164 The Ohio Naturalist. [Vol. IV, Ko. 7, 



winter by specially developed stems, the leaf which represents the 

 active transpiring and food manufacturing organ being usually 

 shed. 



The methods by which the leaves are separated from the stem 

 are various. Some plants like the Hemlock shed them after ihey 

 are several j^ears old. Others like the Pines get rid of the foliage 

 leaves by pruning ofE 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 waj^ 

 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 finall}-, 

 when the cleavage is nearly complete the merest puff of wind will 

 break the woody 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 tissues for some time 

 before it is detached. In many cases 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 awaj' 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 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 gradually ; 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, 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 Elm, Maple, Oak, and Catalpa. In 

 some, however, two cleavage planes are produced, one at the base 

 of the jjetiole and the other at the outer end just at the base of 

 the blade. This is strikingl}- shown in Ampelopsis tricuspidata 

 and A. cordata. The blade drops off some time before the petiole, 

 so that in certain 3-ears a vine of A. tricusj)idata may shed nearl}- 

 all of its blades before the petioles begin to fall making a rather 

 unique appearance. The same adaptation is present in the various 

 species of Grape. There is probably considerable 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 



