192 
/. H. Priestley. 
To understand the possible mechanism by which the water in 
L can be continually supplied to M, it will therefore be necessary 
to discuss two questions, (1) the significance of the endodermis, 
(2) the excretion of water by superficial hydathodes. 
The endodermis. As the water passes inwards from the cortex 
of the root the endodermis is the first continuous, unbroken layer 
of cells encountered (a somewhat similar layer may be present 
near the exterior of the root in the form of an exodermis and 
possibly its function may be interpreted upon somewhat similar 
lines to those adopted in the subsequent discussion upon the 
functions of the endodermis). 
The continuous layer of endodermis in a young and functional 
root encloses a core of tissue in which parenchyma, sieve tubes 
and xylem vessels are tightly packed without intervening air-spaces. 
A very characteristic feature of the endodermis is the 
thickening of the radial walls, which usually have the same 
staining reactions as the lignified xylem walls, but this radial 
thickening is not itself merely lignified. On the contrary, there is 
very considerable evidence (Haberlandt, 6, loc. cit. pp. 368, et seq.) 
that this radial thickening is suberised and impermeable to water. 
The same thickening of the radial wall is visible in longitudinal 
sections of the root and it is easy to establish that this impermeable 
strip is continuous all round the radial transverse and radial 
longitudinal walls of the cell. 
This means that, as the water passes from the cortex of the 
root into the central core through the cylinder of endodermis, it 
cannot follow a path across the endodermis such that it remains 
always in the wall, it must enter the protoplasm. 
De Rufz de Lavison (4, 1911) has brought out this point very 
clearly. He has shown that even when the cells are vigorously 
plasmolysed, the protoplasm is not withdrawn from the suberised 
strip, the plasmolysed protoplast always taking the form shown at 
* pi * in Text-fig. 1. 
De Rufz de Lavison (3, 1910 and 4, 1911) has also shown that 
solutes in the water surrounding the root, if unable to penetrate 
the protoplasm, can diffuse into the root as far as the endodermis, 
diffusing inwards by the walls, but they are then completely stopped 
at the endodermis. Similarly if the root tips are cut across, and the 
cut ends dipped into water so that this is then drawn up through 
the xylem by the ascending current due to transpiration from the 
leaves, if the ascending water currents contain a substance, 
incapable of penetrating the protoplasm, but whose passage through 
