404 



NOTES AND ILLUSTEATIONS. 



on the fluid which it absorbed 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 development, to the summit 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 summit as before ; and in 

 that way a regular circulation 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 phytologists ; 

 but it has been improved by patient investigation and accurate experi- 

 ment, and cleared of all ill-founded analogy to animal life. Hedwig 

 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 "Willdenow, in his principles of 

 botany, has also introduced the subject, and defended the doctrine. (See 

 English Translation, p. 85.) He confidently asserts that he 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 1805 and 1806. By these 

 experiments he will see that it is rendered in the highest degree prohahh, 

 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 prettt/ nearly of this opinion himself, 

 may be gathered from the follov/ing concise summary of Mr Knight's 

 hypothesis, by that acute and ingenious censor : — 



" Although the doctrine of a circulation," says he, " as maintained by 



