272 PHYSIOLOGY. BOOK II. 



1. Fugacious or caducous, which fall shortly after their 

 appearance ; as in Cactus. 



2. Deciduous or annual, which fall off in the autumn ; as 



the Apple. 



3. Persistent, evergreen, or perennial, which remain perfect 

 upon the plant beyond a single season ; as Holly, Common 

 Laurel, &c. 



With regard to the cause of the fall of the leaf a number 

 of explanations have been given, which may be found in 

 Willdenow's Principles of Botany, p. 336. There is, however, 

 only two that are much worth recording ; those of Du Petit 

 Thouars and De Candolle. 



If you will watch the progress of a tree, — of the elder for 

 example, — says the former writer, you will perceive that the 

 lowest leaves upon the branches fall long before those at the 

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

 upon the 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 wood between the bark and the pith. 

 Now how does this elongation take place ? As the bundles 

 of fibres which run from the pith into the leaf-stalk are at first 

 composed only of spiral vessels, it is easy to conceive that they 

 may be susceptible of elongation by unrolling. And in this 

 seems to lie tlie 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 untill they snap, when 

 the necessary communication between the branch and the leaf 

 is destroyed, and the latter falls off. 



De Candolle explains this matter otherwise. The increase 

 of leaves, he says, whether in length or in breadth, generally 

 attains its term with sufficient rapidity ; the leaf exercises 

 its functions for a while, and enjoys the plenitude of its exist- 



