1899] ON THE ANATOMY OF FOSSIL PLANTS 373 



individual stele iu fact has almost precisely the same structure as the 

 single stele of Hcterangium, and like the latter develops a cambium 

 and secondary wood and bast, the xylem being of typically " loose " 

 cycadean nature. The leaf-traces, which pass outwards from the 

 stelar system to pass into the leaves, carry with them some secondary 

 wood. As it passes through the cortex, however, the trace loses its 

 secondary tissues and undergoes splitting into a number of bundles, 

 each of which is collateral in structure. The base of the leaf therefore 

 received a large number of bundles, and this distribution of the bundles 

 is described as peculiar and unlike that in any known plants of 

 cycadean affinities. 



In other species of Medullosct the structure is more complex, there 

 beiug usually a number of small circular steles occupying the central 

 region of the axis, and one or more larger wavy or annular steles or 

 rings of steles, closer to the periphery, and from which the leaf-traces 

 appear to arise. All the steles exhibit secondary thickening. 



A very interesting suggestion in connection with the structure of 

 Medullosct has been made by Mr. W. C. Worsdell in his paper on " The 

 anatomy of the stem of Macrozamia compared with that of other 

 genera of Cycadeae." x The stem of Macrozamia (in common with 

 those of Cycas and Enceplialartos) possesses a number of anomalous 

 rings of collateral bundles, secondarily formed outside the normal zone. 

 The tissues in these zones show the normal orientation, having xylem 

 inside and phloem outside ; but there is frequently found to be present, 

 either between the normal and the first anomalous ring, or between 

 two anomalous rings, a cambium which develops small bundles with 

 reversed orientation, i.e. with xylem outside and phloem inside. In 

 some cases, where the inverted segment lay rather obliquely, it formed 

 an almost continuous zone with the normally orientated segment, thus 

 indicating a tendency towards the formation of a concentric bundle. 

 If these bundles were more completely developed the anomalous zone 

 would be converted into something like what we find in Medullosct, 

 steles with xylem internal and phloem both on the inner and outer 

 side. The author says : " The whole structure, both of the anomalous 

 zones and the tertiary cambiums, recalls strongly that of the stem of 

 the Medulloseae, a fossil group^ with many cycadean affinities ; but it 

 would be rash and premature to suggest here a homology between the 

 two." He gives the suggestion, however, that these anomalous 

 structures are the remnants of some ancient structure once common to 

 a large number of plants, and that this structure consisted of rings or 

 layers of concentric vascular stands. 



Evidence has been brought forward by Eenault and others to 

 prove that the " Myeloxylon " petioles bore distinctly fern-like foliage, 

 referable to the form-genera Neuroptcris and Alcthopteris. Sterzel 

 thinks that Calliptcris, Alcthopteris, Odontopteris, and Neuroirtcris are 



1 Annals of Botany, 1896, vol. x. p. 601. 



26 NAT. SC. VOL. XIV. NO. 87. 



