67 
Lee . — The Morphology of Leaf-fall. 
tudinal sections of the leaf-base about the time of leaf-fail, the leaf easily 
separated along this plane, leaving a clean surface exposed and the under- 
lying tissues quite unprotected. 
But whether the Separation-layer is effective or no, the subsequent 
course of events is the same. In both cases defoliation is followed by the 
continued activity of the cambium, which produces a few more layers of 
cells before ceasing for the winter. The ligno-suberization of the new cells 
does not proceed so quickly or become so complete as in the cells produced 
previous to leaf-fall, and the last layer formed before the cambium ceases 
its activity retains its protoplasm and undergoes changes quite different 
from those previously described. The cells of this layer remain small and 
have the usual form of periderm cells ; their protoplasmic contents are fairly 
abundant, but no starch granules are present. Soon the outer wall thickens 
considerably and gives the reactions for cutin. In some cases the increase 
in thickness is so great that the lumen of the cell becomes almost oblite- 
rated. By the end of January, when this internal cuticle is fully formed, it 
is very conspicuous, and starting from the stem cuticle at the junction of 
the petiole and axillary bud, it passes beneath the Protective-layer, and 
curving downwards, runs parallel with the surface for some distance before 
finally joining the thick outer cuticle of the stem proper, with which it is 
chemically identical (Text-fig. n, and PI. V, Fig. 7, cut., cut}). 
