Q8 THE AMERICAN BOTANIST. 



drop the dwarf Ijranches and smaller twigs with the 

 leaves at the end of each growing season ; so the plant has 

 lio leaves in the winter. But the common way is for the 

 leaves alone to be separated from the branches. A cleav- 

 age plane is formed usually at the base of the petiole and 

 the leaf then falls away. The separation layer is gradual- 

 ly developed between the vascular bundles and epidermis, 

 and finally, when the cleavage is nearly complete the mer- 

 est puif of wind will break the wood^^ strands and carry 

 the leaf away. 



The casting of the leaf, however, is not a sudden pro- 

 cess but preparatory changes are going on in its tissues 

 for some time before it is detached. In many cases antho- 

 cyan and other coloring matter are developed to protect 

 the chlorophyll and protoplasm 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 

 as the white mulberry, which ma^^ ])ut off its entire leaf 

 dress in a single day after a frosty autumn night. There 

 is much difference in the time of casting the leaf. The Ohio 

 buckeye, juneberry, walnuts and hickories are among the 

 first to shed their leaves. The cotton-wood and chestnut 

 oak shed their leaves very gradual!}^ ; and some of the 

 oaks are among the last of the trees to be bare. The 

 shingle oak drops few leaves before late in the winter, al- 

 though the^' dr\^ off, and it is not completely denuded until 

 about the first of April. 



It is interesting to note the several waj'S in which the 

 cleavage planes are produced. In plants wnth simple 

 leaves a separation layer is more commonly formed at the 

 base of the petiole very close to the stem, as in the elm, 

 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. 



