544 Blackman and Matthaei. — On the Reaction of 
number of cells at just a few places along the edges of the 
cuts in a leaf, as shown by the absence of brown colour : 
just at these places and at these only, as we have photographs 
to prove, the occlusion and abscission is very long delayed 
or may not take place at all. 
We conclude therefore that where the factor of excessive 
dryness does not come in, the reaction of the leaf is directed 
against its connexion with the dead cells, and not against the 
free communication of its interior with the open air. Speak- 
ing # teleologically, the leaf is presumably protecting itself 
against the risk of infection which might get a start in dead 
cells which are kept moist by diffusion from living ones. The 
cases in which abscission follows fatal local heating without 
destruction of tissue also point to the same explanation. 
When leaves freely cut open are left exposed to the dryness 
of the summer atmosphere, cells at the edge are soon killed 
off by natural drying, and we afterwards become unable to say 
which factor it is that provokes the subsequent reaction. 
(3) Another problem lies in the causational difference 
between periderm-formation and abscission. Both processes 
seem to arise at corresponding spots in the line of occlusion, 
and both start by division of a more or less definite line of 
cells into daughter-cells. What are the factors that decide 
whether these cells shall dissolve the middle lamella between 
them, round off and separate, or whether they shall remain 
united as a phellogen and a first layer of cork ? 
The relative dryness of the surrounding air appears to be 
the essential cause, but we do not know whether isolation of 
the leaves from the parent plant has any effect. Many 
observers have noted that dryness promotes cork-formation 
in various cases, and Massart 1 showed ingeniously that wounds 
opening to the atmosphere provoke more reaction than similar 
ones opening only internally into the natural hollow of a 
herbaceous stem, and also that only the layers exposed to 
external air became suberized. Massart 2 also quotes some 
very suggestive but inverse observations as to the effect of 
1 Massart, loc. cit., p. 46, Figs. 48-50. 2 loc. cit., p. 58. 
