436 
AMERICAN JOURNAL OF BOTANY 
[Vol. io. 
of the buffer mixtures was 6.95 or more. Dead potato responded to a 
greater degree to staining, but qualitatively showed the same results. 
Using buffer mixtures of 0.002 M secondary sodium citrate and 0.002 M 
NaOH, the greater amount of dye was retained by those pieces from solu¬ 
tions of pH 6.2-6.3 or less while it was lost in washing by those pieces from 
solutions of pH 6.3-6.5 or greater. The acid dyes did not combine to any 
extent with the starch grains found in the cells of the potato-tuber tissue. 
With basic dyes the starch grains were stained, and pieces of potato from 
buffer mixtures of 0.002 M H3PO4 and 0.002 M NaOH stained rather 
uniformly. However, with care in staining and washing, it was demon¬ 
strated that basic dyes like safranin, crystal violet, and methylene blue were 
retained more strongly by potato from the solutions of pH 6.95 or greater 
than by those from solutions of pH 4.9-6.1. 
Discussion 
The analogy between the staining qualities of potato treated with 
different buffer mixtures and that of a protein like gelatin is good if we 
assume the isoelectric point of the potato-tuber tissue to be near pH 6.0. 
The potato-tuber cells, containing other materials like carbohydrates such 
as starch and pentosans and fat globules, and being surrounded by a cellulose 
wall, could not be expected to give such sharp and clear-cut results as the 
ash-free gelatin used by Loeb. The action of the basic dyes illustrates this 
point. It should be noted here that the position of what appears to be an 
isoelectric point as found by the method of water absorption agrees very 
well with that found by the dye method. In both cases it is located in the 
vicinity of pH 6.0. Attention should also be called to the fact that the 
death of the tissue, so far as these experiments gave evidence, did not affect 
the position of the isoelectric point. 
General Discussion 
The results which are reported in this paper on the absorption of water 
and on the absorption and retention of acid dyes by potato-tuber tissue 
can be explained by the assumption that an ampholyte, possibly a protein, 
plays the chief part in these processes and that its isoelectric point is in the 
vicinity of pH 6.0. The absorption of basic dyes by potato-tuber tissue is 
complicated by the fact that the starch, the cell walls, and possibly other 
constituents of the cell such as the pentosans, combine with the basic dyes 
at all the reactions involved. It is of course difficult to conceive of proto¬ 
plasm as consisting of but one ampholyte with a single isoelectric point. 
The point to which we have called attention here may represent the resultant 
of the isoelectric points of several cell constituents. 
While it is not considered that the hypothesis that potato-tuber tissue 
acts, in the processes indicated, like an ampholyte with an isoelectric point, 
