46 J. H. Priestley 
and frond, save in the absorbing region of the root and in regions 
close behind growing apices, is already in the secondary stage (see 
Basecke (i)). In the Eusporangiate Ferns on the other hand, a primary 
stage endodermis is retained throughout life (Mager( 9 )); whilst 
throughout the Ferns a tertiary stage endodermis does not seem to 
occur. 
In conclusion, brief reference may be made to one further point 
in the original paper to which Blackman takes exception. 
It was pointed out that the water issuing from the hydathodes 
terminating leaf veins was sometimes practically pure water, 
although it seems most unlikely that any mechanism exists within the 
root likely to drive pure water into the xylem vessel. A recon¬ 
sideration of the scheme originally suggested by Pfeffer(iO) to ex¬ 
plain the exudation of pure water, and which is re-discussed by 
Blackman ( loc . cit. p. 112), only confirms the writer in the impression 
that a physiological mechanism acting in this manner is exceedingly 
difficult to visualise and so far has no experimental support. On the 
other hand, for the exudation of solutes into the xylem vessel and 
the resultant sap pressure, there is ample evidence, and ready com¬ 
prehension of the physical mechanism involved. Unless, therefore, 
this hypothesis breaks down under employment it seems the natural 
one to adopt. 
The exudation of pure water through the hydathodes terminating 
the veins seemed a possible example of failure of the hypothesis 
but on examination presents no difficulty. It was suggested that 
the solutes exuded into the vessel at its base might be withdrawn 
higher up as the result of adsorption, followed by “physiological 
absorption.” Blackman considers the introduction of such a sub¬ 
sidiary hypothesis as seriously weakening the value of the theory. 
This surely depends upon the extent to which the subsidiary hypo¬ 
thesis is in accordance with the facts and the writer regrets again 
that he failed originally to draw attention to Ewart’s (G) [loc. cit. 
pp. 349-351) earlier convincing experiments upon the adsorption of 
solutes during their passage along the xylem. 
Further discussion of the question must await the publication of 
data, accumulated by the writer in conjunction with Miss Armstead, 
upon the removal of sugar during its passage in solution through the 
xylem. It may, however, be pointed out that the use of the phrase 
“physiological absorption” was not intended to convey the idea of 
any process more “ unknown ” than that which takes place whenever 
solutes, diffusing into cells, accumulate within them because they 
