422 MR. SPENCER ON CIRCULATION AND 
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 oscilla. 
tions acting alone, will draw sap to the oscillating parts as much from above as from below, 
If the resistance to be overcome by a eurrent 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 thieker branches, the back current wiil extend to 
them also; and a further strengthening will result from their absorption of the elabo- 
rated sap. And when the great branches and the stem are bent backwards and forwards 
by a gale, they too will share in the nutrition. It may at first sight seem that these 
parts, being nearer to the roots than to the leaves, will draw their supplies from the 
roots only. But the quantity which the roots can furnish is insufficient to meet so great 
a demand. Under the conditions described, the exudation of sap from the vessels will 
be very great, and the draught of liquid required to refill them, not satisfied by that 
which the root-fibres can take in, will extend to the leaves. Thus sap will flow to the 
several parts according to their respective degrees of activity—to the leaves while light 
and heat enable them to discharge their functions, and back to the twigs, branches, 
stem, and roots when these become active and the leaves inactive, or when their activity 
dominates over that of the leaves. And this distribution of nutriment, varying with the 
varying activities of the parts, is just such a distribution as we know must be required 10 
keep up the organic balance. 
To this explanation it may be objected that it does not account for the downward 
current of sap in plants that are sheltered. The stem and roots of a drawing-room 
Geranium display a thickening which implies that nutritive matters have descen 
from the leaves, although there are none of these oscillations by which the sap is said t0 
be drawn downwards as well as upwards. The reply is, that the stem and roots tend t0 
repeat their typieal struetures and that the absorption of sap for the formation of thet 
respective dense tissues, is here the force which determines the descent. Indeed it must 
be borne in mind that the mechanical strains and the pumping process which they ke? 
up, as well as the distention caused by osmose, do not in themselves produce a ch 
. either upwards or downwards : they simply help to move the sap towards that pes 
where there is the most rapid abstraction of it—the place towards which its motion 1 
least resisted. Whether there is oscillation or whether there is not, the physiol i 
demands of the different parts of the plant determine the direction of the current; 
all which the oscillations and the distention do is to facilitate the supply o na 
demands, _ Just as much, therefore, in a plant at rest as in a plant in motion, © 
rent will set downwards when the function of the leaves is arrested, and when 
= EE to resist that abstraction of sap caused by the tendency of the stem- and 
