ASCENT OF THE SAP IX TREES. 



97 



and (as far as I have had opportunities to examine) of no essential 

 difference; whilst that of different fruits is extremely various. The 

 external vascular parts of the apple and pear, abstracted from those 

 which seem to carry nourishment to the seeds, appear to me to resemble, 

 in some respects, those of the leaf; and, relative to the offspring, I 

 suspect that they perform a somewhat similar office. 



III. ACCOUNT OF SOME EXPERIMENTS ON THE DESCENT OF THE SAP 



IN TREES, 



[Read before the ROYAL SOCIETY, April 21, 1803.] 



IN a memoir which I had the honour to present two years ago*, I 

 related some experiments on trees, from which I inferred, that their sap, 

 having been absorbed by the bark of the root, is carried up by the albur- 

 num, or \vhite wood of the root, the trunk, and the branches ; that it 

 passes through what are there called central vessels, into the succulent 

 part of the annual shoot, the leaf-stalk, and the leaf ; and that it returns 

 to the bark, through the returning vessels of the leaf-stalk. The prin- 

 cipal object of this paper is to point out the causes of the descent of the 

 sap through the bark, and the consequent formation of wood. 



These causes appear to be gravitation, motion communicated by 

 winds or other agents, capillary attraction, and probably something in 

 the conformation of the vessels themselves, which renders them better 

 calculated to carry fluids in one direction than in another. I shall 

 begin with a few observations on the leaf, from which all the descend- 

 ing fluids in the tree appear to be derived. This organ has much 

 engaged the attention of naturalists, particularly of M. Bonnet: but 

 their experiments have chiefly been made on leaves severed from the 

 tree ; and, therefore, whatever conclusions have been drawn stand on 

 very questionable ground. The efforts which plants always make to 

 turn the upper surfaces of their leaves to the light, have with reason 

 induced naturalists to conclude, that each surface has a totally distinct 

 office ; and the following experiments tend strongly to support that 

 conclusion. 



I placed a small piece of plate glass under a large vine leaf, with 

 its surface nearly parallel with that of the leaf ; and, as soon as the glass 

 had acquired the temperature of the house in which the vine grew, 



* See the preceding paper. 



