70 THE AMERICAN BOTANIST. 



A ver\' 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 plant 

 to its support. Evidentlyif the leaf were shed in the usual 

 waj'the whole vine \vouldfall to the ground in the winter. 

 There is a more or less perfect brittle layer formed in the 

 petiole just a little bej^ond 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 

 1, though usualh' frozen before this time. The develop- 

 ment of a brittle layer in the petiole of this plant seems to 

 be quite a modern adaptation. Smilax glauca, S. rotun- 

 difolia and S. bona-nox show- the same peculiarity. The 

 genus Ruhus represents another group of plants which 

 shed their leaves bj-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 adap- 

 tation, however, may have no other significance than one 

 of the possible ways in which the plant was able to get 

 rid of its leaves. Among the species which show this 

 peculiarity' well are Ruhus odoratus, R. strigosus, R. occi- 

 dentalis, R. nigrohaccus and R. invisus. In the common 

 mock orange {Philadelphus coronarius) the cleavage plane 

 is formed a little above the Ijase of the petiole which 

 remains as a protection to the axillary bud. 



There are certain plants which have the habit of cover- 

 ing their axillary buds with the base of the petiole. The 

 sycamore (Platanus occidentalis) presents a verA- perfect 

 example of this adaptation. The reason for such a pecu- 

 liarity is not eas3^ to see. It may be lor protection, or 

 again as in Rhus glabra it may prevent the development 

 of too many lateral buds into branches. But there \nixy 

 be no special advantage whatever. It may be a mere 

 incident to the adjustment of the leaf to the surrounding 

 tissues. Other plants which cover their lateral buds are 

 Cladrastis lutea, Rhus birta, R. copallina, Acer negundo, 

 Ptelea trifoliata, Gleditsia triacanthos, Robinia pseudaca- 



