538 Blackman and Matthaei . — On the Reaction of 
and dead tissue nearly exfoliated in one large piece. In the 
third photograph those circular dead patches which are well 
separated are each surrounded with an effective absciss-layer, 
but in the upper part of the leaf four such dead spots are 
close together and they are being cut out by one common 
absciss-layer. The appearance of this leaf is rather com- 
plicated by the large round hole in the middle of each dead 
area. In this case, circular areas of cells were killed by 
a round piece of hot iron, and sub- 
sequently, but this is of no present 
significance, the centres of these were 
removed by a cork-borer of smaller 
diameter than the piece of iron. The 
circular absciss-layer is formed in the 
sound tissue round the patch and 
so ring-shaped stretches of tissue are 
being exfoliated, and are at present 
attached by one or two points only. 
Where the absciss-layer has to pass 
through a vein the processes take 
place more slowly, but the largest of 
the secondary veins may be effectively 
dealt with, as the sixth photograph 
shows, and in this also the pieces 
are dropping out. We have not ob- 
served any case of the absciss-layer 
succeeding in passing through the 
midrib. It follows then that if a 
strip of dead cells extends across the leaf from the edge 
right up to the midrib it cannot be completely surrounded 
with an absciss-layer and thrown off. It becomes interesting 
to see what reaction will then take place. 
The leaf shown diagrammatically in Fig. 7 furnishes a case 
in point, and it merits detailed consideration. The lamina 
was cut and the marginal cells killed by rapid drying as 
in the other cases, but the treatment was more prolonged, and 
very broad dead brown margins border all the cuts. In the 
