IOI 
Lee . — The Morphology of Leaf -fall, 
with the formation of the Separation- layer, the latter after separation had 
been effected. In many species the relative times of appearance of the 
Separation-layer and the Protective-layer are reversed, and in these the 
provision for protection of the exposed tissue is complete before the leaf is 
cast off. The form of the protective device, so far as the species at present 
examined are concerned, appears to have followed three distinct lines : — 
1. Ligno-suberization without further modification of existing cells. 
2. Ligno-suberization of cells after irregular division has occurred. 
3. Ligno-suberization of cells produced by the activity of a regular 
cambium. 
So far as the present research goes, the characters here enumerated, 
and these alone, are invariable ; and it is on these that the classification of 
leaf-fall phenomena has been founded. 
The system proposed by Tison is very elaborate, and is based on 
(1) the mode of formation of the Separation-layer; (2) the mode of 
formation of the Protective-layer ; (3) the degree of ligno-suberization 
which has been effected at leaf- fall ; (4) the amount of cork produced by 
the cork cambium beneath the scar at the time of defoliation ; (5) the 
origin of the cork cambium of the stem ; (6) the exfoliation of scars on 
dead leaves. Were all the characters just enumerated absolutely invariable, 
it would still be impossible to form from them classes of co-ordinate value ; 
they are far too dissimilar to be comparable. Again, over and over again 
in the above descriptions, it has been pointed out how very variable are 
most of the characters concerned, and one or two variations taken from 
Tison’s paper will be briefly mentioned here. Thus in his second class, 
which is distinguished by the possession of a Separation-layer formed 
without division of cells, and by the absence of ligno-suberization at leaf-fall, 
Tison places Crataegus monogyna , in which the cells of the Separation-layer 
sometimes present division walls , and Euonymus europaeus , in which ligno- 
suberization may occur before leaf-fail. Similarly in Class IV, which is 
distinguished by the irregular division of the Protective-layer, absence of 
ligno-suberization, &c., are placed Ficus Carica and Euonymus latifolius , 
in both of which some ligno-suberization may occur before defoliation. Other 
examples might also be given to show that some at least of the characters 
relied on by Tison are of very doubtful value. In addition the results of 
Lowi’s experiments on the effect of different conditions on the Separation- 
layer show us that the form and extent of the latter depend largely on 
external conditions ; and any one, by repeating von Mohl’s experiment in 
which he placed a healthy branch in a damp chamber, may prove that the 
formation of the Separation-layer can be induced without the production of 
a ligno-suberized layer even in species in which, under ordinary conditions, 
ligno-suberization is complete before leaf-fall. The form of the Separation- 
layer, the time of ligno-suberization, and the amount of cork produced by 
