iQiQJ DORETY—DIOON 253 



are endarch where they separate from the hypocotyl cylinder 

 (fig. 28) ; they become mesarch just after they enter the base of the 

 cotyledon (fig. 27) and maintain that character throughout the 

 greater length of the organ; near its tip, however, they are exarch 

 (fig. 26). In all cases the orientation is collateral ectophloic, 

 although at levels where branching is effected there is always an 

 apparent concentric arrangement where the 2 xylem masses are 

 still in contact, and the phloem masses are swung to right and left 

 of them. Transfusion tissue accompanies the metaxylem. Muci- 

 lage ducts and tannin cells are abundant. 



The root meristem is plainly visible below the hypocotyl plate 

 in embryos of the age shown in fig. 2, but no differentiation has 

 taken place, and of course there is no vascular tissue. The 4 poles 

 of the root are later developed in connection with the 4 protoxylem 

 groups of the hypocotyl vascular plate. 



The plumule consists of 3 or 4 abortive scales inclosing the rudi- 

 ments of the first and second true leaves, sometimes of a third leaf, 

 and the stem tip. There is no means of distinguishing between the 

 stem tip rudiments and those of a new leaf, because the leaf meri- 

 stem grows much more rapidly, and soon overtops the stem tip 

 (fig. 14). The vascular system supplying all these bracts and 

 leaves is complicated by the well known habit of girdling, the 

 details of which have fascinated and baffled many investigators. 

 Although D. spinulosum differs in no way from the other cycads 

 in this respect, its greater size makes a naked eye drawing possible 

 and thus furnishes a means for solution and demonstration. Figs. 

 14 and 15 were drawn from macerated stems. Each node of the 

 stem is, like the Jiodes on a first year stem of foxglove, telescoped 

 within the older one instead of growing above it. The internodes 

 are not elongated because the primary meristem of the stem tip 

 is held in check by the more rapidly growing secondary meristem 

 for each developing leaf. Since each leaf is supplied with strands 

 from cauline bundles in different parts of the stem, those strands 

 which come to it from the opposite side of the stem describe almost 

 a semicircle to reach the leaf; those which arise on the same side 

 as the leaf pass directly into it; and small arcs are described by 

 those strands which arise in intermediate positions. In fig. 14 



