198 MK. H. HAMSHAW THOMAS AND MISS N. BANCROFT ON 



are so characteristic in tlie fossil forms ; the thin areas between the pores and the 

 cnsp-shaped thickenings are the same as are seen in the fossil forms, especially in 

 OtozamiteSt Anomozamites, and T(Bniopteris . 



The polar thickenings of the gnard-cells which are seen in most of the recent forms, 

 and which appear especially marked in Stangeria (where they consist of both woody 

 tissue and cuticle), are almost entirely absent in the fossil fronds, and present a point 

 of difference. 



It is probable that the relations of the guard-cells to the surrounding epidermal cells 

 and subsidiary cells, offer one of the most important points of comparison which can be 

 obtained in the surface view. In the Bennettitalean fronds we have a very constant 

 and characteristic arrangement. On either side of the guard-cells there are two large 

 subsidiary cells of exactly the same diameter as the guard-cells and probably developed 

 from the same mother-cell. [The small cells seen at the poles of the slit in some cases, 

 €. g. in Anomozamites , PL 19. fig. 5, are probably not polar subsidiary cells but thin 

 places above the poles of the actual guard-cells.] In the Nilssoniales group, however, 

 and in recent fronds, the subsidiary cells are very different. They are usually six or 



Yig. 32 



Stomata of Stangeria paradoxa aa seen in surface view after prolonged maceration. Notice that the lateral upper 



thicltenings of the guard-cells show as spindle-shaped patches, while there appear to he empty cell-liko spaces 

 at the poles. Cf. with figures of PtilophyUum and Otozamites stomata. 



more in number, and are arranged more or less radially round the guard-cells. In 

 Stangeria, Bncephalartos, and other forms, we find two large lateral and tAvo smaller 

 polar subsidiary cells, but they often become subdivided. Thus in this respect the 

 Bennettitales are quite distinct from the recent Cycads. On the otlier hand the fronds 

 included in the Nilssoniales may be closely compared with the modern forms in this 

 character, the numerous overarching subsidiary cells of Mlssonia, with their strongly 

 cuticularised points, sometimes coalescing to form a solid ring, being comparable with 

 those of Dioon and Cj/cas. These overarching cells frequently are of the same type 

 as those forming the remainder of the epidermis in both the fossil and the recent forms. 

 It is unfortunate that we cannot get more evidence as to the form and structure of 

 the guard-cells in the Nilssoniales group, though in the case of the example of Ptilo- 

 zamiies figured (PL 19. fig. 16) the guard-cells seem to have been of somewhat the same 

 type as those of the Bennettitales. 



