igo8] THIESSEN—DIOON EDULE 363 



section is irregularly oval. These four groups extend downward for 

 a short distance, where they form the protoxylem of the primary root 

 (a, figs. 4, 5, 8). 



The potential vascular tissue. — Above the vascular plate there is a 

 conical or dome-shaped mass of tissue terminating in the growing 

 point of the stem, sharply marked off from cortex and pith, and con- 

 sisting of long, irregular cells with dense protoplasm and prominent 

 nuclei. The frequent occurrence of mitotic figures shows it to be 

 the most active meristematic region of the stem, being the tissue in 

 which the vascular strands are developed, and therefore the procam- 

 bium (pc, figs. 8, 34, 35). 



The pith in the vascular plate contains scattered and short xylem 

 elements (x, fig. 8), as stated above. Above the plate it is conical 

 and terminates in a tier of cells against the epidermis (pt, fig. 8) ; in 

 this region the cells are thin-walled and very irregular. In all the 

 pith mucilage cells and cavities are abundant. 



The vascular strands. — All the vascular bundles are collateral, 

 except in the upper part of the cotyledons, where they may be regarded 

 as concentric. In the leaf strands the collateral bundle is surrounded 

 by a sheath, which is not well-defined. In the younger strands the 

 few elements are protoxylem, and new elements are added centrifu- 

 gally (endarch), centripetally (exarch), or in all directions (mesarch). 

 The bulk of the bundle is as yet procambial tissue (compare figs. 

 10-15 and 16-21 with figs. 22-25). In the older strands it is difficult, 

 if not impossible, to determine where protoxylem ends and metaxylem 

 begins. 



The vascular strands of the cotyledons (figs. 4-6).— From each of 

 the four protoxylem groups in the vascular plate (px) a strand runs 

 outward for a short distance and then branches, the branches 

 separating at wide angles and continuing outwardly in a horizontal 

 plane until well under the bases of the cotyledons, where they turn 

 abruptly upward into the cotyledons, thus giving four strands to 

 each cotyledon, and in such a way that each of the opposite protoxylem 

 groups on the diagonal perpendicular to the inner faces of the cotyle- 

 dons gives rise to the two inner strands of each of the cotyledons ; while 

 each of the other two protoxylem groups gives rise to the outer strands 

 of the cotyledons in opposite edges, that is, one branch goes into the 



