OF THE SAP-VESSELS. 35 



liar to every one; and a little reflection will satisfy us 

 that such substances must each be lodged in proper 

 cells and vessels to be kept distinct from each other. 

 They are extracted, or secreted, from the common juice 

 of the plant, and called its peculiar or secreted fluids. 

 Various experiments and observations, to be hereafter 

 enlarged upon, prove also that air exists in the vege- 

 table body, and must likewise be contained in appro- 

 priate vessels. Besides these, we know that plants are 

 nourished and invigorated by water, which they readily 

 absorb, and which is quickly conveyed through their 

 stalks and leaves, no doubt by tubes or vessels on pur- 

 pose. Finally, it is observable that all plants, as far as 

 any experimenthas been made, contain acommon fluid, 

 which at certain seasons of the year is to be obtained 

 in great quantity, as from vine branches by wounding 

 them in the spring before the leaves appear, and this 

 is properly called the sap. It is really the blood of the 

 plant, by which its whole body is nourished, and from 

 which the peculiar secretions are made. 



The.great difficulty has been to ascertain the vessels 

 in which the sap runs. Two of the most distinguished 

 inquirers into the subject, Malpighi and Grew, be- 

 lieved the woody fibres, which make so large a part 

 of the vegetable body, and give it consistence and 

 strength, to be the sap-vessels, analogous to the blood- 

 vessels of animals, and their opinion was adopted by 

 Du Hamel. In support of this theory it was justly ob- 

 served that these fibres are very numerous and strong, 



D '2 



