768 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON 'THE 
that of a Cycad, while in Lyginodendron the presence of a pith and distinct bundles 
create a certain similarity. 
The most interesting and remarkable point, however, which our investigations have 
brought to light, is the fact that the vascular bundles in the stem of Lyginodendron 
have exactly the same structure as those in the leaf of existing Cycads. This 
applies also to the leaf-traces and the bundles which form the outer part of the stele 
in Heterangiwm. Both in our fossil genera and in Cycadez the bundles in question 
are collateral and mesarch, the primary xylem being chiefly centripetal, while a 
smaller part is centrifugal. The essentialiy Cycadean structure of the bundles 
illustrated in Plates 21 and 22, figs. 2, 3, 4, 5, and 6 is evident to any botanist. 
This type of bundles among existing plants is limited to Cycadez, and until now 
has only been observed in their leaves, in which it is of universal occurrence.* 
Among fossil plants the same structure occurs most clearly in the leaves of 
Cordaitez,t and also in Cycadoxyez and Sigillaria, though, in the last case, the true 
interpretation of the structure is doubtful.{ In the petiole of Myeloxylon, according 
to SEwARD, the xylem of the bundles appears to have been wholly centripetal, so that 
the agreement with Cycadez is here imperfect.§ 
Poroxylon, according to the investigations of MM. Bertranp and RENAULT, appears 
to have had the Cycadean type of bundle in both stem and leaf. 
We regard it as an essential characteristic of this type of bundles that the centri- 
fugal, as well as the centripetal, portion of the xylem belongs to the primary tissue. 
This was certainly the case in the stem-bundles of both Lyginodendron and Heteran- 
gum, and the observations of one of us leave no doubt that the same holds good for 
the foliar bundles of recent Cycadex. 
The occurrence of mesarch bundles in the stem of Lyginodendron and Heterangium 
at once suggests the view that this structure in Cycads is not a mere peculiarity of 
the leaf, but may rather be an ancestral character which once existed in the stem 
also, but has disappeared from that organ (in relation, perhaps, to the progress of 
secondary growth), while it has survived in the leaf. 
These considerations suggested to one of us (D. H. Scorr) the question whether 
some traces of this structure may not still exist in the stems of recent Cycades. A 
preliminary investigation of this point has yielded the interesting result that, in the 
peduncles of both male and female flowers of Stongeria, the bundles are often 
mesarch. ‘There is in these cases a well-marked centripetal xylem, in addition to 
the normal centrifugal portion of the wood. A full account of these observations 
will be published on another occasion. 
* The foliar bundle of Isoétes is somewhat similar, but not identical. 
+ Renavtt, “Structure comparée de quelques tiges de la Flore Carbonifére,” Clichy, 1879, Plate 16, 
figs. 2-8, 
¢ Rewnavtr, loc. cit., Plate 12, figs. 1,2, and 6. See, however, Soums-Lavsacn, “ Fossil Botany,” chap. xi. 
§ Sewarp, “ On the Genus Myeloxylon,” ‘ Aunals of Botany,’ vol. 7, 1893. 
