90 ACCOUNT OF SOME EXPERIMENTS ON THE 



blossom has fallen, the pulp will appear to be of two kinds : one of which 

 is included within the vessels which carry up coloured infusions ; and this 

 seems to be formed by continuation of the vessels and fibres within the 

 wood. The other part appears to belong, in a great measure, to the 

 bark : it is in very small quantity in the very young fruit ; but, at its 

 maturity, it constitutes much the greater part of the pulp. The vessels, 

 however, which diverge into the external pulp, and probably convey 

 nourishment to it, appear to be continuations of the central vessels, every 

 where, I believe, accompanied, as in 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 cer- 

 tainly 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 from 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. 



Linnaeus and others have attempted to account for the ascent 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 suffi- 

 ciently evident, that the heat applied to the branches of a vine within the 

 stove, cannot expand the fluids in the stems and roots, which grow on the 

 outside. It is also well known, that the degree of heat required to put 

 the sap into motion, in this plant, is not definite, but depends on that to 

 which the plant has been previously accustomed. Thus a vine, which has 

 grown all the summer in the heat of a stove, will not be made to vegetate 

 during the winter by the heat of that stove ; but, if another plant of the 

 same variety, which has grown in the open air, be at any time introduced, 



