34 BULLETIN 64, U. S. DEPARTMENT OF AGRICULTURE. 
leaves, but some physiological defect prevented its prompt transloca- 
tion to the below-ground tubers, it would be laid up in thickened 
branches and aerial tubers. 
An interesting and important line of study in pathological physi- 
ology presents itself in the determination of the ways in which leaf- 
roll potatoes differ from healthy ones. Doubtless a better knowledge 
of the nature of leaf -roll will lead to a determination of its cause. Up 
to the present but little more has been done than to diagnose leaf-roll 
more accurately and separate it from other maladies with which it has 
been confused. 
CAUSE OF LEAF-ROLL. 
The hypotheses as to the cause of leaf-roll are numerous but exceed- 
ingly varied. They have indeed only one point in common — that 
all are as yet unproved. It has been argued by one that leaf-roll 
results from the use of unripe tubers for seed; by another, that it is 
due to the employment of matured tubers for seed; while a third 
believes that seed from prematurely ripened plants is a cause of leaf- 
roll. The disease is attributed by some to a lack of mineral elements 
in the soil, while others advance evidence that it is caused or aggra- 
vated by an oversupply of these same mineral elements. Poor 
cultural methods, lack of seed selection, and varietal degeneration are 
other suggested causes. The struggle between those who believe 
leaf-roll due to fungi and those who think it nonparasitic is nearly 
fought out, with the victory apparently in sight for the latter. Many 
signs now point to the plant breeder as the one who will finally 
triumph over this malady. 
The present-day opinions on the cause of leaf-roll may be briefly 
reviewed. (Appel and Schlumberger, 1911.) 
The relation of fungi to leaf-roll has already been briefly summa- 
rized. Much more on this point will be found in the writings of 
Himmelbaur, of Kock and Kornauth, and of Appel and Schlumberger. 
(See " Bibliography," pp. 44-48.) 
On the question of using mature or immature seed, Hiltner (1905) 
is the leading advocate of the stand that the immature seed stock 
gives an abnormal growth. On later evidence, he limits this to 
those potatoes which are prematurely ripened by drought or other 
untoward circumstances. Against this is to be balanced the very 
extensive use, with good results, of immature tubers for planting. In 
Scotland, particularly, this is held to be the best practice. Hiltner 
(Appel and Schlumberger, 1911) further holds leaf-roll to be the 
result of excessive applications of fertilizer of unbalanced composition 
at the wrong time. He considers that the concentrated salts, espe- 
cially potash salts, enter the roots and cause a disturbance in nutrition. 
Through the presence of these salts in the vessels, the water in them 
is prevented from rising. He thinks that these salts also favor the 
