ON VEGETATION. 729 



register of the age of the tree, are very easily observed when it is cut across; 

 sometimes as many as 400 have been found in firs, and oaks are said to have 

 lived 1000 years. 



Mr. Knight has inferred, from a great variety of experiments, that the sap, 

 either usually or universally, ascends through the wood into the leaves, and then 

 descends through the bark to nourish the plant. The leaves seem to be some- 

 what analogous to lungs, or rather to the gills of fishes: for plants have need 

 of air,and it has been found, that even seeds will not germinate in a vacuum. 

 As the lungs of animals appear to be concerned in forming the blood, so it 

 may be inferred from Mr, Knight's experiments, that the sap first ascends 

 to the leaves through the external i'resh wood or alburnum, and through the 

 central vessels of the young leaves and branches, derived from the alburnum, 

 and accompanied by the spiral tubes; and after being perfected by exposure 

 to light and air in the leaves, it descends in the bark, and serves for the secrcj- 

 tion of the alburnum, and of the internal la\ers of the bark, being conveyed 

 probably by two distinct sets of vessels. The sap, thus prepared by the leaves 

 in the summer and autumn, is supposed to leave its extractive matter in the 

 tree throughout the winter, in such a state as to be ready to unite with the 

 aqueous juices, which ascend from the root, in the succeeding spring. The in- 

 ternal parts of the wood, having served the purposes of vegetation, are hard- 

 ened, and perhaps dried up, so as to be afterwards principally subservient to 

 strength alone. By subsequent experiments, Mr. Knight has also found, that 

 when a branch hangs downwards, the sap still appears to proceed from the 

 part of the bark which is uppermost; so that the direction of the force of gra- 

 vity seems to be concerned in determining that of the motion of the sap. 

 There appears also to be some reason to suppose that mechanical means assist 

 in the protrusion of the sap, and the consequent growth of the tree; for if a 

 tree be more agitated by the wind in one direction than in another, its dia- 

 meter will be greatest in that direction. 



The process of grafting depends on a remarkable property of 'the growth 

 of vegetables; if the cut surface of the inner bark of a small branch, or 

 cutting, be placed in contact with that of the branch of another tree, they 

 will unite sufficiently for the nourishment of the cutting; provided, however, 

 that the nature of the plants be not too diiTerent. Something of the same 



