42 J. H. Priestley 
water from the root hair to the protoplast adjoining the xylem vessel 
depends upon a gradient of absorbing power and not of concentration 
of osmotic substances. Another serious omission in the earlier paper 
was the neglect to point out that Ewart ( 5 ) had already clearly 
shown that such a gradient of absorbing power might well arise as 
a result of the restriction placed upon the distension of the proto¬ 
plasts adjoining the vascular strand. 
The only positive contribution made by the present writer in 
this connection was to show that the mechanical structure of the 
endodermis was extraordinarily well adapted to facilitate the forma¬ 
tion of such a gradient of absorbing power. This is merely an applica¬ 
tion of the earlier results of Schwendener(i2) who investigated so 
thoroughly the mechanical properties of the endodermal cylinder. 
It is, however, essential to distinguish between different types of 
endodermis, as is indicated more fully later. The endodermis in the 
absorptive region of the root always possesses a well-marked Cas- 
parian strip but has no other strengthening devices. That the 
Casparian network itself is a relatively rigid structure is a fact that 
forces itself upon the botanical student at an early stage of his 
studies. As sections of roots are cut, the extra strain thrown upon 
the junction between rigid strip and elastic cellulose wall leads fre¬ 
quently to visible deformation of the wall in the neighbourhood of 
the strip. In many cases the whole central cylinder breaks away 
from the cortex, such breaks almost invariably occurring in the im¬ 
mediate vicinity of the Casparian strip. 
2. The Supply of Solutes 
It seems desirable to state a little more fully the case for the 
mechanism providing the supply of solutes to the xylem. In the 
original statement use was made of a comparison with the excretion 
of sap from the capitate cells of superficial hydathodes, and certain 
experimental evidence was brought forward in support of this analogy. 
The data as to the condition for the excretion of sap by superficial 
hydathodes were supplied by the work of Lepeschkin(8). Blackman 
(■ loc. cit. p. 107) points out that the interpretation given by Lepeschkin 
of his experimental results is very much open to question and that 
the value of the work lies in the data supplied. 
It is therefore emphasised that the only use made of Lepeschkin’s 
work in the earlier paper (Priestley, loc. cit. p. 193) was to utilise his 
data. The only theoretical statement adopted from his work was 
that a different permeability of the protoplast membrane on different 
