OF NATURAL HISTORY, 21 



of beings have veffels deftined to the performance of fimilar offices. 

 In man and quadrupeds, the fluids are circulated by the puliation 

 of the heart and arteries. The juices of plants do not circulate ; 

 but they are raifed from the root to the trunk, branches, leaves, 

 flowers, and fruit, by the fap-vefl"els. The afcenfion of the fap has 

 been afcribed to capillary attradion. But, though no motion is 

 perceptible in the fap-veflels fimilar to the pulfation of arteries ; 

 yet, both the propulfion of the fap, which moves with great force, 

 and the fecretion of different fluids by different parts of the fame 

 plant, imply an adion in thefe vefl"els. In animals, the gall, the 

 urine, the faliva, are all conceded from the general mafs of blood 

 by the adion of particular vefl"els. Fluids of thefe different quali- 

 ties exift not in the blood itfelf : They are created by an incompre- 

 henfible operation of the veflfels peculiar to their refpedive glands. 

 In plants, the fap afcends, and different fluids are fecreted from it 

 by glandular veflels. Here the fame effeds are produced both in 

 the animal and the plant. We muft, therefore, attribute them to 

 the fame caufe, namely, the adion of veflfels. Befides, the fap, 

 which is the blood of plants, moves with a force often equivalent 

 to the weight of the atmofphere. M. Bonnet remarks *, that he 

 has feen, by means of coloured liquors, the vegetable fap move 

 three inches in an hour ; and Dr Hales, in his Statics, has fliown. 

 that the leaves are the principal organs of tranfpiration. He like- 

 wife cnnfiders them to be the infliruments which raife the fap. But 

 it has fince been difcovered, that coloured liquors rife equally high 

 in branches deprived of leaves, and that they do not rife at all in 

 dried plants. Hence the fap of vegetables is not taken up in the 

 fame manner as a fpunge imbibes water, but is forced to afcend by 

 an unknown adion of the veflTels. The fpring of the tracheae may 

 put in motion the air they contain, and that air may have fome in- 

 fluence on the general movement. But, by whatever powers the 



fap 

 * Oeuvres, torn. i. p. 140. 



