4 
Colorado Experiment Station. 
of the surface were badly diseased as coiupared to those deeper in 
the soil. This field study of roots was supplemented later by a 
study of the root cap and by a study of the potato leaf and its 
adaptation to an arid climate. It was decided later to study in detail 
the effect of soil-temperature and soakage ufK>n potato plants grown 
during tlie winter of 1911 and 1912 in the College greenhouse 
(Figure i). 
Figure 1.—Greenhouse experiment to determine effect of soil temperature and 
soakage upon the potato plant. 
Careful study was given to the history and practices of the 
various potato-growing districts, and field evidence was secured 
which seems to the writer conclusive that the leaf-roll disease in 
Colorado had, in addition to these causes—soil-heat and soakage—a 
third cause, probably parasitic and in some way injurious and 
capable of distribution. 
Out of these studies, and out of field studies made later over 
Iowa, an attempt is made in the summary to make somewhat clearer 
the essential conditions for health in potatoes and for permanency 
in potato production. More space is given to detailed reports of 
observation and experiments, but what may be called evidence-in- 
