Mar. 15 1924 
Studies on the Potato Tuber 
829 
Starch. —The quality of starch is determined by the percentage of 
large grains it contains. The ability to produce a large percentage of 
superior grains is, according to Saare (i< 5 ), a variety character, which is 
reasonably constant. In the breeding of varieties for a definite purpose 
a knowledge of the starch content of the different varieties might be an 
important factor which would enable the experimenter to make such 
crosses as would be most promising in results. Parow ( 12 ) found in 
his studies that in the variety Tannenberg the percentage of superior 
grains was as high as 50, while Weddingen yielded only 11 per cent of 
high-quality grains. In American varieties a group classification based 
on the quality of the starch grains could also be easily worked out, espe¬ 
cially if there exist such striking differences as in Green Mountain and 
Cobbler as seen in Plate 2, B, C. Since material for investigation is; 
not available at present, the information to be gained from such a study 
must be reserved for a future publication. 
THE PERIDERM 
The periderm of the potato has been the object of frequent investi¬ 
gation, and pictures of its structure may be found in any of the text¬ 
books on botany. A comparative study of the periderm of different 
varieties, however, began only when differences in the disease resistance 
of certain varieties focused the attention of the pathologist on this 
natural and effective barrier to infection—the potato skin. 
More than 50 years ago Sorauer made a study of the structure of the 
periderm of nearly 75 varieties. He made exact measurements of the 
thickness of the periderm, the number of cell rows, and the size of the 
individual cells. He noticed that deep planting tended to produce 
a thinner skin than shallow culture and that fertilization of the soil had 
the same effect. In his infection experiments with late blight he found 
that fertilization and deep planting produced a higher per cent of diseased 
plants and that the red varieties were more susceptible than the white 
ones. From this he concluded that thickness of the skin is correlated 
with disease resistance. A few years later, however, it was shown by 
Ress and Bretschneider that the very thick-skinned varieties were 
even more readily attacked than the thin-skinned ones, and that thick¬ 
ness of the skin in itself did not insure immunity. In 1908 Kreitz (7) 
extended the work of Sorauer by making extensive investigations on 
the effect of environmental and soil factors on the structure of the 
periderm. Kreitz found that the thickness of the periderm of a given 
variety was not a constant factor, but varied with changes in environ¬ 
mental conditions. In opposition to Sorauer he noticed that dryness 
tended to produce a thin skin and that the different varieties behaved 
variously. Thickness of the periderm acquired in a new locality would 
be retained for a generation or more, even after the tubers were trans¬ 
ferred back to their original home. Finally, however, they showed the 
same kind of periderm as they originally possessed. Application of 
fertilizers gave varying results in that potassium and nitrogenous 
fertilizers tended to produce a thin skin, phosphoric acid fertilizers a 
comparatively thick skin. Since the thickness of the periderm in itself 
appears to be no factor in disease resistance, Kreitz suggested that the 
ability to regenerate periderm cells in wounded places would be of im¬ 
portance in preventing infection with bacteria. In this connection the 
work of Appel (1) is of significance. Appel found that in the variety 
