826 
Journal of Agricultural Research 
Vol. XXVII, No. ii 
Dixon (j) obtained similar figures for the phloem area of potato stolons. 
(A stolon 1.6 mm. in diameter had a cross sectional area of the phloem 
of 0.42 sq. mm.) From experimental evidence and theoretical considera¬ 
tions, Dixon concluded that organic substances could not pass through 
the phloem tissue in such amounts as must pass during the active growth 
of the tuber and that it is the xylem which conducts most of the food. 
However, in the stolon the xylem does not exceed the phloem area, or 
is, at best, only twice the area of the sieve tubes proper. In the tuber 
the xylem is greatly reduced, while the phloem is correspondingly 
increased. For the stolon it may be granted that xylem and phloem 
share equally in the transport of organic substances; in the tuber, however, 
the phloem and the extensively pitted storage parenchyma are probably 
the channels of translocation. 
COMPARATIVE STUDIES 
Differences in the external morphology of the tuber, such as shape, 
color, type of eyes, have been used in the classification of potato varieties. 
It was found, however, that climatic and soil conditions greatly impaired 
the diagnostic value of these characters, and therefore it became neces¬ 
sary to consider the plant as a whole in the classification scheme. An 
anatomy of the potato tuber would be incomplete without at least an 
attempt at a study of such internal characteristics as might aid in the 
task of grouping potato varieties or establishing their relationship. Yet, 
if such a study is to be fruitful, the effect of environment, so potent in 
