536 Blackman and Matt had. — On the Reaction of 
The fourth of the photographs in Plate XXIX shows this 
stage, but only as seen by reflected light, when the line is not 
nearly so obvious. In the photograph there can be distin- 
guished (1) the original cuts between the leaf-veins made with 
a sharp knife, (2) the brown fringes round the edges of the cuts 
produced by cells that have been killed by rapid drying, and 
(3) the fine sharp occlusion-line running round each killed area. 
After a few more days the epidermis on both surfaces of 
the leaf will be found to be cleanly split through, all along 
the track of the line. The splitting extends steadily through 
the substance of the mesophyll and in another week or two 
the separation will be complete, the circumscribed areas will 
drop out, and leave such a leaf as is shown in the fifth 
photograph — a perfectly healthy leaf which has thrown off all 
the dead portions and has its edges closed in again all round. 
We have worked out roughly the histology of the separation- 
process by cutting sections through the lamina at right angles 
to the track of the translucent line. An absciss-layer arises 
by the division of a single row 
of cells running from one epi- 
dermis to the other through 
the occluded spongy paren- 
chyma and the palisade-tissue. 
As soon as the cells along this 
row have divided into two the 
cuticle seems to crack sharply 
across and the contiguous epi- 
dermis-cells in the region of the 
line become separated from 
one another without themselves taking any part in the 
division. Fig. 4, which is rather schematic, shows this 
stage, and it is to be noticed as a constant character that 
the absciss-layer does not form in the middle of the solid 
occluded portion of mesophyll, but always on the side towards 
the dead cells. 
The new cells of the absciss-layer are poor in cell-contents, 
and the two layers of cells soon round off and so separate 
Fig. 4 - 
