24 BULLETIN 64, U. S. DEPARTMENT OF AGRICULTURE. 
This author finds that the phloem strands of leaf -roll plants are 
shrunken and the walls thickened and lignified, resulting in such a 
disorganized condition that the translocation of elaborated food mate- 
rials from the leaves to the tubers for storage is prevented or inter- 
fered with. 
This shrinkage of the phloem strands can be detected before exter- 
nal signs of leaf-roll appear, but it is not present in false leaf-roll due 
to mechanical injury, wet soil, bacteria, overfertilizing with kainit, 
etc., nor in the curly-dwarf disease. It is discoverable first after the 
young shoot from a diseased tuber has broken through the ground 
and formed several leaves. Each new branch is in the beginning 
healthy, but the diseased condition soon manifests itself. It can be 
traced upward, as the plant grows, to the tips of mature diseased 
shoots, and even to the petioles and midribs of the leaves and to the 
flower stems, but not on lateral leaf veins. The same pathological 
condition can be traced downward in the underground portion of the 
stem to the mother tuber, but it rarely appears in the stolons and 
never in the young tubers. 
This shrinkage of the phloem affords an explanation of many of 
the results of leaf -roll, including the thickened stems and formation 
of aerial tubers, which takes place when the products of photosyn- 
thesis can not be translocated. It may be connected with the 
"endurance of the mother tuber'' and with the higher percentage 
of nitrogen in the latter, since these compounds can not move so 
freely in the shrunken phloem. The reduction of growth and the 
lessened yield are attributable to the same cause. The rolling of the 
leaves is a natural reaction of the plant to a stem injury or stoppage. 
The observations of Quanjer led him to the conclusion that leaf- 
roll is hereditary and not parasitic and that the presence of fungous 
mycelium, bacteria, tyloses, and vascular discoloration are not char- 
acteristic symptoms of the disease. 
It is left undetermined how this phloem shrinkage is brought about. 
It has not yet been produced experimentally, nor have remedial 
measures been foimd, but the need is emphasized, as pointed out by 
Sorauer (1913), for more experimental work, under controlled condi- 
tions, on the influence of the several natural environmental factors 
on the potato plant. 
NONCOMMUNICABILLTY OF LEAF-ROLL. 
That leaf-roll is not communicable from diseased to healthy plants 
is the conclusion to be drawn from all available evidence. Appel, 
Werth, and Schlumberger (1910) report grafting a great number of 
diseased sprouts on healthy ones and vice versa. These were put in 
the greenhouse and union took place. The scions gradually died, 
however, after the plants were brought into the open air. During 
