CYCADALES 



lOI 



nected with the central cylinder (fig. 79). There is no girdling of 

 leaf traces in the younger stages, the traces being at first vertical; 

 but with the radial increase of the inclosed leaves and stem tip girdling 

 becomes apparent. The xylem of the leaf traces is endarch, but it 

 becomes mesarch in the base of the leaf and remains so to the tips of 

 the pinnae. In a later study (66) the extrafascicular cambium of 

 the seedling of Ceratozamia was investigated. In over eighty seed- 

 lings, ranging in age from a few months 

 to over two years, only one small extra- 

 fascicular bundle was found, although 

 the central cylinder was surrounded by 

 several groups of cambium cells. If 

 these groups should function in the 

 production of vascular bundles, the 

 result would be strongly suggestive 

 of polystely. The innermost extra- 

 fascicular cambium groups arise in the 

 pericycle, near the transition region, 

 but the others appear later. 



The vascular anatomy of the seed- 

 ling of Dioon edule has been investi- 

 gated by Thiessen (62), and illustrates 

 the origin and course of the leaf traces 

 of cycads, concerning which there has 

 been much misapprehension. The 

 vascular plate of the transition region 

 is four-cornered in section, with a pro- 

 toxylem group at each corner, and, as 

 in Ceratozamia, it is protostelic and soon 

 passes into the siphonostelic condition in the seedling stem. The four 

 protoxylem groups of the vascular plate are continuous with those of 

 the root, which is therefore tetrarch. The primary strands from the 

 intercotyledonary angles of the transition plate are related to the coty- 

 ledons exactly as described above for Ceratozamia, but the primary 

 strand from each cotyledonary angle forks, so that four strands enter 

 the base of each cotyledon (fig. 80). 



The connections of the leaf traces of Dioon are simpler. The 



Fig. 77. — Ceratozamia mexi- 

 cana: diagram of vertical view of 

 vascular supply of cotyledons, where 

 the usually aborting cotyledon has 

 been made to develop; A, B, C, D, 

 the four main cotyledonary bundles. 

 — Made for this work by Sister 

 Helen Angela. 



