Mar. 15, 1924 
Studies on the Potato Tuber 
815 
tubers were, on the whole, normally developed; an examination of the 
periderm, however, showed that it differed from the normal in structure. 
The outer rows of cells were greatly hypertrophied (PI. 8, E» D). The 
enlarged cells were found in all parts of the tuber, even covering the 
lenticels. Since other varieties grown under identical conditions showed 
a normal periderm, no satisfactory explanation was forthcoming for this 
abnormal formation. 
If potatoes are cut and left in suitable surroundings, the surface of the 
cut becomes covered by a new “skin”—a wound periderm. The new 
cells appear in certain places and spread in all directions; the walls are 
laid down in a tangential plane. The cells resemble the normal periderm 
cells and, like the latter, perform the function of resisting evaporation 
and fungus attack. In this capacity the cells of the wound periderm 
appear to be even more effective than the normal periderm cells. Con¬ 
ditions which affect wound periderm formation have been the object of 
a great deal of investigation. The latest contribution, together with a 
review of the older literature, is by Edson and Shapovalov ( 4 ). 
As the stolon enlarges to form the tuber, the ring of bundles begins to 
lose its definite arrangement in groups, and, since the development of 
parenchyma within the bundles tends to split up the separate groups 
still further, the entire vascular tissue becomes greatly spread out to such 
an extent that the vascular ring of the mature tuber can be compared 
only by analogy with the original one. 
Cambium activity becomes evident even before the stolon tip enlarges 
to form the tuber, but the number of secondary elements thus added is 
conspicuously small, and only under favorable conditions does it attain 
the proportions shown in Figure 6. The secondary xylem cells are large, 
mostly porous, vessels with side walls heavily pitted (PI. 2, E). The 
length of the elements varies greatly wherever the cells have been de¬ 
flected from their normal course. Separating the radial rows of vessels 
are parenchyma cells homologous with the rays of the aerial stem. 
These cells rarely contain starch and their walls remain cellulose. The 
new phloem elements differ only in size from the cells of the primary 
tissue, and, like the former, they occur in small groups. The paren¬ 
chyma cells surrounding these phloem groups are smaller than the nor¬ 
mal storage cells, more elongated, and contain only fine-grained starch. 
In their entirety the tissues so far discussed—namely, cortex, pith, and 
vascular ring—form only a small part of the tuber. By far the larger bulk 
of tuber tissue lying between cortex and pith and divided into two 
unequal parts by the narrow vascular ring consists of large polyhedral 
parenchyma cells in which small islets of phloem are embedded. What 
is this tissue morphologically? An inquiry into the ontogeny of the 
tuber should assist in arriving at a satisfactory solution (fig. 3). 
The first change concomitant with tuber formation is an enlargement 
of the stolon in radial direction. This is brought about by successive 
cell division in the region of the pith. As a result of this localized 
growth, the vascular elements become deflected from their normally 
vertical course and are forced to continue in a more or less oblique 
direction. Simultaneous with division in the pith cells are changes in 
the peripheral cortex. To meet the dilation of the center of the stolon, 
the cells begin to divide radially whenever tangential stretching is unable 
to accommodate the constantly widening circumference of the organ. 
At the same time the cortical cells become filled with starch and occasion¬ 
ally dark cells of crystal oxalate appear among the white starch cells. 
