Why Trees Shed Their Leaves. It is very commonly supposed 
that trees shed their leaves as a protection against cold, that is, 
to avoid being frozen in winter. This idea, however, is not cor- 
rect. Winter, so far as the tree is concerned, is not so much a 
season of cold as a season of drought, and leaves are the prin- 
cipal organs by means of which the tree loses water. During the 
growing season, water is being absorbed from the soil by the 
roots, and on the other hand, given off in large quantities from 
the leaves. When winter arrives, and the ground becomes frozen, 
the water is changed to ice, and therefore cannot enter the tree. 
If the leaves remained on, they would continue to give off water, 
and the tree, not being able to make good the loss, would die. 
In pines, hemlocks and other evergreen trees, where the 
foliage remains on over winter, the leaves have a relatively thick 
outer skin (epidermis), are covered with a waxy coat, and have 
their stomata* nearly closed, so that the loss of water is very 
greatly reduced, if not entirely checked. 
In the tropics leaf-fall has no adaptive significance. The 
leaves remain on until old age renders them of little use to the 
tree, and then they are shed. The leaves ot some palms are not 
shed in the manner above described, but the leaf-stalk merely 
breaks off below the blade, leaving the leaf-base, and the lower 
portion of the stalk attached to the trunk. 
Preparation for Leaf-fall. As stated in Leaflet No 8, of this 
Series, the chief function of a green leaf is to make food for the 
plant. Materials taken in from the soil and air are, in the leaf, 
combined into plant food, which passes down the leaf-stalk to 
nourish the buds developing in the axils, and the other living 
parts of the tree. As the season of leaf-fail approaches, this food- 
making gradually ceases, and most of the manufactured food 
passes from the leaves into the branches. Thus a bushel of 
leaves would weigh less (dry weight) at the time of leaf-fail, than 
the same leaves would have weighed in mid-summer. 
In this connection it is interesting to note that the abscission 
layer, formed at the base of the leaf-stalk, does not at first cut 
across the strands of vascular tissue (vibro-vascular bundles) 
through which liquids pass in and out of the leaf from the stem, 
and thus the passage of the contents of the leaf into the branch 
is not interferred with. 
Autumn Colors. It is a very wide-spread belief that the var- 
ied leaf-tints of autumn are caused by frost. Such a belief, 
however, is based on superficial observation, for beautiful 
autumn colors may be seen every fall, weeks before the first frost. 
The exact cause or significance of autumn colors is not well 
understood by botanists. Some have thought that they were 
*Small openings through the epidermis. See Leaflets. Series 1, No. 8. 
