9i 
Lee . — The Morphology of Leaf-fall. 
last signs of the conspicuous nuclei disappear with the completion, before 
leaf-fail, of these chemical changes. 
Numerous fairly regular divisions which occur in the cells of the petiole 
adjacent to the Protective-layer result in the formation of a well-marked 
Separation-layer (S.L.), in which starch is present in considerable quantity. 
After several divisions have occurred, the primary walls begin to swell, 
at the same time taking the haematoxylin and ruthenium-red more strongly. 
Later the middle lamellae between the outer cells become mucilaginous 
and disappear, and with the subsequent rupture of the vascular tissue the 
leaf falls to the ground. A Lignified-layer (. Lig .) is also present. 
Tyloses and gummy lignin appear very early and become very abundant 
in the vessels of the primary and secondary wood at the level of the Protec- 
tive-layer. Tison has already described the manner in which the laticiferous 
elements behave near the region of separation. Latex tubes in these plants 
are very numerous, and as those of the cortex communicate with their 
fellows in the petiole, it follows that a considerable number must traverse 
the Protective-layer. Those parts which are enclosed within the latter 
do not become divided as do the ordinary cells of the Protective-layer ; their 
walls, however, become lignified and probably suberized, though the latter 
process is not at all definite. When separation is taking place there can 
often be observed a direct squeezing out of the latex tube in the region of 
the Separation-layer, the process finally culminating in the complete rupture 
of the tube at this point (PL VI, Fig. 15). 
But the most interesting point in connexion with the behaviour of the 
laticiferous tubes appears to relate to the phenomenon mentioned by Parkin 1 
for other laticiferous plants, i. e. Hevea brasiliensis and Pltuniera actitifolia. 
This author found that if a mature leaf is cut off half-way up the petiole, 
latex exudes copiously from both cut surfaces ; nearer the leaf-base the 
amputation results in copious exudation of latex from one surface only, 
that of the petiole, while if the leaf is cut off flush with the surface of the 
stem, the flow of latex is from the stem only. In the species examined by 
Parkin the only reason he could discover for such behaviour consisted 
in the appearance near the leaf-base of a number of crystal cells which 
seemed to cut off the latex tubes. So far as Parkin could discover there 
was no formation of walls within the tubes themselves. 
Now in M. alba and nigra , as well as in Ficus Carica and Broussonetia 
papyrifera , just before leaf-fall definite walls are formed in the laticiferous 
tubes near the leaf-base, though so far it has been impossible to work out 
the details of their formation. If longitudinal sections are taken of the 
bases of leaves which have a fully-formed Protective-layer and the latex is 
dissolved out by any of the ordinary methods, distinct walls will be found 
traversing the tubes at a varying distance on either side of the Protective- 
1 Annals of Botany, vol. xiv, 1900, p. 205. 
