Leaves to Traumatic Stimulation . 
537 
from one another (Fig. 5), till the two portions of leaf are 
quite free. No further histological change takes place in the 
piece of leaf that falls off, but 
on the edge belonging to the 
main leaf these new cells go 
on multiplying parallel to 
the new edge, till we get the 
appearance of a meristem 
with a number of tiers of 
swollen cells beyond, gener- 
ated by its activity and giving 
theedgeavelvetylook. These 
cells have very thin walls but are cuticularized. Fig. 6 is made 
from a preparation in strong sulphuric acid ; the mesophyll 
has all been dissolved 
and the cuticularized pad 
of callus-like cells is seen 
to be attached to the 
cuticle at both surfaces 
of the leaf, so as to 
close in and protect the 
mesophyll as effectually 
as before. 
Some instructive vari- 
ations of the cutting-out Fi g< & 
process must now be 
considered. If a number of dead areas be produced close 
together there may not be sufficient sound tissue between them 
for the line of occlusion and the absciss-layer to be formed 
at the proper distance from the dead cells. The line of 
demarcation then sweeps round and includes several con- 
tiguous dead areas in one contour. 
The first photograph (Plate XXIX) shows a good case of 
this, as a line of occlusion envelops seven parallel strips of 
dead tissue and only attempts to pass between the upper two 
which are rather further apart. The second photograph shows 
the same leaf at a later stage with the whole patch of living 
Fig. 5. 
