328 
PHYSIOLOGY. 
.BOOK II, 
wither in the autumn, but do not fall olF till the succeeding 
spring ; and, in a third class, they neither wither nor fall off 
the first season, but retain their verdure during the winter, 
and till long after the commencement of anotlier year’s 
growth : these last are our evergreens. Mirbel distinguishes 
leaves into three kinds, as characterised by their periods of 
falling : — 
1. Fugacious^ or cadvjcous^ 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 
JVilldenoid* s Principles of Botany, p. 336. There are, however, 
only two much worth recording; those of Du Petit Thouars 
and De Candolle. 
If you watch the progress of a tree, of the Elder, for ex- 
ample, 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 tlie 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 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 
