73 
Lee . — The Morphology of Leaf-fall. 
change occurs in the cells of the Protective-layer previous to leaf-fall. The 
latter is brought about by the chemical alteration and subsequent dis- 
appearance of the middle lamellae of a layer of cells which is never very 
obvious, and in which no visible preparations occur. After leaf-fall the 
cells near the exposed surface undergo ligno-suberization, and the cells 
beneath the Protective-layer divide to form a cambium, which produces 
a continuous layer of cork in that region. 
A fairly well marked Lignified-layer can often be demonstrated before 
leaf-fall above the Separation-layer. Tyloses are never very numerous, nor 
is there an abundant formation of gummy lignin. 
Celtis occidentalis, Linn. 
In this example there is no essential difference from the method that 
has already been described for Castanea. Reduction of sclerenchyma and 
distribution of starch granules and crystals of calcium oxalate are as usual ; 
while in addition groups of ‘ stone 5 cells are present in the cortex. 
The Separation-layer is formed by the slight division of 1-2 rows of 
cells situated at some distance above the junction of the petiole and 
axillary bud, and separation occurs by mucilaginization and subsequent 
disappearance of the middle lamellae. There is no Lignified-layer, and 
tyloses, though present, are not at all abundant at the time of leaf-fall. 
Soon after the latter has occurred, ligno-suberization commences in the 
lower portion of the Protective-layer, and gradually spreads until the whole 
of the parenchyma in this region — including the living cells of the leaf- 
trace — has become ligno-suberized. The last cells to undergo this change 
are those near the surface ; but later these collapse and become altered in 
the usual way. 
In the first year, the cambium, which arises by division of the cells 
beneath the Protective-layer, gives rise to 1-3 layers of cork and a similar 
quantity of phelloderm (PI. V, Fig. 8, co ., ph .), the latter being distinguished 
by the retention of the living protoplasm and cellulose walls and by the 
presence in many of the cells of a rhomboidal crystal of calcium oxalate ( c . cl). 
The behaviour of these crystal-containing phelloderm cells is very similar 
to that of many of the stone cells in the cortex which also contain crystals. 
In both lignification of the cell-wall occurs, and the lignin, instead of being 
confined to the primary wall, becomes deposited as a film of varying 
thickness on the surface of the crystal (PI. V, Fig. 9). This is very 
noticeable in many of the crystal cells where a connexion exists between 
the primary cell-wall and the film of lignin which encloses the crystals. 
In the second year the activity of the Protective-cambium is continued, 
and results in the formation of a thick layer of cork and a less quantity of 
phelloderm. 
