86 



ON ORGANIZED BODIES, 



clearly the same kind of obstacle in the course of the arteries in this 

 direction, which was proved to exist in the veins in the opposite direction ; 

 and which reverse obstacles we can scarcely ascribe to any other cause 

 than the existence of valves,* 



By this double set of vessels, moreover, possessed of an opposite power, 

 and acting in an opposite direction, the one to convey the sap or vegetable 

 blood forwards, and the other to bring it backwards, we are able very suf- 

 ficiently to establish the phaenomenon of a circulatory system ; and, ac- 

 cording to several of the experiments of M, Wildenow, it seems probable 

 that this circulatory system is maintained by the projectile force of a regu- 

 lar and alternate contraction and dilatation of the vegetable vessels. Yet 

 the great minuteness of these vessels must ever render it extremely diffi- 

 cult to obtain any thing hke absolute certainty upon this subject. Even 

 in the most perfectly established, circulatory systems of animals, in man 

 himself, it is not once in five hundred instances that we are able to acquire 

 any manifest proof of such a fact : we are positive of the existence of an 

 alternating systole and diastole in the heart, from the pulsation given to 

 the larger arteries when pressed upon ; but no degree of pressure produces 

 any such pulsation in the minuter arteries, at least, in a healthy state ; yet 

 we have full reason to believe that the same action of the heart extends to 

 the minutest as to the largest arteries. How much less, then, ought we 

 to expect any full demonstration of this point in the vessels of vegetables, 

 in every instance so much more minute than those of the more perfect ani- 

 mals, and seldom exceeding, as 1 have already observed, a three thousandth 

 part of an inch in diameter ! 



It becomes me, however, to confess, that no experiments which have 

 hitherto been made have detected the existence of either motific or sensific 

 fibres themselves in vegetables, although very high degrees of galvanicr 

 electricity have for this purpose been applied to the most irritable of them, 

 as the dionaa muscipula^ or Venus fly-trap ; oxalis sensitiva ; different 

 species of drosera, or sun-dew ; acacias of various kinds, and other mimo- 

 sas ; and especially the mimosa pudica, and sensitiva, the common sensi- 

 tive plants of our green-houses. Humboldt has uniformly failed ; Rafn 

 appears to have succeeded in one or two instances, but his general want of 

 success prevents us from being able to lay any weight on the single case 

 or two in which he seems to have been more fortunate. 



It should be observed, that the matter of fibrine, or the principle of the 

 muscular fibre, formerly supposed to exist exclusively in animal sub- 

 stances, has lately been detected by M. Vauquelin in vegetables also. Dr. 

 Hales cut off the stems of vines in the spring, and by fixing tubes on the 

 stumps, found that the sap rose in many instances to the height of thirty- 

 five feet. Tubes have been fixed to the large arteries of animals, as near 

 as possible to the heart, in which the blood did not rise higher than nine 

 feet. 



It has long been admitted by botanists in general, that the thorns of 

 plants are abortive branches ; the scales of buds have, in like manner, been 

 regarded as transformed leaves ; and it has lately been conjectured by M. 

 de Candolle, that their petals are not special organs, but stamens in an 

 abortive or transformed state, t 



* Yet Hales and Duhamel seem to have shown, that in the sap- vessels no valves exist ; and 

 that branches imbibe moisture nearly equally at either end. See Thomson's Chemistry, v. 

 385 ; an assertion, however, opposed by various fact.*. See also Smith's Iptrod. p. 57. 60. 



f M<?rn. de 1^ Societe d'Arcneil, torn. iii. ^ 



