318 



fluiii \vliu;h It ahsoiljfcl, as fitted it for the nourishment of the plant. It 

 was supposed also to have the power of propelling the digested fluid, 

 when impregnated with the principles of nutrition, growth, and develop- 

 ment, to the suiTimit of the leaf. From thence it was again returned 

 to the root ; where, mingling with the newly-digested fluid, it was again 

 propelled to the sunmiit, as before ; and in that way a regular circula- 

 tion was maintained. In this process, these propelling vessels were 

 said to be arteries, and the returning vessels were considered as veins. 

 Such is the theory of the circulation of the sap, held forth by the 

 earlier phytologists ; and as it was found to rest on a very slender basis, 

 they did not fail to prop and bolster it up with a multitude of ingenious 

 arguments. 



Of late years, the doctrine has been revived, as mentioned in the 

 text, and supported by some of the most distinguished modern phytolo- 

 gists ; but it has been improved by patient investigation, and accurate 

 experiment, and cleared of all ill-founded analogy to animal life. Hed- 

 wig declared himself to be of opinion, that plants possess a circulation 

 of the fluids, in some sort similar to that of animals. Costi united in 

 the same opinion, and is said to have found it exemplified in the stem of 

 the chara, and other plants. Professor Wildenow, in his Principles of 

 Botany, has also introduced the subject, and defended the doctrine, 

 (Sec Eng. Translation, p. 85.) He confidently asserts, that ho believes 

 a circulation to exist ; because it would be utterly impossible for the 

 leafless tree to resist the cold, if there were no circulation of the fluids. 

 This, as Mr. Keith observes, " is no argument, and therefore merits no 

 reply ;" — yet, we must admit, that it is a presumption^ of which the 

 force IS more easily evaded than invalidated. 



It is impossible, in the narrow compass of a Note, to give a detail of 

 Mr. Knight's ingenious and valuable experiments, to account for the 

 conversion of the alburnum into wood ; but the reader is referred for 

 them to the Philosophical Transactions for 1806 and 180G. By these 

 experiments he will .see, that it is rendered in the highest degree proba- 

 ble, if it be not altogether certain, that a circulation of the vegetable 

 fluids actually exists : for, if it once be admitted, that the descending 

 or proper juice forms not only a new epidermis where it is wanted, and 

 a new layer of liber and alburnum, but that it also partly enters into the 

 alburnum of the preceding year, where it mingles, and is again carried 

 up with the ascending sap, it cannot well be denied, that a circulation is 

 completed. That Mr. Keith is pretty nearly of this opinion himself, 

 may be gathered from the following concise sunmuuy of Mr. Knight's 

 hyi)othesis, by that acute and ingenious censor. 



