Leaves to Traumatic Stimulation . 543 
Being able to obtain the exfoliation-effect with certainty in 
the laboratory we have looked out for signs of it in the field, 
and have seen a number of specimens illustrating it, although 
it has never occurred in response to our experimental lesions. 
In natural specimens the green sound tissue on the distal side 
of the absciss-layer dries up quickly and turns brown before 
the piece drops out, so that the separation appears to be taking 
place at the exact edge of a dead patch as if it were only a 
mechanical rupture between dry and living tissue. 
In the laboratory, further, we have occasionally found that 
leaves which have remained for a long time with their stalks 
in water may proceed to exfoliate small areas which happen 
to have become injected with water ; and rarely, small exfoli- 
ations take place for which there is no obvious cause. We 
hope that further experiments will throw light on these cases. 
III. 
Finally, we may just touch on two or three problems and 
give a few tentative reflections that suggest themselves, in 
relation to the contrasted reactions that have been noted, and 
to the biological significance of the various processes. 
(1) Occlusion of the spaces in a spongy parenchyma would 
seem to be a necessary preliminary to the formation of a 
definite line of meristem in that tissue. Is this the whole 
significance of the occlusion-line, or is the process of direct 
value as a reparative one, closing in again the internal spaces 
of the leaf ? 
Though occlusion has been described as one of the types of 
wound-healing, yet we incline to the other view here, as it 
takes place very locally and is closely linked to the subsequent 
activity, and occurs so well in the leaves kept in damp air, 
which do not require for their continued sound health to have 
their spaces occluded at all. 
(2) The fact that leaves do not react to a clean incision* 
which kills only a minimal number of cells, seems to us to be 
of the first importance. We have several times found that the 
disorganization of rapid drying may fail to kill an appreciable 
