260 J. H. Priestley and L. M. Woffenden 
in the fact that the endodennal barrier in the 3'oung root prevents 
the accumulation of sap essential to the phellogen (see p. 256). 
Schneider-Orelli( 27 ) has made some observations on apple and 
pear fruits which also admit of explanation from this point of view. 
The young fruits react to wounds by forming cork tissue, the ripe 
fruits do not. But this is not merely a question of age because the 
young fruits removed from the tree no longer form cork when 
wounded. The conclusion is natural that the cork formation depends 
upon the sap pressure still active in the young fruit upon the tree 
and no longer operative when the fruit is removed from the tree or 
has ripened. 
Finally a reconsideration of the phenomena of wound healing in 
the case of leaves shows that they fall readily into line from this 
point of view. Blackman and Matthaei( 4 ) have given a general 
account of the wound reaction in leaves. It may be recalled that all 
the various modifications of the process described show the following 
features in common. First a solid mass of tissue is formed at the 
cut surface by outgrowths from adjacent mesophyll cells, this pro¬ 
ceeding pari-passu with the dying of the cut cells and some of their 
neighbours and the suberisation of the walls at this region. Within 
this solid mass of tissue, which seals off the internal intercellular 
spaces from the external air, there now develops a meristem in some 
cases but not in all. Under conditions of considerable moisture this 
meristem layer may act as an absciss layer and exfoliate tissues 
bordering on the injury; under drier conditions it forms periderm. 
From our present standpoint these phenomena make a natural 
sequence. The only point we wish to emphasise is that under certain 
conditions, and in some plants, meristem formation follows the 
occlusion of the air spaces and sealing of the cut surface. This took 
place in Blackman and Matthaei’s experiments, even in leafy 
branches cut off from the tree, but kept with their bases in water and 
their leaves in a moist atmosphere. Under these conditions (Priestley 
and Armstead (21)) it is quite possible for sap pressure to be developed 
within the tissue after the closure of the cut base of the branch by 
suberisation. The subsequent formation of callus, and, in the case of 
Oleander leaves, of crops of adventitious roots, is evidence that such 
sap pressure had indeed developed and therefore meristem formation 
was possible. 
To obtain further evidence we have carried out experiments with 
injured leaves of Prunus Laurocerasus L., and Camellia japonica L., 
in which the cut leaves have been supplied with water, either by 
