POROXYLEAE 



5°7 



formation of periderm took place on the inner border 

 of the cortex, and immediately outside the " primary 

 phloem " — in fact, in just the same position which it 

 occupied in Lyginodendron. In the older stems, the 

 whole of the cortex was thrown off as bark, a stage 

 which has not been observed in the latter genus, though 

 Calamopitys may offer analogies. 



The double leaf-trace, as it passes out through the 

 cortex, preserves its collateral structure, and is accom- 



Fig. 183. — poro.vylon Boyssctii. Transverse section of petiole, -r, primary xylem ; jtr'", 

 secondary xylem ; pli, phloem ; hy, hypoderma. X about 10. After Renault. 



panied by secondary wood and bast on its outer side. 

 The same structure is maintained in the leaf itself. 

 Fig. 183 represents a transverse section of the petiole 

 of P. Boyssetii. The vascular bundles, which have 

 begun to subdivide, are arranged in a transverse 

 series, the phloem facing downwards and the xylem 

 upwards, as is usual in leaves ; they present essentially 

 the mesarch structures of the bundles of Cycadean 

 leaves (cf. Fig. 133, p. 365). The petiole, like the 

 stem, had a strong mechanical construction, owing to 



