8io 
Journal of Agricultural Research voi. xxvn, no. n 
axial direction. Near the apex the cells are radially elongated; the 
tangential walls are short and equal in extent to the vertical walls. More 
distant from the growing point the tangential dimension of the cells 
increases, while the depth remains about the same. The tangential 
walls of the cells are slightly arched; the outer wall is always thicker 
than the inner, and is covered with a thin cuticle. Some of the epidermal 
cells are specialized to form guard cells of the stomates; others enlarge 
and divide to form cover hairs. Both stomates and hairs, however, are 
only sparingly found on the young stolon tip, and, because of subsequent 
development of the periderm, they have only a temporary existence. 
The young epidermal cells are devoid of chlorophyll and crystals, but 
rich in protein and solanin. The latter, a glucosidal alkaloid, is also found 
abundantly in the meristematic regions of the eyes at the time of renewal 
of growth in the spring. 
The cells of the fundamental meristem enlarge without specialization, 
the 'region external to the procambium becomes the cortex, the zone 
inclosed by it becomes the pith. 
The cortical parenchyma of the young stolon forms a comparatively 
broad band of tissue, about 9 cells wide and 345 microns in thickness. 
The typical cortical cell is rounded polygonal, slightly elongated or isodia- 
metric; its two diameters are, on the average, 81 and 62 microns, respec¬ 
tively. Near the outer periphery the cells are sometimes collenchymatous 
and have a greater vertical extent. The cells at the inner margin are 
smaller and grade into the endodermis. Farther from the growing 
region the cells elongate vertically, but often remain isodiametric or 
become broader than deep. After the differentiation of the cortical 
cells from the meristem of the growing region, intercellular spaces form 
between them. These spaces are very narrow and appear in a cross 
section as small triangles. 
Simultaneous with the formation of these intercellular spaces is the 
appearance of the first visible storage product—starch. The first starch, 
usually in the nature of fine round grains, is found in the cells near the 
procambium; later in larger deposits in other cortical cells. In the 
peripheral cells of the cortex, which are usually devoid of starch, protein 
crystals in the form of cubes of varying size are occasionally found; in 
the mature tuber they occur abundantly. Other cells of the cortex are 
conspicuous by a dark deposit which fills the entire lumen. This deposit 
is granular and consists of small crystals of calcium oxalate embedded 
in the nitrogenous remains of the protoplast of the cell. 
The pith of the young stolon is small compared to the area of the 
cortex. The individual cells are vertically elongated and have a slightly 
smaller diameter than the cells of the cortex. Both pith and cortical 
cells are profusely pitted; the pits are minute and arranged in char¬ 
acteristic groups. Intercellular spaces of the nature found in the cortex 
appear close behind the growing region; starch and crystals, however, are 
deposited only at a later period. 
The procambium, the progenitor of the vascular tissue, forms a hollow 
cylinder with small projections into the pith. It is composed of small, 
elongated, thin-walled cells with abundant protoplasm. In their*further 
development these cells either merely enlarge or become highly special¬ 
ized, forming the elements of the vascular system. The first specializa¬ 
tion in the procambium becomes evident in the region of the procam¬ 
bium projections, for, if a cross section of the stolon be examined prior 
to tuber formation, several groups of vascular tissue with gaps separating 
