Aug. 5, 1918 
Anatomy of the Potato Plant 
247 
Simultaneously with the change in the orientation of the protoxylem 
in the hypocotyl, go different, though not less important, changes in 
the phloem. In the root the primary phloem alternates with the xyletn, 
both tissues being arranged in distinct groups. In the stem the arrange¬ 
ment is bicollateral—phloem both inside and outside the xylem. The 
first noticeable change in the hypocotyl consists in the breaking up of 
the two phloem groups of the rootlet, with the formation of three to 
four smaller groups. These phloem strands orient themselves in such a 
way that two or three of the groups come to lie in the center of the 
stem between the now separating xylem groups (fig. 4, b, c). The other 
phloem groups take a position at the periphery of the stele close to the 
endodermis. At this stage the protoxylem groups have moved through 
an angle of 90°, and form a tangential row of cells. The outer phloem 
groups have meanwhile increased further in number so that some of the 
strands come to lie at the outer face of the protoxylem. The central 
groups of phloem have also divided and an increase in the size and 
number of the parenchyma cells in the center of the stele is forcing these 
groups away from the center and causing them to cdme to lie close to the 
protoxylem. The bicollateral condition is thus established (fig. 4, d, e, f). 
STOLON AND TUBER 
About six weeks after the potato tubers have been planted, branches 
arise in the axils of scaly leaves of the subterranean part of the stem and 
grow more or less horizontally outward. These branches, known as 
stolons, remain simple or fork and sooner or later swell at their tips to 
form tubers. 
The stolons, being modified stems, present a typical stem structure 
and the tissues go through the same process of development as do those 
of the latter. In the study of the development of the tubei it is neces¬ 
sary only to note the changes incidental to the enlargement of the stem. 
In the mature stolon, then, is found a ring of vascular bundles consist¬ 
ing of four or five larger groups with a few smaller ones between them 
(Pl. 38, A). The xylem is only weakly developed and is made up chiefly 
of primary elements, among which are a few vessels. The phloem, how¬ 
ever, is extensive, and the groups show the typical arrangement in both 
inner and outer region. The pith and the cortex are made up of large 
polyhedral cells, many of which are filled with crystal sand (calcium 
oxalate). A collenchyma and a specialized epidermis are not developed. 
The first change in the tip of the stolon consists of extensive cell divi¬ 
sion in the region of the pith and to a less extent of the cortex, resulting 
in a swelling of the stolon, which becomes at first oval, later spherical, 
the change from stolon to tuber being quite abrupt. This excessive cell 
division in the region of the pith causes the vascular tissue to bend out¬ 
ward; transverse sections at the proximal end of the tuber show the 
vascular tissue cut obliquely, in places even longitudinally. Potato 
