Physiological Studies in Plant Anatomy 263 
verse walls and with the protoplast firmly attached right across the 
cell to this girdle, the Casparian strip. As this fat impregnated layer 
is very impermeable, and at this stage the protoplasts are also rela¬ 
tively impermeable the endodermis provides a very complete barrier 
to the outward leakage of organic solutes. If a cork layer is now laid 
down the phellogen becomes active within the endodermis and the 
cork is therefore pericyclic in origin. 
As the root grows older, the protoplasts of the endodermis 
become more permeable, but this alteration is usually associated 
with an accumulation of fatty substances all over their surface and 
the consequent formation of a suberin lamella. The cells of the 
endodermis have then passed into the secondary stage (Priestley and 
North (22)). If all the endodermal cells pass over into this stage then 
the endodermis has become a still more effective barrier, preventing 
the diffusion of inorganic salts as well as organic solutes. Within 
such a secondary endodermis a cork layer frequently arises in the 
pericycle, but no case has ever been seen of a phellogen active in the 
cortex outside such an endodermis. 
On the other hand the fat deposits sometimes fail to accumulate 
in certain passage cells which therefore remain uncoated with a 
suberin lamella. The protoplasts of these cells are now readily 
permeable to organic solutes, as may be shown by the ease with 
which they admit the entry of organic dyes, and such an endodermis 
permits the formation of a cork phellogen external to it in the 
cortex. When such a phellogen arises, it always appears just below 
the exodermis which may be expected to act as the necessary 
blocking surface causing the accumulation of the sap flowing from 
the vascular tissues. An example of such a cortical cork layer 
arising outside an incomplete secondary endodermis and beneath a 
sclerenchymatous exodermal region is provided by the aerial root 
of Philodendron eruhescens. Monster a deliciosa Liebm. provides an 
example of an aerial root which as it grows older never produces a 
secondary endodermis. As the protoplasts of the primary endodermis 
become more permeable, cork appears below the exodermis. In a 
root of Monstera, 20 feet long, obtained from the Cambridge Botanic 
Garden, a primary endodermis appeared within a few (8) inches of 
the tip; 27 inches from the tip a cork phellogen was visible and two 
or three layers of cells had been formed by its activity; at the base 
of the root six to eight layers of cork cells were present and a thick 
band of sclerenchyma had also formed outside the endodermis, but 
this endodermis itself remained primary. 
