88 
Lee . — The Morphology of Leaf -fall. 
in the stem, and each is protected by a layer of sclerenchyma. At least one 
canal (Text-fig. 15, res.) accompanies each of the three bundles which pass 
into the leaf, and the sclerenchyma ( scl .) surrounding it does not diminish to 
any great extent at the leaf-base. 
The Separation-layer (Text-fig. 1 5, S.L. and PL V, Fig. 1 1) is formed quite 
early by division of a layer of cells situated at some distance above the upper 
extremity of the stem periderm. The protoplasm first becomes conspicuous 
and starch granules are formed ; numerous regular walls appear, usually 
2-5 in each cell, and the fully-formed Separation-layer, which often has 
a thickness of 2-3 cells, may diminish to one cell at the ventral surface of 
the petiole. 
A conspicuous Lignified-layer ( Lig .) is formed by the lignification of 
the cells immediately above the Separation-layer. The Protective-layer 
(Text-fig. 15 and PI. V, Figs. 11 and 12, L.P.) varies greatly in rela- 
tive time of formation, sometimes appearing with the Separation-layer, 
at other times commencing to form only after the latter is clearly marked 
out. Ligno-suberization begins in the upper part near the Separation-layer 
and vascular bundles and spreads downwards ; it is preceded by the 
irregular division of the cells. The living cells of the vascular bundle 
at this level may also become ligno-suberized before the leaf finally 
separates from the stem. The crystal cells included in the Protective- 
layer undergo lignification, but in other cells the suberization is most 
marked. 
The formation of tyloses in this species is worthy of note. With the 
appearance of the Separation- layer the production of tyloses in the vessels 
commences, and the lumen of each of the latter is later completely 
obliterated (PI. V, Fig. 12, v l ). The resin canals accompanying the 
vascular bundles behave in this respect very much as vessels, for although 
surrounded by thick-walled cells an enormous production of tyloses com- 
mences about the time of formation of the Protective-layer ; and for a Httle 
distance both above and below the latter these structures completely fill 
the cavity of the resin canals (PI. V, Figs. 11, 13, 14). Later these in- 
growths undergo the changes of any part of the leaf-base in which they are 
situated, and in this way help to make more efficient the various modifications 
connected with leaf-fall. 
A very curious phenomenon noticed in connexion with this species 
is the evident individuality of each leaf. In any twig the leaves in which 
the leaf-fail processes are most advanced are not, as in most cases, the 
lowest, but in several instances where two adjacent leaves were examined, 
the one nearer the apex might be on the point of falling, whilst in the other 
not a trace of the formation of the Separation-layer would be apparent. 
Quite analogous with this is the difference in degree of completeness of the 
leaf-fall processes at the time of leaf-fail, for while in some the elements of 
