4 o8 STUDIES IN FOSSIL BOTANY 



or, if it happen to pass between two series, may miss 

 them altogether. The cells of which the sclerotic plates 

 are composed are short, and similar in structure to the 

 stone-cells of many recent plants. The transversely 

 striated appearance, which these plates give to the 

 cortex, often serves to identify Heterangium at once, 

 even on a naked-eye inspection of rough specimens or 

 carbonised impressions (cf. Figs. 159 and 160). 



The outer cortex, or hypoderma, is formed of 

 alternating strands of fibres and parenchyma (Figs. 

 154, hy, and 155, o.c). The structure, as seen in 

 transverse section, is much like that of the same region 

 in Lyginodendron, except that the parenchymatous 

 tracts are narrower ; in tangential section a greater 

 difference is shown, for in Heterangium the fibrous 

 strands take a straight vertical course, and only 

 anastomose with one another at long intervals. 



We have now to consider the distribution and 

 structure of the vascular bundles which supply the 

 leaves. The main points in their longitudinal course 

 were determined by Williamson in his first investigation 

 of the plant in 1872, when he examined a series of 

 eight transverse sections from a piece of stem nearly 

 two inches long. In this length the bases of two 

 leaves were met with, into each of which a vascular 

 bundle was traced. The arrangement of the leaf-traces 

 in this specimen, as well as in others, shows that the 

 phyllotaxis was -§. This, was probably the usual 

 arrangement in the larger stems of Heterangium 

 Grievii; in the smaller specimens a | divergence is 

 found. 



The leaf-trace bundles passed out very gradually, as 



