OCCURRENCE AND SIGNIFICANCE OF PHLOEM 
NECROSIS IN THE IRISH POTATO' 
By Ernst F. Artschwag^r 
Assistant Pathologist, Cotton, Truck, and Forage Crop Disease Investigations, Bureau 
of PUmt Industry, United States Department of Agriculture 
INTRODUCTION 
In a recent publication Esmarch (5)* makes the following generaliza¬ 
tion in regard to the condition of the phloem: 
Necrosis of the phloem is fotmd in both normal and diseased pl^ts and is always 
present in mature organs; if necrosis is observed during the early life of the plant, it 
IS an indication of premature ripening. 
In support of this theory, which is based oh his own observations and 
supplemented by the data of other writers (7), Esmarch compares the 
changes in the phloem of the potato to those tal^g place in the secondary 
phloem of woody plants. Here, with the disintegration of the nucleus 
and the formation of callus deposits over the plates, the sieve tubes 
become inactive. The empty elements, deprived oif theit turgor, col¬ 
lapse and are crushed by the surrounding tissues. As obliteration 
progresses the old phloem often becomes so changed structurally and 
chemically ^ no lohger to resemble its former state. However, while 
the obliteration of the phloem in woody plants, as has been shown by 
numerous investigators (jj, is comprehensible on structural and physio¬ 
logical grounds, siniilar in the phloem tissue of herbaceous 
plants may not be expected, nor is their frequent occurrence reported 
in literature. Indeed, our present knowledge is restricted to two short 
notes by Eopdle and Schuman. : Boodle (4) observed in Helianihus 
annynis ^e occurrence of sieve tubes and companion cells whpse walls 
were ligmfied And! :who^ gave reactions resembling tho^ of 
lignin.., 3chuman,v(<^) men^^^ sclerosis of the phloeru takes place 
in some few WPPdy Composites and giyes and 
as examples. A systematic study of the mature phloem, car¬ 
ried on recently in the botanic^ laboratory of Cornell University, indi¬ 
cates cle^ly that most herbaceous plants do not exhibit any changes in 
the phloem upon hiaturation; where changes do take place, the phloem 
elements become lignified without obliteration. 
A consideration of anatomical changes in diseased plants has often 
been fruitful in providing diagnostic symptoms for the identification of 
plant diseases, provided, however, that the ontogeny of the normal 
parts was fully understood. It is imperative, indeed, to distinguish 
clearly between normally expected tissue changes, such as take place 
1 Accepted for publication July ii, 1922 . 
» Reference is made by number (italic) to “Literature dted/* p. 245. 
Journal of Agricultural Research, 
Washington, D. C. 
jibw 
(237) 
Voi: XXIV, No. 3 
Apr. 2X, X923 
Key No. G-396 
