the Sweet Potato. 211 
weaker ones between, and an irregular, interrupted ring of five 
or six bundles remains. These narrow down one by one 
until the xylem is represented by a single spiral trachea 
(Fig. 3). This trachea is surrounded on all sides by a 
zone of angular, radially arranged, phlem—or phlem-like 
cells; on the outer or centrifugal side this zone is five or six 
cells thick, on the inner side one or two cells thick. In 
longitudinal section the spiral trachea is accompanied by 
numerous sieve tubes and companion cells. Proceeding 
basipetally, the trachea disappears entirely; a little farther 
on, the sieve tubes give place to two or three pro-cambioid 
cells, which in turn are followed by small parenchymatous 
cells, and finally by ordinary pith. This transition takes 
place in the length of half an inch from the end of the spiral 
trachea. Thus the normal stem structure is reached, though 
this may be at a distance of two to five or six feet from the 
root of the plant. At no point do the vascular tissues of the 
two systems connect, though they merge into a common 
circular meristem tissue at the apex of the stem, where neither 
is yet differentiated. 
Hitherto there have been described two undoubted cases of 
ring-fasciation, and two doubtful ones. In 1891, H. de Vries 
(5) described a spadix of Peperomia maculosa 30 cm. high, 
of which the upper 15 cm. were hollow. The lower solid 
portion was 0.7 cm. in diameter, spreading to 2 cm. at the 
top of the tube, where it was split into four lacinee of differ- 
ing size and length. The hollow spadix was covered inside 
and out with flowers, which lay in both cases above their 
bracts, showing that the inner development, like the outer, 
was acropetal. There were two close rings of distinct fibro- 
vascular bundles, one on either side of a ring or hollow 
cylinder of pith, as we have just described in the sweet potato. 
These bundle systems were entirely separate throughout. 
The inner system, with phloem facing the cavity of the stem 
