ON THE TORPIDITY OF PLANTS. 15JJ 



or inner bark by its inferior layer of fibres : tbis double layer of fibro- 

 vascular tissue will be easily perceived by examining a decayed leaf, 

 many of wbicb are found in tbe ditches or road sides in the winter 

 season, in which the cohesion of the upper and lower layers is 

 destroyed, so as to allow them to be easily separated. 



We shall now be prepared to appreciate the explanations of the 

 cause of the fall of the leaf, which have been given by Du Petit 

 Thouars and De Candolle. 



The opinion of Du Petit Thouara is as follows :—" If you watch 

 the progress of a tree, of the elder for example, you will perceive that 

 the lowest leaves upon the branches fall long before those of the 

 extremities. The cause of this may, perhaps, be explained upon tho 

 following principle: — In the first instance, the base of every leaf 

 reposes upon the pith of the branch, to the sheath of which it is 

 attached. But as the branch increases in diameter by the acquisition 

 of new wood, the space between the base of the leaf and the pith 

 becomes sensibly augmented. It has, therefore, been necessary that 

 the fibres by which the leaf is connected with the pith should lengthen, 

 in order to admit the deposition of the wood between the bark and the 

 pith. Now how does this elongation take place ? As the bundles of 

 fibres which nin from the pith into the leafstalk are at first composed 

 only of spiral vessels, it is easy to perceive that they may be suscep- 

 tible of elongation by unrolling. And in this seems to lie the 

 mystery of the fall of the leaf; for the moment will come when the 

 spiral vessels are entirely unrolled, and incapable of any further 

 elongation, they will, therefore, by the force of vegetation, be stretched 

 until they snap, when the necessary communication between the 

 branch and the leaf is destroyed, and the latter falls oflT." 



De Candolle explains the matter otherwise and better ; he says — 

 " The increase of leaves, whether in length or in breadth, generally 

 attains its tenn with sufficient rapidity ; the leaf exercises its functions 

 for a while and enjoys the plenitude of its existence ; but by degrees, 

 in consequence of exhaling pure water, and preserving in the tissue 

 the earthy mattei-s which the sap had carried there, the vessels harden 

 and their pores are obstructed. This time in general arrives the more 

 rapidly as evaporation is more active. Thus we find the leaves of 

 herbaceous plants or of trees which evaporate a great deal, fall before 

 the end of the year in which they were bom, while those of succulent 

 plants, or of trees with a hard and leatherv textiure, which from one 



