Mar. 15, 1924 
Studies on the Potato Tuber 
827 
shaping the external form of the tuber, must be taken into consideration, 
not for one, but for a number of generations. 
In the present study only one generation of plants grown in different 
localities (the States of Maine, New York, Pennsylvania, and Colorado) 
was included. The suggestive results obtained from this preliminary 
study appear to warrant further investigations along this line. 
In this comparative study the following terms, as defined, are used 
for convenience only, and are not considered as having strict morpho¬ 
logical significance: 
Pith: The central region of the tuber extending as an axial strand 
from stem end to apex. 
Medulla: Lateral connections which the pith forms with the buds. 
Cortex: Tissue between cambium and periderm. 
Periderm: The potato skin. 
Stone cells: Cortical cells which possess a thick, lignified wall. 
Pith and Meduula. —The pith forms the axial core of the tuber. It 
varies in thickness, attaining its greatest diameter a little below the 
center. At intervals branches in the form of compressed hollow cylin¬ 
ders leave the pith, take a more or less oblique vertical course, and con¬ 
nect each with one of the eyes. These medullary branches attain vari¬ 
ous proportions, and in cross section exhibit configurations which are 
more or less characteristic of the variety. The medullary branches 
vary in both form and distinctness. To be sure, young growing tubers 
have a less distinct pith and medulla than mature tubers, especially if 
the latter have been in storage for a certain time; yet when examined 
under identical conditions the tubers show differences which are dis¬ 
tinct and form a constant feature of the different varieties. In tan¬ 
gential section the medulla branches appear in the form of open or closed 
horseshoe-shaped areas, and as the radial cut is approached the relation 
of these areas to the pith and eyes can clearly be seen. 
CLASSIFICATION OF VARIETIES 
Medulla distinct and large, antler-shaped; pith commonly large.— 
Cobbler , Rural (PI. 9). 
Medulla distinct, either large or small; pith large.— Ohio , Up-to-date , 
McCormick (PI. 9). 
Medulla branches distinct and fine; pith usually small.— Triumph , 
Green Mountain , Rose (PI. 9). 
Medulla branches fine; pith either large or small.— Michigan , Hebron , 
Pearl (PI. 9). 
Medulla branches indistinct; pith varying in size .—Burbank. 
This classification, based on median cross-section view of tubers, is 
at best only a tentative one. While the medulla configurations are fairly 
constant, the pith is very variable, especially in tubers which are off 
type. Environmental factors appear to be of subordinate importance, 
since marked local variations are very pronounced; in other words, 
tubers grown in Maine and New York, for example, show no more vari¬ 
ation than those from a given field in either State. 
Cortex. —The cortex forms a narrow band of tissue limited extern¬ 
ally by the periderm, internally by the cambium of the vascular ring. In 
the region of the eyes the cortex becomes narrow and disappears com¬ 
pletely as the vascular tissue branches out into the buds of the eye. 
Aside from the variations in the region of the buds, the width of the cor¬ 
tex throughout the tuber shows a more or less pronounced fluctuation. 
