553 



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 refil the half-emptied vessels with sufficient 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 moderat- 

 ing 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 switch- 

 ing 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 several trials. Similar trials brought out 

 similar effects in other structures. The various petioles and herba- 

 ceous shoots experimented upon for the purpose of ascertaining 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 cases 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 mechanical action is not here 

 assigned as the sole cause of circulation, but as a cause co-operating 

 with others, and helping others to produce 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 circumstanced 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 for- 

 mation of new tissue in growing parts ; the seco.nd is the loss by 

 evaporation, mainly through adult leaves ; and the third is the loss by 

 extravasation, through compressed vessels. Only so far as it pro- 

 duces this last, can mechanical 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 con- 

 tinually restores the equilibrium continually disturbed ; and of these, 



