102 Lee . — The Morphology of Leaf-fall. 
the cork cambium before leaf-fall are all highly variable, and cannot be 
relied upon to form the basis of any type. 
The species which Tison has classified under Type io appear to be 
more appropriately referrable to other classes, from which indeed they seem 
to differ only in the deep-seated nature of the periderm ; and the species 
comprised in his last class (n), while they may differ in other ways, agree 
in that the scar (or dead leaf) is usually thrown off some time during the 
second year. In the present work the last-named peculiarity has not been 
regarded as of sufficient weight to justify the erection of a type. This is 
because the morphological distinction is not regarded as equal in value to 
the other characters selected, and also because the class as at present con- 
stituted affords an easy transition to the ordinary type in which the leaf-scar 
is exfoliated by formation of periderm. If the class were limited to those 
cases in which the dead leaves remain on the tree during the winter, the 
case for a separate class would be much stronger. But it is not so limited, 
and therefore the conclusion is arrived at that it is better to trust to the 
morphological distinctions which depend on the cells forming the Protective- 
layer than to rely on the time of exfoliation of the scar. 
A consideration of the facts, and especially of the experiments and 
observations already alluded to, which tend to show that the actual stage 
which the leaf-fall processes may have reached at the time of defoliation is 
largely influenced by external conditions, leads to the conclusion that any 
scheme for the classification of leaf-fall phenomena must be based, not on 
the features presented at any given time, but rather on the structures which 
are the ultimate result of leaf-fall. On these lines the simplest and perhaps 
the most logical scheme appears to be the one now advanced, in which the 
first class consists of species in which the Protective-layer is formed by the 
ligno-suberization of cells which have not previously undergone division ; 
the second, of species in which division precedes ligno-suberization ; while the 
third class comprises species in which the Protective-layer is produced by 
the activity of a regular cambium, and is therefore quite secondary in origin. 
In the first two classes there is a gradual transition from species in 
which the Protective-layer is entirely unchanged at the time of leaf-fall to 
-others in which ligno-suberization is complete at that time ; while in the 
third class, of which only two examples have been described, the Protective- 
layer is fully formed at the time of defoliation. 
The fall of leaflets in compound leaves has not yet been extensively 
studied, but from various observations which have been made the rule may 
be laid down that the leaf-fail structures in leaflets are of the same type as 
those which occur at the base of the parent leaf, but that they are usually 
much simpler. A Separation-layer is invariably present, a Lignified-layer 
may be produced, and the Protective-layer may or may not be well marked, 
but ligno-suberization is seldom well advanced at leaf-fall. 
