Physiological Studies in Plant Anatomy 63 
concerned only with the question of the provision of the solutes in 
the xylem sap. Certain experimental evidence based on vital staining 
has been given (Priestley, loc. cit.) for the existence of relatively 
permeable protoplasts that might prove a possible source of these 
solutes. This evidence is first briefly reconsidered. Such relatively 
permeable protoplasts may be found throughout the vascular strand, 
and the next section of the paper is devoted to experiments demon¬ 
strating the development of exudation pressures by stem or leaf, 
when placed under suitable conditions. 
Subsequently data are given as to the quantity and nature of the 
solutes present in the xylem, and the possibility examined of the re¬ 
absorption of these solutes as they move along the vascular strand. 
There is, finally, a short discussion of the significance of these exuda¬ 
tion processes as part of the mechanism concerned with the rise of 
water in the plant. 
I. Experiments with Stains and Living Tissues 
The essential idea of these experiments is that the penetration of 
the complex molecule of a dye into a protoplast may give some 
indication of the permeability of the membrane of that protoplast, 
especially if subsequent experiment shows that the stained proto¬ 
plast is still capable of plasmolysis. In this case the protoplast mem¬ 
brane evidently retains its semi-permeability and Osterhout has 
shown ( 17 ) (loc. cit. p. 144) that this behaviour is a good index of its 
continued vitality. 
In the earlier paper (Priestley, loc. cit. p. 193) it was shown that 
certain cells internal to the endodermis were exceptionally per¬ 
meable to various dyes introduced into the xylem, and the analogous 
behaviour of the apical cells of multicellular excretory hydathodes 
was emphasized. Further work has confirmed these earlier observa¬ 
tions and the experiment has withstood the test of repetition by 
a large class of students. 
A wider experience of experiments of this type has brought to 
light one very interesting fact. When acid and basic dyes are used 
a comparison of the results will often reveal striking differences. 
Thus with Vida Faba L. if o-i per cent, solutions of acid green or 
light green F.S., both acid dyes, are drawn up the xylem vessel of 
the root, the protoplasts of certain cells of the pericycle, especially 
the cells just opposite the protoxylem group, will be stained. If 
malachite green, a basic dye, be used instead, these protoplasts will 
