396 Journal of Agricultural Research y 0 ]. xxxi, no. 4 
other buffer mixtures, light, age, death, temperature, and dissolved 
materials, should be made. The main thesis, however, that plant 
tissue acts much like an amphoteric colloid with definite isoelectric 
points appears clear, substantiated as it is by earlier work by the 
writers. Stearn and Stearn ( 9 ) have come to somewhat similar con¬ 
clusions in their work on the staining of bacteria. 
At the same time several facts also show that plant tissue does not 
act completely like a simple protein such as gelatm. The overlapping 
of the final equilibrium points noted in the experiments described here 
with all the tissues, except that of F. lycopersici , would not be expected 
with gelatin. It must be remembered, however, that protein and 
other materials of plant tissue are inclosed in a cellulose wall, and 
that the reaction in the cell may not be that of the solution in which 
it is immersed. The shifting of the so-called isoelectric point caused 
by ions other than the hydrogen and hydroxyl ions, noted m the earlier 
experiments on water absorption by potato-tuber tissue (5) in dilute 
buffer mixtures, would not occur if we were dealing with the isoelec¬ 
tric point of gelatin. The overlapping of the zones of absorption and 
retention of basic and acid dyes m plant tissue placed in solutions of 
different hydrogen-ion concentrations, noted by Rohde (7) and by 
one of the writers for living tissue, and by Naylor 4 for dead tissue 
does not occur with pure gelatin. The absorption of both anions 
and cations on both sides of the isoelectric point must occur, else 
organisms could not exist as they do over a comparatively wide range 
of hydrogen-ion concentration. This is not what would be expected 
from a substance like gelatin. 
What happens in the simple buffer mixtures used in the experiments 
reported in this paper is probably mucli similar to what Rohde found 
to be true for the influence of hydrogen-ion concentration on the ab¬ 
sorption of dyes, and pictured so clearly in the curves he presents. 
On the acid side 9 f wnat we have called the isoelectric pomt, both 
anions and cations are taken up, the former, however, more rapidly 
and in greater amount. The further we move toward greater acidity 
away from the isoelectric point, the more anions and the fewer cations 
are absorbed. On the alkaline side of the isoelectric point the situa¬ 
tion is reversed. 
In plant tissue we are probably dealing with several amphoteric 
colloids which have different isoelectric points. Owing to their am¬ 
photeric properties, these substances may interact with one another 
and form one or more amphoteric compounds which may be necessary 
to life. That the constituents of the cell do not all act alike, at least 
when dead, has been shown by Naylor. With sections of root 
tips he has found that the cytoplasm, resting nucleus, chromosomes, 
and nucleolus possess different isoelectric points. 
We may also be considering not the isoelectric point but the point 
of maximum undissociated ampholyte molecules, the p maximum of 
Michaelis (3). The isoelectric point, as shown by Michaelis, is not 
affected by salt formation, but the p maximum is affected by salt 
formation. The hydrogen-ion concentration for it may coincide with 
the isoelectric point, or may lie on either side of it, depending upon the 
dissociation constants of the salts formed. This possibility is sug- 
■*Naylor, E. E. the effect of the hydrogen-ion concentration upon the staining of plant 
tissue by basic and acid dyes. 1924. [Unpublished thesis, Univ. Mo.) 
