PALEOBOTANY AND EVOLUTION. 113 



stele, there is a multilayered pericycle made up of short- 

 celled parenchyma, containing nests of sclerous cells and 

 numerous secretory sacs. In most cases the pericycle 

 tissues are succeeded towards the exterior by a deep-seated 

 periderm, which arose at an early stage of secondary 

 thickening from a phellogen developed in the pericycle. 

 The extra-stelar region of the stem may be conveniently 

 described as being made up of an inner and outer cortex ; 

 the former consists of parenchyma with scattered secretory 

 cells and occasional sclerous nests. The outer cortex 

 presents a very characteristic appearance, being made up of 

 alternate radiating bands of dark sclerenchyma, and lighter- 

 coloured, thin-walled, parenchyma containing secretory sacs. 

 From the peripheral cells of the cortex plint emergences 

 are here and there developed. 



Mention has been made of petioles attached to the 

 Lyginodendron stems. The nature of these appendages is 

 of special interest as regards the affinities of the plant. 



In 1872 Williamson suggested a possible connection 

 between certain structures described under the generic name 

 of Ed7'axyloii and the stem of LyginodcudTon. Sub- 

 sequently the genus Rachiopteris was substituted for 

 Edraxylon, and thus the Lyginodeiidi^on leaves were in- 

 cluded among true fern fronds. There is now direct proof 

 that Rachiopteris aspera Will, is the petiole oi Lyginodendron. 

 The form of the ultimate segments of the frond is that long 

 known to palaiobotanists under the provisional generic 

 name Sphenopteris. The petiole is made up of a ground- 

 mass of parenchyma with alternate radiating strands of 

 sclerenchyma and thin walled tissue at the periphery. Long 

 secretory sacs traverse the inner fundamental tissues, and 

 externally the cortex exhibits gland-like out-growths or 

 emergences similar to those in Lyginodendron stems. The 

 vascular bundle of the leaf-stalk is of the typical concentric 

 form, and usually consists of a mass of V-shaped xylem 

 containing several protoxylem groups surrounded by a thin- 

 walled and small-celled phloem. External to the phloem a 

 more or less well-marked pericycle has been recognised, but 

 no distinct bundle-sheath. In addition to the structural 



