XXIX] LYGINOPTERIDEAE 47 



found in a selection of trees and shrubs examined by Miss 

 Brenchley. The secondary wood of the stem swells below the 

 point of exit of a branch and frequently a fairly large amount 

 of wood occurs in the pith when a branch is given off: this 

 anomalous wood may help to close the branch-gap. 



ii. Leaf. 



In his account of Lyginopteris stems published in 1873 WiUiam- 

 soni suggested that the vascular bundles met with outside the 

 xylem-cyHnder might be the leaf-traces of large fronds and 

 expressed the opinion that the 'stems or petioles' previously 

 described by him under the generic name Edraxylon might belong 

 to Lyginopteris. A year later he substituted the name Rachi- 

 ofteris aspera for the petioles previously referred to Edraxylon 

 and inclined to the view that this type of Rachiopteris may be 

 the petiole of the Carboniferous fronds known as Sphenopteris 

 Hoeninghausi Brongn., an inference based to a large extent on 

 the occurrence of emergences on Rachiopteris aspera (fig: 404, E) 

 preserved as petrifactions like those on impressions of Sphen- 

 opteris Hoeninghausi as figured by Brongniart (figs. 404, A; 

 405, D, D'). In 1890 Williamson was able to demonstrate the 

 truth of the surmise that Rachiopteris aspera and Lyginopteris 

 oldhamia are respectively the petiole and stem of the same 

 plant, which he believed to be an arborescent fern^. The 

 petioles of Lyginopteris fronds, which may reach a diameter of 

 1 cm., are attached by a broad base to the stem, and as already 

 suggested by the number of internodes traversed by each leaf- 

 trace, the leaves are comparatively far apart. A transverse 

 section of a petiole is shown diagrammatically in fig. 405 A. The 

 hypodermal stereome is a prominent feature, but the narrow 

 radial plates of the stem-cortex tend to be replaced in the rachis 

 by broader and confluent masses of strengthening elements : 

 the upper surface of the petiole is slightly grooved. Glandular 

 and spinous emergences are often very abundant, as in the section 

 reproduced in fig. 404, B. A glandular emergence is seen at a 

 in fig. 405, A. The spinous emergences may be compared with 



1 Wimamson (73) A. p. 403. ^ WilKamson (90). 



