i6o 
Journal of Agricultural Research 
V'ol. XXVII, No. 3 
pericycle opposite the phloem groups. As the root enlarges, cells are 
produced by the cambium more rapidly in the inner region, and the 
cambium cylinder becomes symmetrical. Occasionally the irregularities 
in the activity of the cambium ring continue to be present throughout 
the growth of the tuber, causing the formation of deep invaginations 
which are especially characteristic of certain varieties, as Nancy Hall 
and Belmont. 
The endodermis of the young root is very prominent, but gradually 
the cells become stretched in the tangential plane and show signs of 
disorganization. The cells of the cortex increase in number to accom¬ 
modate the widening circumference of the growing organ. The inter¬ 
cellular spaces grow larger, whereby the tissue acquires a very loose 
texture; the epidermal cells become torn and lignmed. 
Those parts of the sweet potato roots which do not become thickened 
undergo similar changes in their ontogeny. But early during the 
differentiation processes, the cells between the protoxylem points and 
the large central cell become lignified (fig. i). Miss McCormick ( 4 ) 
states that a certain amount of xylem is developed in all young roots; 
but when a root enters upon tuber formation, the xylem mass is broken 
up by the development of a parenchymatous sheath between its elements. 
This observation, however, was not verified. The enlargement of the 
young roots into fleshy roots is initiated at a very early stage in the 
development of the root, and always precedes the differentiation of 
xylem on the inner face of the protoxylem groups. 
With the differentiation of the cambium cylinder, the cells around the 
protoxylem groups begin to show a more regular arrangement. Soon a 
cambium becomes distinct around each of the groups. This develop¬ 
ment of secondary cambiums is not limited to the protoxylem groups, 
but becomes a general phenomenon. (Fig. 6). Simultaneously with 
the formation of secondary cambiums around the protoxylem groups, a 
new meristematic zone develops around the central cell. At first only 
one cell wide, the number of cell rows of this new cambium increases 
rapidly and soon forms a tangential band of appreciable width. At a 
somewhat later stage a number of cells in close proximity to the primary 
cambium mature into xylem. They, in turn, become surrounded by a 
secondary cambium which continues activity for a limited period. In 
the later development of the root there appear, independent of the vas¬ 
cular groups, cambiums in the form of bands or circles, producing xylem 
toward the center and phloem toward the periphery. In the original 
phloem groups new cambiums may also arise, which produce secondary 
elements in the regular manner. 
The primary cambium cylinder is meanwhile actively dividing, form¬ 
ing xylem and phloem, but mostly thin-walled storage parenchyma. 
The xylem elements are few in number, yet show a certain regular arrange¬ 
ment in radial rows. Around each of the newly formed xylem cells a 
complete or partial cambium may develop, which produces new elements 
in the regular manner. Often, when the amount of tissue produced by 
the secondary cambium is considerable, as can best be seen in the region 
of the large central cell of the young root (PI. 1, B), a tertiary cambium 
develops around a number of the secondary elements and increases in the 
same way as did the group from which it arose. Sections through fleshy 
roots, which exhibit in mature condition the typical ridged structure 
previously mentioned, show the ridges traversed by vascular bundles. 
(Fig. 4.) These bundles originate in the primary cambium cylinder, 
