206 
Parkin.—Observations on 
(line C-D in the diagram) the reverse takes place, and latex 
oozes out only from the stem side. 
3. On cutting through the leaf-stalk higher up (such 
as E-F in the diagram), latex exudes freely from both 
surfaces. 
4. By making the incision follow as closely as possible 
the place where the absciss-layer will eventually be formed 
(indicated by the broken line in the diagram), it is almost 
possible to sever the base of the petiole without any exudation 
of latex appearing on either surface. 
5. In a leaf which has not yet reached maturity , these 
peculiarities are not observable; latex exudes from both 
surfaces alike, when the petiole is cut across at the base 
(such as A-B in the diagram). 
6. When the base of the short stalk (petiolule) of the 
leaflet of a mature leaf is cut through, latex appears on 
both surfaces. These leaflets are disarticulated by means 
of absciss-layers just as the petiole is, but do not resemble 
the latter in the peculiarity of the exudation of latex. 
These observations point to an obliteration of the cavities 
of the laticiferous vessels at the region of the petiolar base, 
on the leaf attaining maturity. Since the place of interruption 
to the flow of latex appears to correspond with the position 
of the absciss-layer, the idea naturally suggests itself that the 
special cells composing this layer are formed on the maturity 
of the leaf, and cause the closure of the laticiferous vessels 
at this point. A microscopic examination, however, shows 
no definite layer of cells at all in this position. The absciss- 
layer only seems to be formed about the time that the leaf 
changes colour previous to its fall. 
There are two marked differences between the microscopic 
appearances of the young and mature leaf-base. As the leaf 
assumes its adult condition, certain cells making a more or 
less broken layer across the base of the petiole become 
sclerosed to form ‘ stone 5 cells; there is also a marked 
deposition of cluster-crystals right across this region. Such 
structural changes suggest a closure of the laticiferous vessels 
