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BOTANICAL GAZETTE 



fig. 8. In this study no attempt was made to trace the relation- 

 ship between the vascular system of the stem and that of the root, 

 as has been done by Scott and Brebner (17) and others in some 

 plants having bicollateral stems 



Primary structures.— The smallest roots of /. Batatas are 

 frequently triarch or tetrarch; but the large roots are polyarch, 

 chiefly pentarch or hexarch. In 

 the young root there is usually a 

 solid arrangement of vessels with 

 no thin-walled parenchyma 

 between them, and the phloem is 

 well denned between the rays. 

 Laticiferous cells are early dis- 

 tinguished (fig. i,lc). The peri- 

 cycle consists of a single layer of 

 cells. The endodermis has sharply 

 defined Casperian bands, and the 

 remaining cells of the cortex are 

 large and have conspicuous inter- 

 cellular spaces between them. 



Secondary structures.— 

 Cambium is organized very soon, 

 and there is formed an extensive 

 secondary growth which in every 

 respect is like the secondary 

 growth common in roots (fig. 2). The secondary xylem forms 

 solid masses around the primary xylem; but wherever a lateral 

 root has been formed, there is a break in the secondary xylem 

 directly opposite the ray, for the lateral roots connect directly 

 with the protoxylem points. These breaks, like greatly widened 

 medullary rays, extend for some distance on each side of the 

 place of attachment of the lateral root. It was long ago shown 

 (22) that there may be as many rows of lateral roots as there 

 are protoxylem points, and this explains the definite rows of 

 lateral roots seen even in the fully matured potato. The amount 

 of the secondary xylem probably is largely dependent upon 

 conditions for growth. One root, which had grown above ground 



