xxxin] AETisiA 247 



occurs in the white Jasmine, in Geropegia peUata, and some other 

 flowering plants. An interesting case is that of the tree Groundsel, 

 Senecio praecox D.C.^, of Mexico: in this plant, which grows in 

 arid districts, the pith serves as a water-store and as the water 

 is dTg,wn off the thick turgescent discs contract and form thin 

 transverse diaphragms separated by wide spaces, as is also the 

 case on drying in some succulent Euphorbia stems. It may be 

 that in Cordaites the medullary region also served as a water- 

 reservoir and the depth of the medidlary discs would vary according 

 to the state of their contents. 



The earher writers regarded the pith-casts as stems with 

 scars of amplexicaul leaves: Artis^ described specimens from 

 the English Coal Measures as Sternbergia, one of which he stated 

 to be 6 ft long ; a few years later Sternberg^ proposed the name 

 Artisia and this has been generally used on the ground that 

 Sternbergia is the name of a recent flowering plant. A specimen 

 of Artisia transversa (Art.) from the Coal Measures of Yorkshire 

 is shown in fig. 466, B, and similar specimens varying considerably 

 in diameter up to about 10 cm. are abundant in European and 

 American Coal Measures. The prominence and depth of the 

 transverse ridges, the presence or absence of anastomoses between 

 adjacent discs are, as Zeiller* says, of very doubtful value as 

 specific characters. Dawson in 1846^ spoke of Artisia as probably 

 the pith of a tree, a view suggested to him by Mr Dawes. In 

 1851 Williamson^ published a description of some specimens in 

 which a pith-cast, Artisia approximata Lind. and Hutt., was 

 enclosed by wood showing very clearly Cordaitean characters. 

 Further demonstration of the true nature of Artisia was supplied 

 by Grand'Eury from St !^tienne material. If the generic name 

 Artisia is applied to all pith-casts showing the transverse ridges 

 and grooves like those seen in fig. 466, B, it must be remembered 

 that it is not safe to assume a connexion with Cordaites or Meso- 

 xylon. A Liassic species described by Lignier' from France as 



' Harshberger (98). ^ ^rtis (25) A. PI. vm. 



' Sternberg (38) A. p. 192. For examples of Artisia, see also Renault and 

 ZeiUer (88) A. p. 581, PI. Lxv. 



4 Zeiller (06) B. p. 189. » Dawson (46) p. 135. 



6 Williamson (51). ' Lignier (95) PI. vn. fig. 13. 



