CYCADALES 



99 



persist above the plate more or less, in one case for a distance of 

 1.6 mm., above which a siphonostele is established. The develop- 

 ment of the tetrarch root is delayed, its four protoxylem poles being 

 inserted upon the cotyledbnary bundles, and the entire xylem system 

 of the root being 

 bordered periphe- 

 rally by cambium. 

 The single cotyle- 

 don, which is often 

 lobed at tip, is mul- 

 ti fascicular, usually 

 with alternating 

 mucilage ducts, but 

 all the vascular 

 strands are derived 

 from three bundles 

 that connect with 

 the vascular plate 

 (figs. 76, 77). 



These three 

 strands are not of 

 the same rank, and 

 their relation to the 

 vascular plate of the 

 transition region 

 should be under- 

 stood. Two of the 

 angles of this plate 

 are directed toward 

 the center of the 

 two cotyledons (the 

 usual abortion of one cotyledon in Ceratozamia does not affect this 

 orientation), and therefore may be called cotyledonary; and the 

 other two are in the plane between the two cotyledons, and there- 

 fore may be called intercotyledonary. The median vascular strand 

 that enters a cotyledon is connected directly with the adjacent 

 cotyledonary angle or pole of the vascular plate, and therefore is a 



Fig. 75. — Ceratozamia mexicana: transverse section 

 of vascular plate of seedling, showing the protostelic con- 

 dition; px, protoxylem; X225. — After Sister Helen 

 Angela (59). 



