156 ME. H. Hi^MSHAW THOMAS ASD MISS N. BANCEOJFT ON 



may be valuable from the phylogenetic standpoint has not yet been worked out in 

 any detail, nor has any attempt been made to compare modern plants with their 

 fossil ancestors with regard to this character. One of us (Tliomas, 1911) has shown, 

 however, that some of the Carboniferous Calamites possessed the transverse bands of 

 thickening which are so characteristic of the guard-cells of the modern Equisetums, 

 and it seemed important to make further investigations of this kind. An especially 

 favourable opportunity is presented in the Cycadophyta ; the modern representatives 

 of this group possess considerable variety of frond form, while the fossil fronds are also 

 varied, and are abundant in a fairly good state of preservation in Mesozoic rocks. 

 Prom a purely paloeobotanical standpoint the work offers some attractions, because we 

 have so often to deal with isolated fossil fronds which possess few points of distinction 

 apart from external form ; the possibility of classifying them and obtaining information 

 as to their affinities by means of a study of tlieir epidermal structure seems important. 

 So long ago as 1856 Bornemann made a brief comparative study of the epidermal 



characters of recent Cycads for the purpose of identify 



of fossil Cycadean 



fronds from the Lettenkohle beds of Thuringia, but his studies of the fossil forms were 

 not very critical. He thought that the Mesozoic fronds were nearly related to if not 

 generically identical with, some of the modern forms, and therefore expected that the 

 study of the latter would at once facilitate the identification of the fossils. We are 

 now tending to regard the fossil Cycads as more and more widely separated from 

 their recent representatives., and we may at the outset expect the similarities in their 

 structure to be few. 



Recent work on the fossil and living members of the Cycadophyta has raised them to 

 a very important position among the Gymnosperms. The investigation of the Carboni- 

 ferous Pteridosperms, and the recent discoveries with regard to the Bennettitales have 

 caused many botanists to seek for the origin of the Angiosperms amongst the ancestral 

 plexus of Cycadean forms, much attention having been focussed on the Mesozoic 



plants of this affinity. It is, therefore, important to utilise every form of evidence 

 as to' the relationships of these plants. 



The method of enquiry adopted in the present paper is, first, the detailed investigation 

 of the epidermal structures found in the modern Cycads, and the attempt to find a type 

 of stomatal structure characteristic of the group, the departures in detail from this type 

 being noted. Subsequently, the epidermal structures of all the available Mesozoic 

 fronds are described, and an attempt is made to estimate their relations in this respect 

 to one another and to the modern fronds. In studying the fossil fronds, although we 

 have many beautifully preserved examples of their cuticles, yielding much information 

 in surface view, we have as yet been unable to get satisfactory preparations showing the 

 guard-cells of the stomata in section ; we are thus placed in an unfavourable position 

 for comparing them with the recent structures. 



