9^ MORPHOLOGY OF GYMNOSPERMS 



cambium produces a vascular cylinder as prominent as the primary 

 one; the second develops much smaller and more widely separated 

 bundles; and this continues with diminishing constructive power 

 until the outermost cylinder is recognized only by the appearance of 

 a small bundle here and there. These cortical bundles are produced 

 in Cycas, Encephalartos, Macrozamia, and Bowenia; and in Cycas 

 they are concentric. In Macrozamia and Bowenia Worsdell (20, 30) 

 discovered a tertiary cambium, appearing between the successive 

 secondary cambiums, and giving rise to small intermediate bundles 

 with reversed orientation, the xylem being directed toward the xylem 

 of the next outer bundles. This suggested that the original structure 

 of all the cortical bundles of these genera 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. The cortical concen- 

 tric bundles of Cycas, the concentric bundles in the peduncles and 

 leaves of several genera, the evidence of incomplete concentric bundles 

 in the cortical cylinders of Macrozamia and Bowenia, all indicate 

 derivation from an ancient stem type in which layers of concentric 

 bundles were developed, such a type as exists among the Cycado- 

 filicales (23). 



In Encephalartos and Macrozamia a system of bundles is also 

 developed in the pith, forming a dense network, each bundle being 

 accompanied by a mucilage canal in contact with the phloem. Some 

 of the smaller bundles pass out through the leaf gaps, the xylem and 

 phloem strands Joining the corresponding elements of the primary 

 cylinder, and the mucilage canals passing on to join the cortical canal 

 system. 



An account of the anatomy of the seedlings of Ceratozamia, Dioon, 

 Microcycas, and Zamia will furnish a basis of comparison. Cerato- 

 zamia is peculiar in the fact that one of the cotyledons is abortive, but 

 it has been made to develop by eliminating the influence of gravity 

 (58). The seedling has been investigated by Sister Helen 

 Angela (59), the outline of whose results is as follows. The vascu- 

 lar plate of the transition region is irregularly four-cornered, all of 

 the xylem being in the center, sometimes in a solid mass, sometimes 

 interspersed with pith cells (fig. 75). This protosteHc condition may 



