6 Weiss, XenopJiyton rndiculosiim (Hick). 



oblique walls when seen in longitudinal section {P/ate XII., 

 Fig. i). In such sections, the demarcation between the 

 inner cortex and the middle cortex is more distinct than 

 in transverse sections. The middle cortex, which is as 

 rarely preserved in Stigmaria as in lepidodendroid stems, 

 is conspicuous in XenopJiyton, both by its massiveness 

 and by the excellence of its preservation. Hick regarded 

 this middle cortex, or " radicular tissue " as he called 

 it, as intercalated by secondary development between 

 the outer and inner cortex. But though we have 

 evidence that certain secondary changes take place within 

 the middle cortex, there is not, I think, sufficient evidence 

 to regard it as due entirely to secondary growth. On the 

 contrary, it corresponds closely with the middle cortex of 

 the stem of the Lepidodeiidrecs which, except in Lepi- 

 dophloios Juliginosus, is so rarely preserved. It also 

 corresponds in position with the lacuna left b}' the 

 defective tissue in Stigmaria between the central c\'linder 

 and the outer cortex, and with the similar space left in the 

 stigmarian rootlets by the defective middle cortex. 



The irregular and patchy appearance of this tissue in 

 XenopJiyton is due to the passage through it of the vascular 

 bundles which are to supply the rootlets, and it is around 

 these bundles more particularly that secondary changes 

 in the cortex seem to take place. Otherwise, the middle 

 cortex seems composed of short polygonal or square cells, 

 with a certain amount of intercellular space sometimes 

 showing that growth characteristic of this layer in the 

 Lepidodendrecs (more particularly in LcpidopJiIoios) where 

 it often appears made up of interwoven filaments, like a 

 sclerotial tissue of a fungus {Plate XL, Fig. 3). 



In some places the middle cortex seems to possess 

 definite powers of secondary growth, and its cells often 

 show a radiating arrangement around the rootlet bundles. 



