CYCADOXYLEAE 489 



leaves and branches are present, and, as the two organs 

 appear to correspond in position, it is not improbable 

 that the latter were axillary. It seems that the 

 phyllotaxis was ■§. 



Where the bundles of a leaf or branch pass out, the 

 external vascular cylinder is interrupted, and its edges 

 are incurved, to unite with two of the internal vascular 

 bands (see Fig. 179, x', pk'). The latter are thus 

 always continuous, at some point of their longitudinal 

 course, with the normal vascular ring. This, it will be 

 remembered, is precisely what happens in some speci- 

 mens of Lyginodendron oldhamium, where the anomalous 

 medullary wood and bast are likewise connected, at the 

 leaf-trace gaps, with the normal vascular tissues (see 

 p. 372). In Ptychoxylon, however, the conditions are 

 more complicated, for there are often two or three 

 concentric systems of internal vascular arcs, joining on 

 at different levels, to the external ring (see Fig. 179, 

 x' ', ph")} The character of the wood and bast of each 

 ring is very parenchymatous, like that of Cycadoxylon, 

 or of a recent Cycad. The phloem, which is external 

 in the case of the normal zone, but internal in each of 

 the anomalous bands, is beautifully preserved, and even 

 the details of the sieve -tubes have been made out. 

 There appears to be no indication in this genus of any 

 primary centripetal wood in the stem. 



The leaf-traces, however, appear, from M. Renault's 

 description and figures, to have essentially the same 

 structure as in Lyginodendron ; the trace consists of two 

 bundles, side by side ; in each bundle there is a large 



1 Compare the double anomalous zone in Cycadoxylon robustum, 

 Fig. 178. 



