828 
Journal of Agricultural Research voi. xxvn, no. h 
These differences may be local or affect an entire side, so that one longi¬ 
tudinal half of the tuber has a broad cortex, the other half a narrow one. 
External irregularities also tend to affect the width of the cortex; but 
an external swelling may not always indicate a correspondingly wider 
cortex; on the contrary, this tissue in such regions is often narrower 
(PI. 9). 
CLASSIFICATION OF VARIETIES 
Wide cortex (&-n mm.). 
Medium cortex (6-7 mm.). 
Narrow cortex (3-6 mm.). 
Hebron. 
Michigan. 
Early Ohio. 
McCormick. 
Up-to-date. 
Burbank (seedling). 
Cobbler. 
Rose. 
Burbank. 
Green Mountain. 
Triumph. 
Rural. 
Spaulding Rose. 
The varieties which are most likely to possess an irregular cortex are: 
McCormick, Hebron, Pearl, and Michigan. In the Burbank group, the 
varieties have a fairly even cortex, except the Burbank seedling, 4 which 
shows great irregularities. The latter variety differs further by having a 
broader cortex than the other members of the group. The Green Moun¬ 
tain and Rural types have as a rule a very regular cortex, though in the 
Green Mountain the width may sometimes vary. Climatic conditions 
appear to influence the development of the cortex, inasmuch as rapid 
growth in the early development of the tuber tends toward the formation 
of a broader cortex. However, it is safe to assume that a wide or narrow 
cortex, if constant in a number of specimens, is characteristic of the 
variety regardless of the external conditions under which the tuber was 
grown. 
Stone Ceees. —When tubers of certain varieties are sectioned through 
the region of the bud, large lignified cells are noticed. The occurrence of 
these so-called stone cells is so constant that potato varieties can be 
divided into two classes—those in which stone cells are present and those 
in which they are wanting. While the presence of stone cells is a varietal 
characteristic, there is a great deal of variation in their number. The 
Rural group is distinguished by developing stone cells very sparingly, and 
in some varieties, like Heavy Weight, they seem to be absent altogether. 
CLASSIFICATION OF VARIETIES 
Stone cells present. 
Stone cells wanting. 
Numerous. 
Sparingly developed. 
Peachblow. 
Burbank (seedling). 
Superba Irish. 
Triumph. 
Russet Burbank. 
Rural. 
Rural No. 1. 
Carman I. 
Green Mountain. 
Triumph. 
Early Eureka. 
Pearl. 
Charles Downing. 
Carman III. 
Gold Coin. 
Rose. 
Heavy Weight. 
Ohio. 
McCormick. 
Burbank. 
Hebron. 
Russet Rural. 
White Rose. 
Spaulding Rose. 
Russet Burbank. 
Cobbler* 
4 Name under which William Stuart is carrying one of the Burbank strains. 
