334 Prof. Vaucher on the Fall of Leaves. 



is still more distinctly marked in the pericarps. The different 

 ways in which these pericarps open at the moment of maturity, 

 and the constancy of the mode of opening in the same spe- 

 cies, cannot be explained without having recourse to the 

 supposition of a peculiar organization — to a primitive solder, 

 similar to that which retains the leaf-stalks in their places. 

 In short, says Professor Vaucher, the same traces of stric- 

 ture or tightening may be perceived on the exterior covering 

 of a great number of pericarps ; and even seeds do not sepa- 

 rate from the feeble peduncles which support them but by 

 analogous means. 



But it may be asked, continues M. Vaucher, how the fall 

 of leaves is determined ? — Why, if there be an original sepa- 

 ration of the leaf-stalk and stem, do not the leaves fall as soon 

 as they appear ? — and why, on the contrary, do these leaves, 

 so intimately united to the stem, fall at the approach of win- 

 ter ? — The reason is, 1. That there exists between the leaf- 

 stalk and the stem a substance which unites them, and which 

 botanists call parenchyma. While this substance is impreg- 

 nated with vegetable juices, it fulfils its vital functions, ad- 

 hesion is maintained, and any attempt to remove the leaf 

 produces laceration ; but in autumn, the interposed parenchy- 

 ma having dried up, ceases to preserve the continuity with 

 the stem, and the leaves necessarily fall. 2. Because the 

 fibres which envelope the vessels in the stem or branch are not 

 of the same nature as those which penetrate the leaf-stalk. At 

 their first developement the difference is not manifest, but in 

 autumn the first are hardened, while the others remain herba- 

 ceous — the first continue to live, while the others die, and 

 there is in consequence a natural separation. Besides, the 

 twig and the branches increase in diameter, while the leaf-stalk 

 contracts in drying till the separation is complete. It must 

 be recollected, however, that this difference of increase in the 

 stem and leaf-stalk is not the chief cause of the fall of leaves ; 

 it is but one of the accessory circumstances. The true and 

 only cause is the solution of continuity, and this depends 

 primarily on the difference of organization. Without this 

 difference leaves would never separate from their stems in a 

 manner so general and uniform. They would on the con- 



ll 



