23 ^ Jownal of Agricultural Research voi. xxiv, No. 3 
in the phloem of woody plants, and induced abnormal states, the latter 
alone being truly pathological. The apparent discrepancies noted by 
different investigators may, and usually do, become intelligible if we 
consider all pathological changes and trace the causes of the various 
types of necrotic conditions, if possible, to their sources. 
A classical example of such discrepancies is afforded by the researches 
on the pathological anatomy of the potato in connection with leaf roll. 
To be sure, the views of the different writers are substantiated by obser¬ 
vations, but since statistical data are incomplete at best, only a pur¬ 
poseful and systematic investigation of the problem can provide adequate 
data, even though factors which govern plant growth be not altogether 
neglected in the analysis of results. 
To arrive, at some definite basis as to what constitutes a healthy 
potato plant from the viewpoint of the anatomist, and under what con¬ 
ditions ^e phloem will remain normal, an investigation of a large number 
of plants of both cultivated varieties and indigenous South American 
forms was carried on. Such studies have been in progress since the 
sunder of 1916, and the data obtained have been embodied in part in 
, wlier publications (j, and have been extended and partly modified 
in ;tHe present paper. Further work on the pathological anatomy of 
potato dfeeases will doubtless bring forth new interesting facts and 
greatly aid .in analyzing these complicated potato disorders of which 
the cause is only vaguely understood. 
THE NORMAL PHLOEM 
The vascular tissue of the potato plant shows a bicollateral arrange¬ 
ment of its elements, a condition most clearly seen in the larger stem 
bundles (fig. i). Tlie primary phloem, external to the cambium, is 
made up 01 small groups of cells more or less continuous; the groups 
constituting the inner phloem are very variable in size and scattered. 
Groups of primary phloem appear also in the interfascicular region, 
where they may be seen on both sides of a well-developed cambium. 
Through branching and anastomoses the individual groups in each region 
communicate with one another, while through branch and leaf gaps a 
simil^ connection is effected between the inner and outer phloem. 
Dimng the early vegetative development only groups of primary 
phlpein are seen in a cross-sectional area of the stem. However, when 
tuber formation is under way, the cambiuni gives rise to a broad band 
of phloem (fig. 2). The amount of this secondary phloem varies with 
the ioeation. In the nodal region the amount exceeds that found in the 
intemode, a^nd in a given cross-sectional area, the larger amount is always 
found on the face of the larger stem bundles. 
While secondary phloem elements become differentiated and take part 
in the translocation processes, the primary phloem groups remain active 
until the plant is mature, llie walls of the cells thicken slightly, and 
occasionally callus deposits coyer the plates of the sieve tubes. Other¬ 
wise, there are no noticeable changes, either structural or chemical, 
characteristic of the phloem of the mature plant. 
Local necrotic changes in the parenchymatous tissue, however, may 
be observed in any potato plant. The diseased areas are usually re¬ 
stricted and do not extend vertically for any appreciable distance. In 
the distal Stem region, especially the node, such small pathological areas 
are of frequent occurrence, but they usually disappear in the maturing 
