XXV] CLEPSYDROPSIS 449 



at the other the bay is open and has two protoxylem groups. 

 The latter represents the earliest stage in the production of 

 secondary bundles : at a later stage the bay is closed by the 

 elongation of the edges, the enclosed group of phloem is 

 vertically extended, and the protoxylem strands are more 

 widely separated. The curved band of xylem becomes de- 

 tached as a curved arc and divides into two (fig. 310, A). In 

 a single section of this species one often sees several strands of 

 xylem enclosed in a common cortex with the main vascular 

 axis; these are the xylem bundles of lateral pinnae, Meta- 

 clepsydropsis duplex shows the method of branching of the 

 petiole vascular axis which has already been noticed in Diplo- 

 labis and Zygopteris. In reference to this feature, Williamson 

 wrote in 1872 — " I know of no recent fern in which the secondary 

 branches of the petiole are thus given off in pairs, which pairs 

 are distichously arranged on the primary axis, and each of 

 which secondary petioles sustains ternary ones arranged dis- 

 tichously." By slightly altering the primary stele of this type 

 of frond, by narrowing of the constricted portion of the hour- 

 glass and extending the lateral groups of xylem obliquely 

 upwards, the form of stele shown in fig. SIO, A, would be 

 converted into the Diplolahis type (fig. 308, C). 



Glepsydropsis. 



Unger' instituted this genus as a subdivision of Corda's 

 family Rhaciopterideae^, the name having reference to the 

 hour-glass form of the vascular axis^ The type-species 

 C. antiqua (fig. 308, A) is spoken of as the commonest fossil 

 plant in the Devonian rocks of Thuringia. In some sections 

 the xylem has the form seen in fig. 308, A, in which an 

 invagination of thin-walled tissue occurs at each end; in 

 other sections (fig. 308, A') the bays become islands in the 

 xylem. Solms-Laubach speaks of Unger's species as Rachio- 

 pteris (Clepsydropsis) antiqua. P. Bertrand*, who has recently 

 described Unger's plant, while recognising that G. antiqua and 



1 Unger and Bichter (56) p. 165. ^ Corda (45) A. p. 83. 



' KUrj^iSpa, water-clock. " P. Bertrand (09) p. 127. 



