64 J. H. Priestley and Dorothy Armstead 
remain unstained whilst the protoplasts of cells immediately adjacent 
to the xylem will be stained. 
Experiments upon the same plan show the presence of readily 
permeable protoplasts around the vascular strand in the stem also, 
and in so far as this method can be trusted it supplies definite evi¬ 
dence that various cells bordering upon the xylem strand throughout 
its course show such permeability to dyes that they may be expected 
to permit the passage of various solutes to the sap within the xylem. 
II. Exudation Pressures in Stem and Leaves 
Wherever the xylem contains solutes, if the protoplast of the 
surrounding tissues are in contact at the exterior with a water 
supply less concentrated than the sap within the xylem, an exudation 
pressure should presumably be generated. In the root the conse¬ 
quence of such exudation pressures must be an upward stream of 
sap because the closure of the xylem channel at the root apex prevents 
all movement in that direction. 
Under suitable conditions, therefore, it should be possible to 
demonstrate the existence of exudation pressures in stem or leaf. 
Thus if a piece of stem be sealed at one end and immersed in water, 
sap under positive pressure might be expected to emerge from the 
open end of the xylem strand. Such an exudation pressure might 
well be only temporary, as in Angiosperms there is usually no 
functional endodermis in stem or leaf, so that leakage of solute from 
the vascular strand will be continuous, and the cause of the exudation 
pressure thus gradually disappear. 
In the first experiment a piece of stem of Helianthus annuus L. 
was stripped of its cuticle, one end dipped into melted wax and the 
whole piece subsequently immersed in water. The open end was 
attached by rubber tubing to a piece of capillary glass tubing, the end 
being freshly cut and the tubing attached under water, so that the 
experiment started with a water meniscus visible in the capillary tube. 
After twenty-four hours the meniscus had sunk out of sight be¬ 
neath the rubber tubing owing to absorption of this small quantity 
of water by the cells around the xylem, and the apparatus was nearly 
dismantled; but, fortunately, it was left for forty-eight hours longer, 
when the meniscus was again seen, and rose rapidly to the top of the 
capillary tube to fall again a few days later, presumably owing to the 
outward diffusion of solutes from the xylem. This led to a whole 
series of further experiments with various stems, both herbaceous 
