164 
The Ohio Naturalist. 
[Vol. IV, No. 7, 
winter bj" specially developed stems, the leaf which represents the 
active transinring and food manufacturing organ being usuallj’ 
shed. 
The methods b}' which the leaves are separated from the stem 
are various. Some plants like the Hemlock shed them after they 
are several years old. Others like the Pines get rid of the foliage 
leaves by pruning off dwarf branches of a certain age. Some like 
the Bald Cypress and Tamarix drop the dwarf branches and 
smaller twigs with the leaves at the end of each growing season ; 
so the plant has no leaves in the winter. But the common way 
is for the leaves alone to be separated from the branches. A 
cleavage plane is formed usually at the base of the petiole and 
the leaf then falls away. The .separation layer is gradually devel- 
oped between the vascular bundles and epidermis, and finally, 
when the cleavage is nearlj’ complete the merest puff of wind will 
break the wood}’ strands and carry the leaf away. 
The casting of the leaf, however, is not a sudden process but 
preparatory changes are going on in its tis.sues for some time 
before it is detached. In many ca.ses anthocyan and other color- 
ing matters are developed to protect the chlorophyll and proto- 
plasm while the food material is being transferred to the stem. 
After the cleavage plane is formed a heavy frost will help to 
break away the fragile woody strands which still hold the leaf in 
place. This is very apparent in such trees like the White Mul- 
berry, which may put off its entire leaf dre.ss in a .single day after 
a frosty autumn night. There is much difference in the time of 
casting the leaf. The Ohio Buckeye, Juneberr}', Walnuts, and 
Hickories are among the first to .shed their leaves. The Cotton- 
wood and Chestnut Oak shed their leaves verj’ gradually ; and 
some of the Oaks are among the last of the trees to l)e bare. The 
Shingle Oak drops few leaves before late in the winter, although 
they dry off, and it is not completely denuded until about the 
first of April. 
It is interesting to note the several ways in which the cleavage 
planes are produced. In plants with simple leaves a separation 
layer is more commonly formed at the base of the petiole very 
close to the stem, as in the Him, Maple, Oak, and Catalpa. In 
some, however, two cleavage planes are produced, one at the base 
of the petiole and the other at the outer end just at the base of 
the blade. This is strikingly .shown in Ampelopsis tricuspidata 
and A. cordata. The blade drops off some time before the petiole, 
so that in certain }’ears a vine of A. tricuspidata may shed nearl}- 
all of its blades before the petioles begin to fall making a rather 
unique appearance. The same adaptation is pre.sent in the various 
species of Grape. There is probably con.siderable advantage to 
the plant in such an arrangement, for the food in the large peti- 
ole, which is in much less danger of freezing than the blade, may 
