May, 1904.] 
Deciduous Leaves. 
^65 
thus have a longer time to be withdrawn into the stem. In the 
Catalpa, for instance, the blades often freeze and dry np in the 
fall while the petioles are still green and active. It \vould evi- 
dently be better if the u.seless 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 Creeper and the 
various Buckeyes are cut off some time before the petiole. Pin- 
nately compound leaves ha\ e various peculiarities. In such forms 
as Rhus glabra the leaflets are separated by cleavage planes but 
no transverse cleavage joints are formed in the main rachis which 
persists for some time. In other forms, like in Fraxinus quad- 
rangulata 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 off piece 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 remaining on the tree for some time. Next the side 
branchlets begin to fall, and Anally the whole rachis is separated. 
One may well a.sk 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 extent 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 into the stem. 
A very interesting condition is present in the Green Briers. 
The leaf of Smilax hispida has two tendrils near the base of the 
petiole and these, of course, hold the jdant to its support. Evi- 
dently if the leaf were shed in the usual way the whole vine 
would fall to the gouna in the winter. There is a more or less 
perfect brittle layer formed in the petiole just a little beyond the 
two tendrils where the leaf finally breaks off, leaving the petiole 
base with the tendrils intact. Most of the leaves hang on until 
after December i, though usually frozen before this time. The 
development of a brittle layer in the petiole of this plant seems to 
be quite a modern adaptation. Smilax glauca, S. rotundafolia, 
and S. bona-nox show the same peculiarity'. The genus Rubus 
represents another group of plants which .shed their leaves by a 
break in the petiole, leaving the base on the stem. In this case 
there are no tendrils and the only apparent advantage to the plant 
is the protection of the bud or tender part in the axil of the leaf. 
The adaptation, however, may have no other significance than one 
of the possible way's in which the plant was able to get rid of its 
