Leaves to Traumatic Stimulation. 539 
upper part of the leaf these dead strips extend right to the 
midrib, but in the lower part they just fall short of it. The 
position of the resulting absciss-layers is shown by the curved 
black lines, and everywhere, even in the upper part where they 
outline isolated tongues, one in each sound compartment of 
the leaf, the actual separation of tissues has taken place as 
definitely as if produced by a sharp scalpel. 
Here the absciss-line no longer runs a parallel course with 
the edges of the dead area. In the few places where it is 
approximately parallel it stands much further off than in the 
previous cases, which we think is related to the more extensive 
injury. Except at one point the absciss-lines do not run out 
to the edge of the leaf, but they curve round in the middle of 
the sound strips of tissue and join one another to form larger 
or smaller tongues, retaining a small amount of tissue inside 
their curves and leaving large masses of green healthy tissue 
outside to be exfoliated with the dead. This certainly has 
the appearance of being a radical surgical treatment of in- 
juries which are too extensive to be dealt with in detail, and 
we believe it really has this sort of significance. 
In the upper part of the leaf even this attempted radical 
cure fails, because the dead strips extend right to the midrib 
and so in no way can they be thrown off. In the lower part of 
the leaf there are very narrow tracts of sound tissue left round 
the central ends of the dead strips, and in all these cases the 
absciss-line extends along them from one tongue to another 
and so the dead tissue is all cut off, as on the left half, 
below, though with great sacrifice of sound tissue as well. 
The leaf, by a continuation of this treatment, would be reduced 
to a midrib bearing a few almost isolated tongues of sound 
tissue. 
Below the lowest injury the absciss-line makes its way to the 
edge and a quarter of the leaf is hanging, attached only above. 
It is further very interesting to note that where the absciss- 
line passes along the narrow bridges of sound tissue between 
the successive dead strips and the midrib it has to be formed 
extremely close to the dead cells, and not at the usual con- 
