Physiological Studies in Plant Anatomy 137 
papers of this series, others are still under investigation. We hope, 
however, that this discussion has proceeded far enough to show the 
wide interest of the botanical questions involved. 
Summary 
1. This paper presents an attempt to re-investigate the micro¬ 
chemistry and structure of the endodermis to see how far the facts 
support the view recently advanced as to its function in the plant. 
2. Until now micro-chemical studies of the endodermis have been 
made mainly on sections of tissues. A method is described of using 
cylindrical strips of endodermis from Potamogeton perfoliatus L. 
which has been found much more convenient. 
3. At the outset, certain stages in endodermal development are 
defined, as they need separate consideration both as regards structure 
and function. 
4. The embryonal stage is not discussed, as the cells have the 
properties characteristic of those of the apical meristem, discussed 
in another paper of this series. 
5. The primary stage is characterised by the presence of the 
Casparian strip. The rest of the wall is at this stage cellulose bounded 
externally by a normal middle lamella. 
6. The Casparian strip consists of impregnating substances of 
various kinds, including derivatives of fatty acids and substances 
giving the reactions characteristic of lignified membranes. These are 
embedded in a basal wall substance which is of unknown composition. 
7. The unknown basal substance present in the Casparian strip 
is characterised by its great resistance to concentrated acids and 
alkalis; it contains some nitrogen but is not a protein. It is probably 
allied to the substance that occurs in the cell membranes of the apical 
meristems of many roots. 
8. The undulation characteristic of the Casparian strip can be 
produced both by swelling the cell membranes in concentrated 
sulphuric acid and by contracting them by heating in strong potash. 
It is due to the unequal expansion or contraction of the different 
constituents of the membrane. 
9. The primary endodermis will permit the passage of w'ater and 
of solutes capable of diffusing through the protoplasts, but the 
Casparian strip greatly impedes the passage both of water and solutes 
through the cell membranes. 
10. The secondary endodermis is characterised by the presence 
of a suberin lamella, which always lines the inner tangential wall 
