwards, now by switching and now by straining with 
the fingers. After the lapse of an hour I found that the dye had ascended the oscillating 
shoot three times as far as it had ascended the stationary shoot—this result being an 
average from severaltrials. Similar trials brought out similar effects in other structures. 
The various petioles and herbaceous shoots experimented upon for the purpose of ascer- 
taining the amount of exudation produced by transverse strains, showed also the amount 
of longitudinal movement. It was observable that the height ascended by the dye was 
in all eases greater where there had been oscillation than where there had been rest—the 
difference, however, being much less marked in succulent structures than in woody ones. 
It need scarcely be said that this mechanieal action is not here assigned as the sole 
cause of circulation, but as a cause cooperating with others, and helping others to pro- 
duce effects that could not otherwise be produced. Trees growing in conservatories 
afford us abundant proof that sap is raised to considerable heights by other forces. 
Though it is notorious that trees so cireumstanced do not thrive unless, through open 
sashes, they are frequently subject to breezes sufficient to make their parts oscillate, yet 
there is evidently a circulation that goes on without mechanical aid. The causes of 
circulation are those actions only which disturb the liquid equilibrium in a plant, by 
permanently abstracting water or sap from some part of it; and of these the first is the 
absorption of materials for the formation of new tissue in growing parts; the second is 
the loss by evaporation, mainly through adult leaves; and the third is the loss by extra- 
Vasation, through compressed vessels. Only so far as it produees this last can mecha- 
nical strain be regarded as truly a cause of circulation. All the other actions concerned 
must be classed as aids to circulation—as facilitating that redistribution of liquid that 
continually restores the equilibrium continually disturbed ; and of these, capillary action 
May be named as the first, osmose as the second, and the propulsive effect of mechanical 
Strains as the third. The first two of these aids are doubtless capable by themselves 
of producing a large part of the observed result—more of the observed result than 
IS at first sight manifest; for there is an important indirect effect of osmotic action 
which appears to be overlooked. Osmose does not aid circulation only by setting up, 
Within the plant, exchange currents between the more dense and the less dense solutions 
