COMPARATIVE ANATOMY OF GYMNOSPERMS 305 



peculiar ; for, instead of passing directly from the central cyl- 

 inder into the leaf, they usually pursue a circular course, so 

 that they reach their corresponding leaf on the opposite side of 

 the stem from their point of origin. In Zamia I have observed 

 this arrangement of the traces even in the seedling; but in 

 Cycas, according to Mettenius, 1 the leaf-traces of the young 

 plant at first pursue a direct course, although at a later stage 

 girdles are present. During their cortical course the foliar 

 traces often undergo more or less complex anastomoses. The 

 structure of the strands in the cortex, and even in the base of 

 the petiole, is often concentric. 



Fig. Ill, T, is from a photograph of a cortical bundle of 

 Cycas revoluta. The center of the bundle is composed almost 

 entirely of the large tracheids of the primary wood, which is 

 surrounded by the radially arranged secondary wood and 

 phloem. Higher up, in the lower part of the petiole, the bun- 

 dles lose most of their secondary wood and assume mesarch 

 structure. This is well seen in Fig. Ill, U, which may be 

 compared with Figs. 100, K, and 110, P. A striking feature 

 of the bundle at this stage is that the primary wood is mostly 

 centripetal, and has consequently a markedly cryptogamic ap- 

 pearance. 



Before discussing further the significance of the peculiar 

 structure of the foliar traces of the C'ycads, it will lie con- 

 venient to refer to an interesting discovery made by Scott. 7 

 Mesarch bundles have been found by him in the central cylinder 

 of the peduncle of the cone of Stangeria paradoxa and certain 

 other Cycads. The conservatism of reproductive organs is rec- 

 ognized by the universal use made of them in botanical classi- 

 fication. It is Scott's opinion that in the conservative repro- 

 ductive branches fi. e., cones) of certain living Cycads the an- 

 cestral type of bundle is retained. Hence he believes that the 

 cauline central cylinder of the more or less remote ancestors 

 of the living Cycads must have had a structure similar to that 

 of the stem of Lyginodendron. This hypothesis is borne out 

 by the fact that the course of the leaf-traces in the cones of 

 Cycads is the same as in the seedling of the genus Cycas, and 

 in the vegetative stems of the extinct group of Cycad-like Ben- 

 nettitales; for they pass directly into the leaves (sporophylls) 

 and do not form girdles. Jeffrey 10 has pointed out a similar 



