POTATO WILT, LEAF-ROLL, AND RELATED DISEASES. 33 
scribed, and the effect on the plant is the same, though possibly in 
the western cases there have been more pronounced abnormalities 
in stolon and tuber formation than are described in the German 
literature. These effects are illustrated in Plate VIII, figure 2, and 
Plate IX, figure 2, which show the numerous stolons, often thick 
and white, bearing many small tubers, frequently strung along like 
beads. The few tubers which attain any size are generally clustered 
around the base of the stem, as in Plate IX, figure 2, This clustering 
is characteristic of leaf -roll. Kornauth and Reitmair (1909) say: 
"The stolons are greatly shortened. Many times the tubers are 
attached directly to the stem." 
AERIAL TUBERS. 
Aerial tubers are very frequent, and there is often a thickening of 
the upper stem and leaf petioles which seems to be another result 
of the plant's efforts to store starch above ground. (PL IX, fig. 1.) 
This is a distinct phenomenon from the formation of aerial tubers 
due to lesions on the stem caused by Bhizoctonia, for the leaf-roll 
cases show no trace of fungous injury. Neither of these characters 
is constant, however. Mr. Fritz Knorr informs us that "in 1911 the 
greater percentage of the plants took on this stoloniferous character 
and a smaller portion developed the aerial tubers; this year (1912) the 
reverse was the case. We had but few of the stoloniferous plants 
and very many of the aerial tubers." 
These are reactions of the plant to the abnormal physiological 
conditions accompanying the leaf-roll, which are in turn influenced 
more or less by moisture and food supply and by weather factors. It 
is easy to understand how aerial tubers are produced by the fungus 
Rhizoctonia, which causes lesions on the stem near the soil line and 
thus prevents the translocation of starch from leaves to tubers, for 
we can produce the same result by a mechanical injury, i. e., "gird- 
ling" the stem or by rooting a cutting from a potato shoot in such a 
manner that no node is covered by soil and stolons can not, in conse- 
quence, be formed. 
In those leaf-roll diseased plants which form aerial tubers there 
are no below-ground fungus lesions, and some other force, such as the 
phloem shrinkage described by Quanjer, must be acting to hinder the 
storing of starch in the tubers. 
There is evidence, as mentioned in the paragraph on the relation 
of enzyms to leaf -roll, which suggests that there may be unusual 
katabolic activities going on in the diseased plants, which would 
consume the carbohydrates formed in photosynthesis, leaving little 
or none to be laid by in the tubers during the period of leaf-roll 
prevalence. If, at a later date, under the influence of favorable 
weather, for example, an excess of starch was again formed in the 
