Prof. Vaucher on the Fall of Leaves. 333 



a horse-shoe — in others a heart, the segment of a circle, &c. 

 but always similar in trees of the same species. But if the 

 leaf-stalk be attempted to be broken elsewhere than at its or- 

 dinary point of separation, the fibres are lacerated and torn, 

 and proof is thus afforded that means for this separation have 

 been previously prepared by Nature at one exclusive point, 

 without reference to exterior causes. 



The fibres of a leaf-stalk, in place of being a simple pro- 

 longation of those of the twig, are therefore, in M. Vaucher's 

 opinion, separated from it at the point where this cicatrix is 

 seen. There appears, indeed, no real continuity between 

 them ; and the temporary union which connects the leaf-stalk 

 with the twig, is merely kept up by a kind of adhesive sub- 

 stance, which, when the purposes of the leaf to the parent 

 plant are served, is dried up or dissolved. This adhesive sub- 

 stance is probably formed by some portion of the parenchyma 

 interposed between the two systems of fibres. While this pa- 

 renchyma is under the influence of the vegetable action, the 

 adhesion is maintained ; when this action ceases, the union is 

 dissolved, and the leaf falls. 



The point of separation is also to be perceived exteriorly in 

 the form of a circular ring, at the point which separates the 

 leaf-stalk from the stem. This ring is easily perceptible in 

 most trees. It is particularly marked in the leaf-stalks of 

 compound leaves, the fall of which present more varieties in 

 their appearance than simple leaves. In the Aralia spinosa, 

 for instance, it divides the principal leaf-stalk and its depen- 

 dent petioles into many parts. In the great Chesnut, the ring is 

 seen at the base of the leaves. In the Walnut, this appearance 

 explains how the odd leaflet remains adhering while the others 

 detach themselves ; and in the green leaves of the Clematis 

 may be remarked all the appearances which precede their fall. 

 At the same time, it may be observed, that the solution of 

 continuity which takes place in compound leaves is not of the 

 same nature as that which occurs in simple leaves. 



This natural separation, however, is not a phenomenon pe- 

 culiar to the leaves of arborescent stems. It is equally seen 

 in the peduncles which support the male flowers of a great 

 number of plants, such as the walnut, the willow, &c. and it 



