extreme base of the leaf, where it joins the branch. The contents 
of these cells soon die and the result is a region of weakness, so 
that a slight force, such as a gentle breeze, a flying bird or insect, 
a falling fruit, or even the weight of the leaf itself, is sufficient 
to cause it to fall away. In some trees, as for example, the 
sycamore or plane tree {Platitius) , the buds form directly in the 
leaf-base, which fits like a cap over the end. In such a case, a 
slight enlargement of the bud after the corky layer had formed, 
would be sufficient to dislodge the leaf. In a similar way, the 
swelling of buds which grow in the leaf-axil, just above the leaf- 
base, may rupture the tissue. The corky layer serves as a callus 
to heal over the wound at once, preventing loss of water and sap, 
and the entrance of spores or germs which might in time result 
in injury to the tree. The scars where leaves were attached may 
be seen on the branches at any season of the year. 
Deciduous Trees and Evn gree?is. All trees shed their leaves 
sooner or later. In fact, this is one of the several characters by 
which the botanist distinguishes a leaf from a branch on perennial 
plants, such as trees and shrubs. Branches (save for accident) 
usually persist, but leaves are shed sooner or later. In some 
plants with large, pinnately compound leaves, like the elder 
{Sambucus) , the tree of heaven {Ailanthus) , or the Hercules’ 
Club ( Aralia ), the main axis of the leaf, bearing the leaflets, 
closely resembles a branch bearing leaves, but its true leaf 
character is clearly shown, not only by the bud in its axil, but 
also by it falling away when the leaves fall. 
In the case of the five-leaved ivy ( Ampclopsis) the leaflets of 
the compound leaf usually separate first from the main leaf-stalk 
{petiole), leaving the latter projecting in a conspicuous manner. 
Later the leaf-stalks themselves aie shed. The compound leaf 
of the horse-chestnut is also similarly dismembered during leaf- 
fall. 
Not all trees, however, shed all their leaves every year, or all 
at once. Our own evergreen trees, are indeed evergreen, but not 
by the persistence of the same foliage. Individual leaves may 
remain on for as long as three years or more, but one may always 
see on the ground, under all evergreens, the numerous leaves 
(needles) that have fallen. 
Leaf-fail in the Tropics. In tropical countries the absence of a 
season like our winter makes unnecessary any periodical shed- 
ding of foliage, such as occurs in the temperate zone; neverthe- 
less, the leaves do not persist, but are falling, a few at a time, 
throughout the year. It is very interesting to see trees in the 
tropics with a mixed foliage, composed in part of old leaves 
ready to fall, of new leaves just expanding, and of mature leaves 
in the full vigor of their activity. 
