264 J. H. Priestley and L. M. Woffenden 
One further case deserves mention. Mylius(i 7 ) has described a 
polyderm formation which is practically a type of multiple secondary 
endodermis. Within the original secondary endodermis a phellogen 
becomes active and cuts off two or three rows of cells. One of these 
rows then becomes converted into a secondary endodermis by the 
deposit of a suberin lamella upon every cell, the meristem then 
becomes active within this and the process may be repeated many 
times. A magnificent example of such a polyderm is provided by 
the raspberry, Rubus Idaeus L. 
In the stem, Douliot(6) shows that the cork may be superficial, 
pericyclic or intermediate in origin. In view of the very numerous 
cases to be examined a causal explanation of the position taken by 
the phellogen can only be very tentative in nature. The following 
observations seem, however, to be relevant. In all the cases of 
pericyclic cork so far examined, with one exception, the stems have 
been found to possess a complete secondary endodermis within which 
the phellogen arises. Examples are provided by most dicotyledons 
growing upon British peat (Priestley and Hinchliff ( 25 )), by Escal- 
lonia rubra, Saxifraga rotundifolia and Camellia japonica. Many 
underground rhizomes, as for instance several species of Geum, 
possess a secondary endodermis, within which a cork layer arises, 
the activity of this phellogen often leading later to the exfoliation 
of the cortex. All these cases are obviously in line with the general 
hypothesis put forward in this paper. 
The exception is provided by species of Ribes in which no func¬ 
tional endodermal barrier can be found outside the deep seated 
phellogen. But experiment in which dyes such as acid green were 
forced under pressure into these stems showed that these dyes failed 
to leak into the outer cortex, save in the region of the nodes, even 
before any cork barrier prevented their diffusion. Further examina¬ 
tion led to the conclusion that the superficial layers of the stem of 
these plants were really petiolar in nature (compare Saunders ( 26 )) 
and save at the nodes were still delimited from the inner regions of 
the stem by the presence of a cuticle which could be traced by 
careful staining with Sudan III. The phellogen in this plant is 
not pericyclic in origin and its apparently deep seated origin is 
due to the presence of this envelope of petiolar tissue upon the 
stem. 
The case of stems without a functional endodermis now requires 
consideration. In these stems it is a very suggestive fact, well 
established by the researches of Devaux( 5 ), that periderm formation 
