CUTICLES OE CYCADEAN FEONDS. 



109 



In comparing the general epidermal cells of the recent and fossil types we may 

 notice that there is nothing among the modern forms which is really comparable to the 

 very sinuous cells of Ptilophyllum and Anomozamites. The cell-walls of the Stangeria 

 epidermis are somewhat sinuous, and this is of interest when taken with the forking 

 nervation of its pinnae, another point of comparison with the Bennettitalcs. If the 

 sinuous cell-walls are to be regarded as a feature of any phylogenetic importance, it 



may indicate the relationships of the Bennettitales with the Pteridosperms and ferns. 

 On the other hand, the straight walls of the Nilssonialean epidermal cells, together with 

 the shape of the cells, are additional points of agreement with the majority of the 



modern Cycads. 



We may summarize generally the relationship between the fossil and modern 

 Cycadean fronds as indicated by this enquiry, as follows. The cuticular structures 

 indicate that the Bennetittalean fronds form an easily recognizable group containing 

 the majority of the fronds previously described as Cycadean, which are quite distinct 

 from the modern fronds and from those of the Nilssoniales group. The latter, which 

 includes the genera I^ilssonia, Cienis, and Ftilozamites, are somewhat closely allied to 

 the modern fronds, and may probably be regarded as the true Mesozoic Cycads and the 



ancestors of the modern forms. 



On the other hand, the Bennettitalean stomata show some important resemblances to 

 the Cycadean ones, and may well point to a parallel line of evolution from a common 



stock. 



It is scarcely possible at the present time to make any detailed comparison betvvcer 

 the epidermal structures which have been described in this paper and those of the other 

 groups of Gymnosperms, since in no case have the latter been studied in sufficient 

 detail. A few general comparisons may, however, be attempted. It seems probable 

 that of the other Gymnosperms, all of which possess guard-cells with the same charac- 

 teristic pear-shaped section, the Gnetales are the most closely allied in the form of their 

 cuticular structures. In fact, it would seem that the Bennettitalean cuticles are more 

 closely alHed to those of some of the Gnetums than to any other group, the points of 

 comparison being the frequent arrangement of the stomata in rows, the relations of the 

 guard-cells to the subsidiary cells, and the method of thickening of the guard-cells. 

 In Gnetum Gnemon the walls of the epidermal ceUs are often sinuous. We have 

 been able to examine the cuticles of Welwitschia and Gnetum Gneinon, and the 

 stomata in these forms have also been described by Takeda* and others, but other 

 species need to be examined before the comparison can be pressed. 



The stomatal structures of Ginlgo f are somewhat similar to those of NiUsonia, but 

 differ in the smaller and more constant shape and number of the subsidiary cells and in 

 the very slight thickening of the guard-cells. The stomata of the Conifers % while 

 possessing guard-ceUs which are thickened in a somewhat similar way, seem to differ 



from those of the Cycadales and Bennettitales in the uniformity of the thickening and 



• Takeda (1913), p. 350, text-figs. 1-5, pi. xxix. figs. 2-6 

 t Seward (1911'), pi. v. figs. 69-02. 

 X Mahlert (1885), figs, on Taf, 1. 



