180 ME. H. HAMSHAW THOMAS AND MISS N. BA^S^CHOPT ON 



distinct species. We cannot here enter into this question in detail, but it may he 

 remarked that the existence of seyeral species of fronds is rendered probable by the 

 discovery of a number of different species of male and female flowers which probably 

 belonged to some of the plants bearing these Ftilophyllimi fronds. Three or four forms 

 of male flowers and five or six species of female flowers of the Williamsonia type 

 are now known. Besides these we know of two or three other types of flower which 

 probably belonged to plants bearing fronds of some of the other forms placed in the 

 Bcnnettitalean group in the present paper. In view of the uncertainty of connecting 

 "flowers" with fronds, it appears to be better, at present, to use the generic term 

 Williamsonia to apply only to reproductive structures, while referring to the fronds 

 tmder the names of their form genera. Thus we shall refer to Williamsonia pecten 

 (Phill.) fronds as JPtilophyllimi pecten and to Williamsonia gigas fronds as Zamia gigas, 

 Lindl. & Hutt. 



We may now proceed to the description of the cuticles of fronds of Ftilo2ihyllum 

 peclen (Phill.) *. The cuticles of the upper and lower epidermis are very distinct (see 

 PI. 20. fig. 1). The upper epidermis is composed of uniform cells showing no traces of 



and having no stomata. The walls of the cells show an extremely 



a characteristic in the Bennettitalean group ; each wall is bent into a regular series of 

 folds which sometimes almost touch each other. The appearance of this cuticle is seen 

 in the photograph reproduced on Pi. 20. fig. 2. 



The lower epidermis is composed of cells with a somewhat similar outline, usually 

 rather thinner than those of the upper epidermis and with larger lumina. There is a 

 marked differentiation betw^een the areas above the veins wdiich possess no stomata and 



intermediate portions where the stomata 



three rows (PL 20. fi 



The cells above the veins exhibit a somewhat more elongated form tban the rest. A few 

 circular hair-scars are sometimes seen. Near the margin of the leaf the cells become 

 more elongated and their walls thicker. 



The Stomata. — In each square millimetre of the lower surface between 50 and 60 

 stomata occur; this number seems fairly constant in the specimens examined. The 

 individual stomata are roughly circular, with a diameter of about '039 mm. The guard- 

 cells are very regularly orientated, so that the slit lies transversely, at right angles to the 

 veins of the pinna. The regular arrangement of the stomata is a characteristic feature 

 of the JPtilophi/llum-Oiozamites group of fronds; in other genera thev are irregularly 

 scattered. The guard-cells do not seem to be appreciably sunk. On either side they 

 have a small subsidiary cell {s, text-fig. 27), on to w^hich the sinuous walls of the 

 epidermal cells abut. The actual structure of the ffuard-cells is somewhat difScult to 



make out from the surface view of the remains of their cuticularised portions (PI. 20. 

 fig. 3). From the analogy of the modern stomata {cf. text-fig. 26} it seems probable 

 that the structures seen in our preparations do not represent the outlines of the cells 

 round the stomatal pore but only parts of their walls w hich were differently thickened at 



♦ For methods of obtaining cuticular preparations see Thomas (1912), also Nathorst (1912) and Eather (1908). 



In this section the term cell-wall is used for the ridges on the cuticular film which now represent the walls of the 

 original cells. 



