Physiological Studies in Plant Anatomy 253 
meristematic activity of parenchyma occupying very different posi¬ 
tions in different plants. An analysis of these occurrences of the 
phellogen is attempted in the succeeding pages and it will be found 
that in every case good reason can be assigned for the assumption 
that the phellogen activity had to be preceded by a slow accumula¬ 
tion of sap either from the exudation activity of adjacent parenchyma 
or more usually from the adjacent vascular strands. The accumula¬ 
tion of the sap can in the majority of cases be traced to a previous 
blocking of a parenchymatous surface, occurring in close proximity 
to the region where the phellogen subsequently arises. This blocking, 
frequently due to deposition of suberin or cutin upon surfaces 
exposed to air, has often been reported but so far as we are aware 
its causal connection with the subsequent meristem activity has not 
previously been emphasised. Our approach to the subject from this 
point of view has led us to pay special attention to the chronological 
order in which the various processes associated in cork formation 
occur. 
We are hopeful that the simple causal sequence thus suggested 
in connection with the process of cork formation may prove of value 
in interpreting the results of practical experience in such processes 
as pruning, grafting and the cutting of potato sets before planting. 
We shall be the more gratified if that hope proves well grounded 
because the prosecution of the work was only rendered possible 
through a grant made to the University of Leeds by the Ministry of 
Agriculture. 
In the exposition of our special standpoint in reference to cork 
formation, it is a simplification to begin with the study of wound 
cork. Having established by reference to other investigators and by 
our own further experiments our case for a causal sequence in the 
case of wound cork formation, a brief discussion will show how the 
same general chain of causation may apparently be traced in the 
formation of cork in the cushion left upon leaf-fall and in the normal 
production of cork upon a perennial stem. 
I. The Formation of Wound Cork 
In one case of great practical importance, the tubers of Solanum 
tuberosum L., many statements are available concerning the pro¬ 
cesses following upon wounding, and these agree as to the main 
features. Appel (l) makes it very clear that if the cut surface of the 
potato is left at room temperature in a moist atmosphere, the first 
stage in healing is the deposition of an impermeable brown deposit 
