194 
J. H. Priestley . 
Text-fig. 2 represents 
sufficiently the conditions prevail¬ 
ing in one of these excretory 
hairs, where we have a series of 
cells with permeable cellulose 
walls, lined by semi-permeable 
protoplasmic membranes, and 
with internal osmotic concentra¬ 
tions such that C4 is less than C3 
is less than C2 is less than Cl, 
but C4 is greater than the osmotic 
concentrations of the epidermal 
and mesophyll cells surrounding 
it. 1 
Fig. 2. Diagram of multicellular 
hydathode of Phaseolus mnltiflorus. 
and solutes will continue so long as there are sufficient solutes to 
maintain Cl higher than C2 and also high enough to generate a 
hydrostatic pressure sufficient to force water and solutes through 
the upper surface of the cell. It is important to notice that in a 
hydathode functioning in this manner (compare the hydathodes of 
Colocasia anti-quorum , see p. 198) the excretion, if arising from the 
causes explained above, will never be pure water. In fact it is 
difficult to see how the excretion of pure water under such 
conditions could be harmonised with thermodynamics; water 
would be moved and work done, apparently without energy being 
expended, as the osmotic concentration within the cell would then 
remain unaltered. 
The solutes in the excretion were usually inorganic. 
Lepeschkin found for example, the following percentages of 
inorganic ash in the excretions from superficial cells of the plants 
named :— Phaseolus 0*4%, Abutilon 0*5%, Nicotiana 0*1%, Poly podium 
0*2%, Camellia 0*5%, Lathyrus 0*5%. He found organic substances 
in the excretions in the following cases;— Lathy rus odoratus (alkali 
salts of organic acids); Vida sativa and Polypodium aureum 
(glucose). The excretion was alkaline, except in the case of 
Lathyrus, CaHCO a was usually present in a relatively large 
proportion amongst the inorganic salts. 
Returning to our present problem of root pressure, it will readily 
be seen how the data cited above enable us to fashion a hypothesis to 
overcome our difficulty of the passage of water from the cells (as L, 
Text-fig. 1) bordering on the xylem vessels, into the xylem vessels. 
1 Bayliss (2 loc. cit. p. 163) describes the osmotic concentration in these 
hairs as decreasing from base to apex, but undoubtedly he has quoted 
Lepeschkin incorrectly. 
