7i 
Lee. — The Morphology of Leaf -fall. 
triangular mass of cells intervenes, and later becomes ligno-suberized and 
contributes to the bulk of the Protective-layer (Text-figs. 12 and 13, Proj . /.). 
The cells of the Separation-layer do not increase very much in size, but the 
walls of the mother-cells begin to swell, the middle lamellae between the 
cells of the outer layers gelatinize and finally disappear, and the leaf is 
freed by the rupture of the vascular bundles. Throughout the whole process 
the formation of tyloses in the vessels is very scanty, only a few small in- 
growths being observed in the vessels of the protoxylem. With the first signs 
of activity in the leaf-base, however, there is an abundant supply of gummy 
lignin, which lodges in all the vessels in the region of the Protective-layer, 
and finally results in the complete blocking of these conducting elements. 
About the end of the second year, the activity of the Protective- 
cambium ceases, and a new one is formed by division of the cells in the 
cortex beneath. At the same time a new phellogen appears in connexion 
with the stem-periderm, and the two becoming continuous a regular layer 
of periderm is produced. 
Short descriptions of other species examined will now be given, and 
these will be arranged under their respective types. 
Class I. 
In the following plants all stages will be found, from the state of events 
seen in Castanea sativa to that described for Ribes sanguineiim , and in many 
cases all transitions are to be found within the same species. 
Hibiscus syriacus, Linn. 
The slender, cylindrical petiole is rather swollen at the point of attach- 
ment, and has a row of hairs running up the ventral surface. At the leaf- 
base there is the usual reduction in the patches of sclerenchyma which 
accompany the three vascular bundles in their passage to the leaf. Starch 
granules and compound crystals of calcium oxalate are distributed as usual, 
and mucilage cavities are also present. 
The Separation-layer is formed by the division of 1-3 rows of cells, 
situated at a little distance above the upper extremity of the stem-periderm, 
and distinguished by their dense protoplasmic and starchy contents. The 
walls of the uppermost cells swell and become mucilaginous, and separation 
occurs by the solution of the middle lamellae. No Lignified-layer is present 
above the Separation-layer. Tyloses and gummy lignin are abundant in 
the vessels at the leaf-base before separation occurs. Although easily 
distinguished by reason of the abundance of its cell contents, the Protective- 
layer shows no chemical or other change previous to leaf-fall. As soon as 
the latter has taken place, ligno-suberization, preceded by the disappearance 
of the starch granules, commences near the exposed surface and spreads 
rapidly downwards. Lignification is most marked in the lower cells of the 
