FALL OF THE LEAF. 203 



does the ordinary periderm. Between the cell-rows of this, the 

 flattened and attenuated ends of the vessels can later on be still 

 recognised. The dead ends of the vascular bundles project, to 

 the number usually of 5 or 7, out of the shield-like leaf- scar. 



The Bastard Acacia (Robinia Pseud- Acacia) serves admirably 

 for the purpose of this study, the processes in which are essentially 

 the same in most Dicotyledons, Gymnosperms, and even in many 

 Monocotyledons. The exact sequence of events, and the time of 

 initiation, however, vary materially. In Ferns, on the other 

 hand, the leaf -scar merely dries. 



If strong leaves of Gymnocladus canadensis, or of Ailanthus 

 glandulosa are laid in a damp, dark chamber, the former in about 

 forty- eight hours, the latter in four days, lose their leaflets on the 

 slightest touch. Longitudinal sections through the leaf base 

 shows that an absciss layer has been formed. Such an absciss 

 layer commences its formation also at the base of the common 

 leaf -stalk at about the sixth or seventh day. Under these condi- 

 tions, however, a periderm is not formed under the absciss layer. 

 Fraxinus excelsior (the Ash) and Juglans regia (the Walnut) can 

 also be used in this experiment. The end leaflet of Juglans is not 

 exfoliated. 



