CUTICLES OE CYCADEAN EEONDS. 



197 



IV. COMPARISON OF EECENT AND FOSSIL CUTICLES. 



The difficulty of comparing tlie epidermal structure of the recent and fossil fronds is 

 considerable owing to the impossihility of obtaining good sections through the latter. 

 It is, therefore, necessary in most cases to employ only the information obtained from 

 surface view, and since in most cases the different types of stomata are chiefly to be 

 distinguished by their appearance in transverse section, our work becomes much more 

 difficult. It is possible, however, to notice a number of points of resemblance, and in 

 fact many interesting comparisons can be made. 



In the first place we may notice that the fossil fronds, like the modern ones, exhibit 

 some xerophytic tendencies in the thickness of their cuticles, in the frequently sunken 

 stomata, and in the very small number of stomata per square millimetre, the numerical 

 distribution of stomata in both being remarkably similar. These facts may indicate 

 that the xerophytic characters of the modern Cycadean fronds are ancestral. 



Comparisons can be drawn between the shape and structure of the guard-cells. Tlie 



o 



h stomata of Bennettites Morierei figured by Lignier, indicate that 

 Bennettitalean stomata had a structure of the general Gyranosperm type, and thai 

 guard-cells showed the characteristic Cycadean form. The appearance in surface ^ 

 of the ffuard-cells of several eenera, when they are not obscured by overlying eel 



^ 



somewhat characteristic. It is well seen in Stangeria, Bowenia, and somctnnes m 

 Zamia, Macrozamia, and Encephalartos . The guard-cells are thickened in a peculiar 

 way with a smaU oval thin patch round the pore, on either side of which is a spindle- 

 shaped or crescentic thickened mass. There is a thin patch above each end of the 

 guard-cells, but a solid or Y-shaped thickening at the points of contact of the guard- 

 cells. The resulting shape, which may be seen from the figures, seems to be fairly 



for all modern Cycads, and is very characteristic, being modified in the 



of 



the deeply sunken forms. The same type of stoma is seen in most of the fossil fronds 



examined. 



The Bennettitalean stomata possessed guard-cells with very peculiar thickenings, but 

 which are referable to the same general type as exhibited by the modern fronds. We 

 had a central thin region round the pore and two fusiform or crescentic patches on 

 either side (compare the figures of Zamites, PL 19. fig. 2, and Otozamites, PL 19. fig. 3, 

 with figures of Zamia, text-fig. 21, and StangeHa, PI. 17. fig- !)■ 



We liave to remember, however, that in looking at the recent stomata the most 

 noticeable thickening is due to the lignification of the guard-cells, while in the fossil 

 forms presumably nothing but cutin remains. In order to get preparations of the 

 recent structures which are really comparable with the fossils, we may get rid of the 

 woody tissue by continued boiling in Schultz's solution ; in the resulting preparations 

 little but cuticle will be left. Preparations made in this way show considerable 

 similarity with the fossil forms (^/.text-fig. 32 with figures on Plate 19), the woody 

 thickening seems to have been accompanied by a certain amount of cuticularisation 

 which is especiaUy seen at the poles, while on either side of the pore the thickenings 

 on the upper surface of the guard-cells are seen as the spindle-shaped patches which 



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