370 
POPULAR SCIENCE REVIEW. 
Why, then, do the leaves fall ? is the query to which we 
purpose in the present article to supply a response as far as we 
are able to do so. 
In attempting to give an answer to the question, it maybe as 
well to go a little into detail, which will serve the more clearly 
to render intelligible what really is known about the fall of 
the leaf, and will also indicate certain points which up to the 
present time have scarcely received a satisfactory explanation. 
We may premise that all the many reasons that have from 
time to time been given to account for the process of defoliation 
may be ranged into those which are of a purely mechanical 
nature, and which are very much under the influence of external 
conditions of climate and the like, and those which are of a 
structural or organic character, being only secondarily mecha- 
nical in their action, and less directly influenced by external 
conditions. 
There are such differences in the length of time that leaves 
remain attached to the stem, such variation in the method of 
detachment, that it would seem probable that the causes pro- 
ducing these varied results may themselves be diverse, that in 
one case one cause may be potent, in another instance the result 
may be due to some other agency, while in a third series, per- 
haps, the effect may be the result of the joint action of more 
than one factor. Thus, while some leaves fall off comparatively 
soon after their expansion, such as those which are technically 
termed caducous or deciduous, others are persistent, while to a 
few, which seem never to be shed, the term evergreen, which is 
generally inappropriate, may fairly be applied. Where leaves 
fall off very early, it will be found generally that they serve the 
purpose of protection merely ; such are the small rudimentary 
scaly leaves which envelop the leaf-buds in most of the trees 
and shrubs of this country. Formed at the close of the year 
around the young bud, they shield and protect it during winter, 
and when the returning warmth of spring urges the dormant bud 
into growth, the scales are pushed off by the constantly increas- 
ing pressure of the rapidly enlarging bud. On the other hand, 
where, as in the Araucaria, the leaves are never shed, their in- 
ternal organisation approaches in many respects to that of the 
stem ; indeed, it becomes a question in such a case whether the 
so-called leaves are not merely portions of the stem. Between 
these extremes there is a large class of deciduous trees or shrubs, 
which shed their leaves, some sooner, some later, but at a toler- 
ably regular, determinate period, according to the particular 
species, and according, as we shall see presently, to external 
circumstances. Thus, while the lime, in the southern parts of 
Britain, loses its leaves early in September, the walnut soon after- 
wards, to be followed in their turns by the elder and the horse- 
