342 Mr. Knight’s Experiments on 
the leaf, with minute ramifications of the tubes of the bark. 
The substance of the core is similar to that of the silver grain of 
the wood, of which it may possibly be a continuation. 
The force with which the sap has been proved to ascend, by 
Hales, banishes every idea of mere capillary attraction. The 
action of the spiral tubes appears much more adequate to the 
effects produced ; and I readily admit the supposed action of 
these, wherever they are found ; but I have so often attentively 
searched in vain for them, with glasses of different powers, in 
the root, in the alburnum, and in the bark, that I cannot but 
question their existence in those parts. Attached to the central 
vessels, in the annual shoot, in the fruit-stalk of different trees, 
in the tendril *of the vine, in the leaf, and in the seed, the spiral 
tubes certainly exist, and are in most cases visible, without the 
aid of a lens. But, as I have not been able to discover them in 
other parts of the tree, and as the different authors I have looked 
into have not distinguished those I call the central vessels trom 
the common tubes of the alburnum, nor marked the difference 
in the organization of the annual branch, and annual root, I 
must venture to call their accuracy here in question, though 
with great deference for their opinions. 
Linnzsus and others have attempted to account for the as- 
cent of the sap, by the expansion of the fluids within the vessels 
of the plant, by the agency of heat. But the sap rises under a 
decreasing, as well as under an increasing temperature, during 
the evening and night, (if it be not excessively cold) as well as 
in the morning and at noon; and it is sufficiently evident, that 
the heat applied to the branches of a vine within the stove, can- 
not expand the fluids in the stems and roots, which grow on the 
outside. It is also well known, that the degree of heat required 
