78 J. H. Priestley and Dorothy Armstead 
to the fact that Dixon and Joly(H) in their experiments on the co¬ 
hesive force of liquids had neglected to take account of temperature 
effects, she proposed an alternative theory of the ascent of sap based 
on a mechanism invoking exudation pressures in the root. But no 
theory based on exudation pressures in root or stem can replace 
Dixon and Joly’s hypothesis based on the consequences of tran¬ 
spiration from the leaf. So long as the sap is ascending under negative 
pressure it must be driven by a force exerted from the highest point 
it reaches—the leaf. 
However, in some plants at all times (as in submerged water 
plants) and in many plants at some times (as in deciduous trees in 
the spring) ample scope can be found for the utility of exudation 
pressures whilst observation and experiment supply full evidence of 
their existence. Reinders(2i) has published a most clear and interest¬ 
ing discussion of the relation between the transpiration hypothesis 
of sap ascent and the hypotheses that assume the assistance of 
exudation pressures along the path of the ascending water column. 
He points out, as Copeland ( 7 ) had previously done, that the out¬ 
standing difficulty in the way of fully accepting the cohesion tran¬ 
spiration hypothesis, was the failure of its exponents to deal with the 
lack of experimental demonstration of the gradient of pressure in 
the path of the water column, that must be present if the column 
was entirely raised by the action of forces exerted in the leaves. 
In some interesting preliminary experiments Reinders showed 
that if a series of manometers were placed upon cut branches at 
different heights of the same tree, the pressures recorded were quite 
without reference to the vertical position of the manometer in the 
series, but that if the tract of stem was killed that was bearing the 
branches with the manometer, the pressure readings then recorded 
a suction gradually decreasing from the upper region towards the 
base of the stem. These experiments retain their suggestiveness, 
although further experiment has led Reinders (22) to realise the great 
experimental difficulties that lie in the way of obtaining significant 
records of intermediate pressures in the sap column from the readings 
of such manometers. 
Reinders’ experiments seem to us to provide valid support for 
the position taken up in this paper that whilst the transpiration of 
the leaves remains the main force actuating the ascent of water in 
freely transpiring land plants, yet exudation pressures due to 
parenchymatous tissues associated with the xylem tract, are always 
potentially possible. 
