6 MORPHOLOGY OP SPERM ATOPHYTES 



more widely separated bundles, and so with diminishing con- 

 structive power until the outermost cylinder is recognized only 

 by the appearance of a small bundle here and there. In Cycas 

 these cortical bundles are concentric, as are the bundles of the 

 peduncles and leaves of Cycas, Stangeria, Bowenia, and some 

 species of Zamia and Ceratozamia. Moreover, in Macrozamia 

 and Bowenia Worsdell discovered what may be called a tertiary 

 cambium, developed between the successive secondary cam- 

 biums, and giving rise to small intermediate bundles with re- 

 versed orientation that is, with xylem directed toward the 

 xylem of the next outer bundles. This suggests that the origi- 

 nal structure of all the cortical bundles was concentric, and that 

 in the layers of concentric bundles the meristem of the inner 

 portion of each bundle became gradually less functional, until 

 in most cases the concentric bundles have become collateral. 



Such facts as the cortical concentric bundles of Cycas, the 

 concentric bundles in the peduncles and leaves of several genera, 

 the evidence of incomplete concentric bundles in Macrozamia 

 and Bowenia, all point to an ancient stem type in which layers of 

 concentric bundles were developed. It is important to note, as 

 Scott 24 has suggested, that such a stem type is indicated in sev- 

 eral large extinct groups, as Lyginodendreae, Poroxyleae, Pro- 

 topityeae, and Medulloseae, representing the so-called Cycado- 

 filices, and that the living Cycads constitute only a special group 

 of what was once an extensive alliance. In Cycas, probably 

 the most primitive of the living genera, the concentric character 

 of the cortical bundles is retained ; in Macrozamia and Bowenia 

 remnants of the inner part of the concentric bundles exist ; while 

 in still other genera vestiges of the primitive organization once 

 common to the stem also survive only in peduncles. The fact 

 that the concentric bundle is a prominent feature of the Pterido- 

 phytes, and that it is the prevailing one in Paleozoic vascular 

 plants, emphasizes the general ancient as well as Pteridophyte 

 character of the Cycads. 



In Encephalartos and Macrozamia a system of cauline bun- 

 dles is alao developed in the pith, forming a dense network trav- 

 ersing the pith in every direction, each bundle attended by a 

 mucilage canal in contact with the phloem. Certain smaller 

 bundles pass out through the pith rays, the xylem and phloem 

 strands joining the corresponding elements of the primary cyl- 



