CIRCULATION AND FORMATION OF WOOD IN PLANTS. 587 



for sap will, other things being equal, cause currents towards the 

 most rapidly-growing parts— towards unfolding shoots and leaves, 

 but not towards adult leaves. Next we have evaporation, acting 

 more on the adult leaves than on those which are in the bud, 

 or but partially developed. This evaporation is both regularly 

 and irregularly intermittent. Depending chiefly on the action 

 of the sun, it is, in fine weather, greatly checked or wholly 

 arrested every evening; and in cloudy weather must be much 

 retarded during the day. Further, every hygrometric variation, 

 as well as every variation in the movement of the air, must 

 vary the evaporation. This chief action, therefore, which, by con- 

 tinually emptying the ends of the capillary tubes, makes upward 

 currents possible, is one which intermits every night, and every day 

 is strong or feeble as circumstances determine. Then, in the third 

 place, we have this rude pumping process above described, going 

 on with greater vigour when the wind is violent, and with less 

 vigour when it is gentle — drawing liquid towards different 

 parts according to their degrees of oscillation, and from diffe- 

 rent parts according as they can most readily furnish it. And 

 now let us ask what must result under changing conditions from 

 these variously-conflicting and conspiring forces. When a warm 

 sunshine, causing rapid evaporation, is emptying the vessels of the 

 leaves, the osmotic and capillary actions that refill them will be 

 continually aided by the pumping action of the swaying petioles, 

 twigs, and branches, provided their oscillations are moderate. Under 

 these conditions the current of sap, moving in the direction of least 

 resistance, will set towards the leaves. But what will happen when 

 the sun sets ? There is now nothing to determine currents either 

 upwards or downwards, except the relative rates of growth in the 

 parts and the relative demands set up by the oscillations ; and the 

 oscillations acting alone, will draw sap to the oscillating parts as 

 much from above as from below. If the resistance to be overcome 

 by a current setting back from the leaves is less than the resistance 

 to be overcome by a current setting up from the roots, then a 

 current will set back from the leaves. Now it is, I think, tolerably 

 manifest that in the swaying twigs and minor branches, less force 

 will be required to overcome the inertia of the short columns of 

 liquid between them and the leaves than to overcome the inertia of 

 the long columns between them and the roots. Hence during the 

 night, as also at other times when evaporation is not going on, the 

 sap will be drawn out of the leaves into the adjacent supporting 

 parts ; and their nutrition will be increased. If the wind is strong 

 enough to produce a swaying of the thicker branches, the back 

 current will extend to them also ; and a further strengthening will 

 result from their absorption of the elaborated sap. And when the 

 great branches and the stem are bent backwards and forwards by a 



