CIRCULATION AND FORMATION OF WOOD IN PLANTS. 585 



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 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 

 juicy plants with collapsible structures bleed so rapidly when cut, not 

 only from the cut surface of the rooted part, but from the cut sur- 

 face of the detached part the elastic 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 fur- 

 thers 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 struc- 

 tures throughout the rest of the plant thrust their liquid contents to- 

 wards the place of diminished pressure. This, indeed, may very pos- 

 sibly 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 con- 

 sumption, is probably very great. We must therefore regard the aid 

 which mechanical strains give as being one of several. Oscillations 



lary action and the process just 



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 cur- 

 rent of sap is produced. There remains the rationale of the descend- 

 ing 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, as gravitation acts equally on all 

 liquid columns contained in the stem, it is not easy to see why 

 it should produce downward movements in some while permitting 



