54 2 Blackman and Matthaei . — On the Reaction of 
the original cells takes place, only septation, and this is the 
typical £ wound-cork 5 of Frank k 
(6) The exfoliation by an absciss-layer described by us in 
Prunns Laurocerasus forms a sixth type of reaction. 
Here then we have six modes of reaction, apparently very 
distinct from one another, yet on reflection there is no doubt 
that the leaves of the Cherry-Laurel show processes that sug- 
gest comparison with each one of them. The absence of 
structural reaction ; the occlusion of spongy parenchyma ; 
the outgrowth of bulging cells at the edge after exfoliation ; 
the development of the solid pad of tissue in which the meri- 
stem forms; and the periderm-formation in attached leaves, are 
five processes, having many points in common with the first 
five types of reaction respectively. In addition there is also 
the exfoliation-process. 
The variations in the reaction seem to be correlated with 
the nature of the surrounding conditions, and it is clear that no 
further progress can be made in sorting-out these different 
processes without bringing the problems from the field into 
the laboratory where the conditions can be carefully controlled. 
For this to be successful, leaves of marked vigour and vitality 
are essential, and those of the Cherry-Laurel promise well 
in this respect. We hope to carry further the investigation of 
the controlling effects of external conditions. 
It may be mentioned here that only fully grown leaves have 
been used in all cases, and as most of the work was done 
in the early summer, the leaves used were necessarily those of 
the previous year : in J uly, however, the newly matured leaves 
give similar results. Very young leaves possess a considerable 
power of regenerating lost tissues, as distinguished from the 
carrying out of mere healing processes, and in them the epi- 
dermis may divide as well as the mesophyll and new typical 
epidermis may be generated. This normally happens with 
these fenestrated leaves which develop large perforations as 
they unfold 2 . 
1 Frank, loc. cit., p. 6o. 
2 Massart, loc. cit., p. 55, Fig. 47, also p. 30. 
