ASCENT OF THE SAP IN TREES. 93 



If the motion I have supposed the silver grain to possess, in the living 

 tree, be more than you think can be properly admitted to belong to 

 vegetable life, I will request your attention to the power of moving in the 

 vine leaf, on which I have made many experiments. It is well known 

 that this organ always places itself so that the light falls on its upper 

 surface ; and that, if moved from that position, it will immediately 

 endeavour to regain it : but the extent of the efforts it will make, I 

 have not anywhere seen noticed. I have very frequently placed the leaf 

 of a vine in such a position, that the sun has shone strongly on its under 

 surface ; and I have afterwards put obstacles in its way, on whichever 

 side it attempted to escape. In this position the leaf has tried almost 

 every method possible to turn its proper surface to the light ; and I have 

 several times seen one which, having tried during several days to 

 approach the light in one direction, and having nearly covered its under 

 surface, by bending its angular points almost to touch each other, has 

 unfolded itself again, and receded farther from the glass, to approach 

 the light in an opposite direction. As the whole effect here produced 

 appears to arise merely from the light falling on the under surface of the 

 leaf, I cannot conceive how the contortions of its stalk, in every direction, 

 can be accounted for, without admitting, not only that the plant possesses 

 an intrinsic power of moving, but that it also possesses some vehicle of 

 irritation ; and, without this, it will I think be difficult to explain how 

 the heat applied to the branch of the vine, within the stove, can put the 

 sap in the roots and external stem into motion. It may be objected, 

 that these are always ready when the branch calls for nourishment, 

 and that they are no way affected by the internal heat. But this I 

 cannot admit to be the case ; because I have found that the stem 

 suddenly becomes extremely susceptible of injury from cold, as soon as 

 the branch begins to vegetate ; and that its whole powers will be 

 paralysed for some days, by exposure for a few hours to a freezing 

 temperature. 



I have had very frequent opportunities of observing a remarkable 

 power in trees, of transferring their sap from one tube to another ; for I 

 have often intersected, in the trunk, every tube which led to a lateral 

 branch, and still this branch has derived a considerable portion of 

 nourishment from the trunk. And if the tubes of an annual shoot of 

 the oak be traced downwards in the autumn, they will be found to pass 

 along the layer of wood of the preceding summer, without any apparent 

 communication between them and the tubes of any former year's growth. 

 Yet the sap rises through the whole of the white wood ; and it must be 



