242 
Journal of Agricultural Research 
Vol. XXIV, No. 3 
nodes are shortened and often appear almost telescoped. Withal, the 
morphological changes are of a type akin to the symptoms of leafroll 
and foot diseases. 
An examination of a section through a swollen area discloses to the 
naked eye a dark discoloration of ^e tissues. A microscopic study 
shows further that both vascular and cortical tissues are affected and that 
aside from necrotic changes there are regions of abnormal cellular activity. 
Sections through the advancing margin of a young pustu|e show that the 
first evidence of pathological changes is found in the cambium region. 
The xylem has matured irregularly, giving the peripheral region of the 
tissue a jagged appearance. In places one may observe mature groups 
of xylem embedded in parenchymatous tissue and completely separated 
from the vascular ring. In the more central part of the swelling the 
tissues appear completely disorganized. In cortex and pith are small 
groups of brown cells surrounded by concentric rings of thin-walled 
periderm cells. This latter type of necrosis is, however, frequently met 
with in normal plants, or is found in connection with other injuries. 
The phloem tissue, in the affected areas, shows the same pathological 
condition observed in general. The cell wall and content show a brown¬ 
ish discoloration, and in extreme cases entire cells or cell complexes may 
be obliterated. 
NECROSIS IN CONNECTION WITH STEM STREAK 
Potato tubers of the variety Irish Cobbler which were grown in the 
greenhouse developed extensive stem lesions on both young and maturing 
shoots. The lesions may best be referred to as streak,” the affected 
areas being elongated and brownish in color. Their advance along the 
stem is acropetal. In the nodal region the browning is especially pro¬ 
nounced and extends into the lower part of the petiole. 
The lesions are the result of necrotic changes in the cells of the collen- 
chyma and adjacent tissues. The hypodermis is often still green while 
the collenchyma cells underneath are brown and in a state of oblitera¬ 
tion. Soon, however, being cut off from the water supply, both epidermis 
and hypodermis die and the walls take on the same brown color as the 
cells underneath. In advanced lesions the necrotic areas extend through 
cortex and vascular tissue into the pith. In the nodal region the entire 
pith is reduced to a cavity lined with obliterated and discolored paren¬ 
chyma cells. If the epidermis from an advanced lesion is removed and 
cleared in chloral hydrate, the microscopic examination shows scattered 
through the homogeneous mass of brown cells small areas of dark- 
colored cells. These areas usually cluster around a stomate and extend 
centripetally into the pith. 
The beha^dor of the phloem cells is of special interest in this connection. 
Severe necrosis may be observed in both inner and outer phloem (PI. 2). 
The cells are brownish in color, the lumen distinct or partly closed by the 
pressure of the surrounding tissue. Characteristic intercellular spaces are 
also formed, and progressive lignification of the type diagnostic of leafroll 
is not uncommon. The necrotic phloem groups, however, seem to be 
restricted and limited by the extent of the external symptoms, there 
being no regularity either in their vertical or lateral distribution. It still 
remains to be shown whether this disease is definitely connected with 
leafroll, and, if in part distinct, whether certain aspects of the disease are 
not the result of secondary infection by the leafroll virus. 
