Journal of Agricultural Research 
Vol. xiv. No. e 
230 
Just as the vessel is the most important element of the xylem, so is 
the sieve tube the principal and most interesting element of the phloem. 
In the potato the tubes have a cylindrical shape, with end walls strictly 
transverse. A single sieve plate occupies nearly the entire transverse 
wall, and no plates are found in the radial and tangential walls. The 
plate appears to be perforated by a large number of circular pores, as is 
seen clearly in Plate 33, B, and in the enlarged view (PI. 33, D). Sieve 
fields as such do not exist in this plant; each large pore of the plate repre- 
sents the sieve field of forms with primitive phloem. There is little 
variation in the size of the sieve tube in both inner and outer phloem. 
The length of the individual sieve-tube segment is, on the average, 138 /*; 
the diameter of the lumen varies from 17 to 32 ju. Whenever branching 
of the bundle and anastomosis occurs, the size of the sieve tube varies 
much more (PI. 34, A, B). At such places the elements are short and 
comparatively broad (PI. 35, A, B). When the sieve-tube mother cells 
undergo division, the larger of the two cells formed becomes the sieve 
tube proper; the smaller one, retaining its nucleus, becomes the com¬ 
panion cell. The number of companion cells formed by a single sieve- 
tube mother cell varies, but as a rule it is not more than one. Some¬ 
times the mother cells do not undergo division, and thus there may 
occur a series of sieve tubes without companion cells. Besides sieve 
tubes and companion cells, we find conducting parenchyma in the phlo¬ 
em. These cells are not always distinguishable from the tubes in cross 
section, since they have about the same size and the same delicate walls. 
However, in radial section, they are seen to be elongated, rectangular 
cells, with end walls bearing simple pits very unlike the multiperforate 
end walls of the sieve tubes (PI. 32, B). 
The phloem fibers are long and awl-shaped, with much thickened 
secondary walls which later become lignified. A small lumen is usually 
present in these cells, but pits are wanting. The diameter of the cells 
varies greatly, fluctuating within the limits of 19 and 40 ix (PI. 46, A, B). 
The cells of the cambium (PI. 32, D) are of the general shape and pro¬ 
portions of tracheids; the ends are pointed, the terminal walls following 
an oblique tangential course. In radial section the sloping character of 
the end walls is not apparent. In cross section (PI. 30, A) the cambium 
cells are rectangular, with the greater diameter in the tangential direc¬ 
tion. Secondary medullary rays, of course, arise in the cambium from 
typical cambium cells which undergo a definite number of transverse 
divisions. These medullary initials persist in the cambium and are seen 
in tangential section to constitute a part of that tissue (PL 32, D). 
The remaining cells of the vascular cylinder, the endodermis and the 
pericycle, resemble closely the cells of these tissues as usually found in 
herbaceous dicotyledons. The elements of the pericycle are cylindrical 
parenchymatous cells which vary greatly in size. The endodermis as 
shown in Plate 29, D, is composed of a single layer of cells which differ 
