THE FALL OF THE LEAF 277 



stance. The corky layer is often formed months 

 before the fall of the leaf. It is usually a little lower 

 down than the separating layer. 



Van Tieghem and Guignard also tell us, though I 

 think that they did not first discover the fact, that the 

 changes in the leaf which precede its fall may be 

 artificially induced any time after Midsummer. It is 

 only necessary to cut a branch, and keep it in a still, 

 moist atmosphere. Shutting it up in a botanical 

 collecting-box is a very convenient method, which will 

 cause the complete formation of a separating layer in 

 a week or less. It is not, therefore, necessary to stay 

 at home during the long vacation in order to study the 

 phenomena of defoliation with all the conveniences of 

 our own laboratory or study. 



Van Tieghem and Guignard observe that when the 

 middle tier of cells in the separating layer deliquesces, 

 the exposed cells of the neighbouring tiers begin to 

 bulge. This points to their turgid condition. Our 

 authors believe that increasing turgidity at length 

 causes the two tiers to press against each other with 

 sufficient force to rupture the vessels and fibres. Thus 

 the last attachment of the leaf is severed, without 

 shock and it may be in perfectly still air, and the leaf 

 falls gently to the ground. 



Mohl observed that during a frost in early winter 

 many leaves fell though the air was perfectly still. 

 On close examination he found that the sap in the 

 separating layer had frozen to a thin plate of ice, 

 which forced the tissues apart, just as ice in the 

 crevices breaks up the clods. When the ice melted 

 after sun-rise the leaves fell at once. 



