588 APPENDIX C. 



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 to 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 descended from the leaves, 

 although there are none of those oscillations by which the sap is said 

 to be drawn downwards as well as upwards. The reply is, that the 

 stem and roots tend to repeat their typical structures, and that the 

 absorption of sap for the formation of their 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 keep up, as well as the distention caused by osmose, do 

 not in themselves produce a current either upwards or downwards : 

 they simply help to move the sap towards that place where there is 

 the most rapid abstraction of it — the place towards which its motion 

 is least resisted. Whether there is oscillation or whether there is 

 not, the physiological demands of the different parts of the plant 

 determine the direction of the current ; and all which the oscillations 

 and the distention do is to facilitate the supply of these demands. 

 Just as much, therefore, in a plant at rest as in a plant in motion, 

 the current will set downwards when the function of the leaves is 

 arrested, and when there is nothing to resist that abstraction of sap 

 caused by the tendency of the stem- and root-tissues to assume their 

 typical structures. To which admission, however, it must be added 

 that since this typical structure assumed, though imperfectly as- 

 sumed, by the hot-house plant, is itself interpretable as the in- 

 herited effect of external mechanical actions on its ancestors, we 

 may still consider the current set up by the assumption of the 

 typical structure to be indirectly due to such actions. 



Interesting evidence of another order here demands notice. In 

 the course of experiments on the absorption of dyes by leaves, it 

 happened that in making sections parallel to the plane of a leaf, with 



