CUTICLES OF CTCADEAJs^ EEONDS. 181 



two levels. The appearance observed when preparations of stomata are seen in surface 

 view (text-fig. 27 ; PI. 19. fig. 1) is as follows : — On either side of the central slit-like 

 pore are two elliptical or hemispherical structures ; they are somewhat flattened where 

 they abut the pore, and have rounded ends, appearing, in fact, rather like an ordinary 

 guard-cell appears in surface view. Between these and the subsidiary cells lie two other 

 thickened patches, more or less hemispherical in shape, and apparently somewhat over- 

 lying the central structures. They possess a very characteristic outline, and have, like 

 the central ones, a deep brown colour ; they often show peg-like extensions towards the 

 centre (see text-fig. 27). Very similar patches are seen in almost all the stomata of this 

 roup. They often have a slightly shorter diameter (measured in the polar plane) than 

 the subsidiary cells, but are longer than the central structures; in some cases they 

 appear to be attached to the latter, but in other preparations appear to be free t'rora 

 them at the ends, only lying over them in the centre (cf. PI. 19. fig. 1). 





Eig. 27. 



Stoma of Ptilophjllum ^pecten, showing subsidiary cells (s) 



and thicUeninss of guard-cells. 



The interpretation of these structures would be somewhat difficult but for our know- 

 ledge of the thickenings possessed by the guard-cells of modern forms. In the surface 

 view of these we nevei- see the outlines of the guard-cells, but only the form of their 

 thickenings or woody lameU^. In Soicema the thickening on the upper face of each 

 guard-cell forms a hemispherical structure at each side of the pore, while the lamellae 

 on the lower faces contribute the projecting pieces seen at the poles {cf. text-fig. 26). 

 In the present case we seem to be dealing with a very similar structure. It is clear that 

 the central patches bordering on the slit do not represent the actual guard-cells. They 

 are often extended into sharp points at the poles, arc much smaller than the subsidiary 

 ceUs, and do not abut directly on to them; moreover, on this view the presence of the 

 intermediate patches cannot be easily explained. It seems likely that we see m the 

 fossil fronds two pairs of thickening patches or lamellae belonging to the upper and lower 

 sides of the original highly inclined guard-cells. These lamellae were somewhat similar to 

 the woody lamellae of the modern forms, but were more strongly cutinised. The central 

 lamella would be originally on the lower side of the cells and would be much shorter 

 than in the modem structures. The outer and more thickened patches would represent 

 the upper lamella of the guard-cells, which, however, overlapped the lower lamella, to a 

 much smaller extent than in Stangeria and :Botcema, but are almost identical in shape 



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