CTCADACE^:. / 



cycadean leaf " from the Lower Greensand, which he suggests may 

 possibly represent a frond of Sennettites ; the specimen referred to 

 is not in organic connection, nor in any close association, with 

 a stem, and therefore no satisfactory conclusion can be drawn as 

 to its real nature. As yet we can only reply to the question as to 

 what was the precise form of Bennettites leaves by mere guesses, 

 founded on no surer basis than a vague suspicion of probability. 

 The leaf-scars on the surface of the stems suggest a frond of 

 cycadean habit ; and in all probability many of the Mesozoic leaves 

 which we are accustomed to connect with true cycadean stems 

 should be referred to Bennettites. To include all cycad-like fronds 

 in the Cycadacea as defined for existing species, would almost 

 certainly result in assigning many fossil leaves to a wrong position. 

 Possibly the better plan would be to assign such fossil fronds as 

 may reasonably be referred to cycadean plants, to some more 

 comprehensive Natural Order than that of the Cycadacece. 



This brings us to the question of intermediate forms, and the 

 association of cycadean structure with several of these synthetic 

 types lends an increased interest to the past history of cycads, and 

 at the same time enhances the difficulty of systematic treatment. 

 The Upper Carboniferous genus Myeloxylon (Stenzelia, Goppert, 

 Myelopteris, Renault), found in England, France, and Germany, 

 has been assigned by several writers to the Filiemee, and placed 

 in the Marattiacea or Ophioglossacea ; others prefer to include 

 it with the cycads. The structure of the vascular bundles of 

 Myeloxylon petioles * is in some respects typical of recent cycads ; 

 the spiral protoxylem elements being on that side of the xylem 

 facing the phloem. The bundles are collateral in form, and often 

 accompanied by mechanical or stereome elements. The fundamental 

 tissue contains numerous secretory canals, and in some cases strands 

 of stereome. One of the most readily recognized features is the 

 hypodermal tissue, made up of alternating bands of thick walled 

 fibres and thin parenchymatous cells. Occasionally the petiolar 

 axis is found to be branched, and small Pecopteris-like pinnules 

 have been observed attached to a slender Myeloxylon midrib. This 

 discovery by Renault of pinnules in connection with Myeloxylon 



1 Seward (1). References given to other papers ; see also Zeiller (1), p. 290, 

 pi. xxvii. fig. 1. 



