8 i6 
Journal of Agricultural Research voi.xxvii.no. h 
Soon after cell division in the pith is at its height and the above-de¬ 
scribed changes are going on in the cortex, the pericycle and the Mark- 
krone become the regions of greatest growth. The cells of the pericycle 
surrounding the primary groups of the outer phloem begin to divide and 
to enlarge rapidly, and as new cells are formed the older ones become 
filled with starch and soon become continuous with the cortex; in fact, 
they appear like wedges of the latter thrust into the zone of phloem tissue. 
However,, their appearance, while the continuity of the endodermis still 
exists, permits of no other interpretation than that these cells are pro- 
cambial in origin. Gradually, as the result of continuous cell increase 
in this tissue region, the cells of the endodermis fail to keep pace with the 
Fig. 2.—Cross section of vascular tissue of mature tuber. X 73. (Drawing is a copy of photomicrograph.) 
increasing circumference of the vascular tissue. Gaps appear here and 
there, gradually widening till finally all traces of this cell layer are lost 
in the starch-filled parenchyma, and only the outer circle of phloem groups 
indicates its former position. As a result of the activity of the pericyclic 
cells, the phloem groups become spread out; the individual strands become 
more widely separated from each other. vNew groups are formed as the 
tuber enlarges, and, grouped in larger or smaller constellations, they 
appear to the naked eye as dark dots surrounded by a halo of white 
starch cells. Detailed microscopic study shows the smaller groups to be 
made up of aggregations of the elements found in the ordinary primary 
phloem groups; i. e., a few sieve tubes with their companion cells and a 
number of conducting parenchyma elements. The cells surrounding 
