THE AMERICAN BOTANIST. 69 



SO that in certain years a vine of A. tricuspidata may shed 

 nearly all of its blades before the petioles begin to fall mak- 

 ing 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 arrange- 

 ment, for the food in the large petiole, which is in much 

 less danger of freezing than the blade, may thus have a 

 longer time to be withdrawn into the stem. In the catal- 

 pa, for instance, the blades often freeze and dry up in the 

 fall v^hile the petioles are still green and active. It w^ould 

 evidently be better if the useless blade were cut off by a 

 cleavage plane so as not to hinder the work of the petiole- 

 In compound leaves the leaflets are usually shed singly. 

 The leaflets of such palmate leaves as in the Virginia creep- 

 er and the various buckeyes are cut off some time before 

 the petiole. Pinnately compound leaves have various 

 peculiarities. In such forms as Rhus glabra the leaflets 

 are separated ,by cleavage planes but no transverse cleav- 

 age joints are formed in the main rachis which persists for 

 some time. In other forms, like in Fraxinus quadrangu- 

 lata and Staphylea, not only are the leaflets cut off by 

 cleavage planes but there is a series of cleavage joints 

 formed in the rachis at the insertion of each pair of leaflets 

 and thus the main rachis of the leaf drops offpiece by piece. 

 Decompound leaves often form an elaborate system of 

 separation layers. A good example of this is shown in 

 the leaf of the honey locust. First the numerous leaflets 

 drop off, the main rachis and the side branchlets remain* 

 ing on the tree for some time. Next the side branchlets 

 begin to fall, and finally the whole rachis is separated. 

 One may well ask the meaning of such an elaborate system 

 of cleavage planes when one amputation at the base of 

 the petiole would be sufficient. There is no doubt but that 

 the green rachis and petiole may continue, to a limited ex- 

 tent at least, the process of photosynthesis ; and as 

 stated above, by means of a gradual cutting away of the 

 large leaf surface the more exposed parts are removed first 

 and there is a better opportunity for the withdrawal of 

 the food present in the stem. 



