560 OYCADOPHYTAN FRONDS [CH. 



that the double 'midrib' is an untrustworthy character and that 

 the stomata are not always arranged as in the fronds examined 

 by Nathorst. Nathorst states that in Psevdocycas Steenstrupi 

 and P. insignis the midrib though usually double may be single : 

 the same variation occurs in P. Safortae. Miss Holden found 

 that an impression made from a pinna of Cycas siamensis showed 

 a double hne on the upper face and a single rib on the lower; 

 that is Pseudocycas and Cycas characters occur on a single pinna. 

 On drying, the ridge on the lower face of the lamina collapses and 

 a groove is formed bounded by two ridges. 



In a recently published and interesting paper on 'Some 

 Xerophytic Leaf-structures in Mesozoic Plants,' Dr Halle^ makes 

 some important additions to our knowledge of Pseudocycas. He 

 agrees with Miss Holden's views with regard to the use of the generic 

 names Pseudocycas and Gycadites, but dissents from her inter- 

 pretation of the median groove as the result of shrinkage of a 

 midrib. Part of the evidence presented by Halle is based on the 

 structure of the leaf-cuticle as shown in microtome sections^. He 

 found that a distinct median groove is always present on the 

 pinnae of Pseudocycas insignis and other species. If the groove 

 were due to collapse of the tissue of a true median rib the presence 

 of stomata — which in P. insignis are confined to the middle 

 line of the lamina — below the vein would be an unusual feature. 

 He found no indication of a median vein or of any vascular tissue. 

 All that is revealed by an examination of the cuticle is that the 

 lamina has a weU-defined groove bounded by prominent ridges, 

 and in Pseudocycas Roemeri the groove is deeper and is formed by 

 the bending-back of the lamina. The stomata are practically 

 confined to the groove and in P. insignis and P. Steenstrupi an 

 additional protection against excessive transpiration was afforded 

 by elongated papillae. 



In the sinuous walls of the epidermal cells and in the structure 

 of the stomata Pseudocycas agrees with the Bennettitalean type 

 as defined by Thomas and Bancroft^^ The ' midrib ' of the pinnae 

 marks the position of a pronounced groove and we have no in- 

 formation with regard to the venation. As Halle says, protection 



1 Halle (15). ^ pjg ^n., xiii. 



* Thomas and Bancroft (13). 



