262 J. H. Priestley and L. M. Woffenden 
the same season as the leaf falls, Tison’s data suggests that the 
suberised cap of tissue is formed unusually early so that the leaf 
scar is blocked before the sap pressure has ceased to be a significant 
internal factor with the advent of the autumn. 
One small point of difficulty arises from a consideration of 
Tison’s observations upon those compound leaves in which absciss 
layers are formed not only at the base of the main petiole but also 
at the base of the leaflets. In two cases, Juglans regia L. and Pyrus 
aucuparia L., he describes a periderm, very slight in character, as 
forming at the base of the leaflet scars, in the tissue of the petiole 
after its fall from the stem. This phenomenon was unexpected, but it 
is a point to which it is hoped to direct further attention as 
opportunity offers. So far, in the fallen petioles collected in this 
country, no trace of such a periderm has been found, but in 
Fraxinus excelsior L. a cork meristem has been found at the base 
of leaflets which have fallen before the main petiole. Probably 
closer examination will show that all cork layers at the base of 
leaflets have arisen whilst the meristem was still able to receive 
supplies from a petiole still in communication with the stem. 
III. Normal Cork and Lenticel Formation 
In the light of the previous discussion of wound cork and of the 
periderm arising below a leaf-scar the problem of normal cork 
formation upon stem and root may be visualised from an unusual 
angle. In any species the position of the cork phellogen seems quite 
definite but in different species this meristem arises in very different 
positions: it may be nearly superficial or even arise in the epidermis 
or it may be as deep seated as the pericycle. This question of the 
position in which the periderm arises obviously requires analysis to 
see whether it admits of correlation with a supply of sap from the 
vascular tissue and a blocked surface external to the phellogen 
which provides for an accumulation of this sap. 
In the root the phellogen may occur in one of two alternative 
positions, either just within the endodermis (the usual position 
—de Bary(2), loc. cit. p. 553) or just within the exodermis. A very 
extensive analysis of cases will be required before any generalisation 
can be advanced with confidence to cover all cases of periderm 
formation in the root, but the following tentative conclusions have 
been found to cover all cases so far studied. When the growing root 
is young, the endodermis is in the primary stage (Priestley and 
North (22)) with fatty substances deposited in the radial and trans- 
