the Sweet Potato. 2 1 1 



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, phloem— or phloem-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 ordinar>' 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 Pcperomia 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 lacinese 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 



