Leaves to Traumatic Stimulation . 545 
prolonged exposure to damp air on parts that normally form 
only cork. Thus in damp air the lenticels of Sambucus will 
grow out into tufts which consist of long multicellular filaments 
produced by the underlying phellogen ; and even the phellogen 
under the leaf-scars on branches of Populus may react in the 
same way, although many layers of cork have been previously 
produced by if. The intumescences on Hibiscus , worked out 
by Miss Dale 1 , seem to belong to the same category in their 
causation. 
This stimulation of a phellogen to grow out into filaments 
seems somewhat analogous with the stimulation of a meristem, 
so that the sister-cells become turgid and tend to round off 
from one another as in abscission. As soon as an abscission- 
line is formed in a dry atmosphere the parts of the leaf cut off 
outside it dry up, and so the line appears to have arisen close 
to the dead patch and not back in the sound tissue as it has 
done in reality. 
(4) The distance from the lesion at which the reaction 
takes place seems to be constant for a given amount of injury, 
as the photographs all show, and to be further back the 
greater the killed area, but other factors also come in, as with 
the leaf in Text-Fig. 7. A curious case is given by M assart 2 
in which a reaction takes place at several centimetres from the 
seat of the injury. If an internode of Impatiens Sultani be 
cut through at the top no change takes place there, but 
a reaction takes place at the very bottom of the internode, 
where abscission takes place — as in leaf-fail— and the whole 
internode is thrown off. Also with many leaves, if the midrib 
be destroyed, the leaf becomes cut off at the normal position 
at the base of the petiole and a leaf-scar is formed there. 
(5) Finally, the addition of the new phenomena which we 
have described to the previous stock of known reactions to 
traumatic lesions complicates very much the attempt to 
distinguish definitely between callus and wound-cork. Frank 3 
defines wound-cork as being formed by the passage of pre- 
1 Dale, Philos. Trans. Roy Soc., vol. 194, p. 163, 1901. 
2 loc. cit., p. 61. 3 Frank, loc. cit, pp. 59, 60. 
