CUTICLES OF CYCADEAiS" TEONDS. 



175 



Nestler (1895, pp. 360, 364) gives a table showing the numbers of stomata per square 

 millimetre in a representative selection of species. The Ceratozamias (with from 24 to 

 31 per sq. mm.) and the Macrozamias (29 to 31) are seen to have the fewest, and the 

 Dioons {B. edule, 81, and D. angmtifolia, 72) the most numerous stomata. The 

 investigated species of Cycas (with from 49 to 56 per sq. mm.), JEncephalartos (41 to 52), 

 Stangeria (54), and Zamia (43 to 60) have very similar numerical distribution of 

 stomata, and are intermediate between the Ceratozamias and Macrozamias on the one 

 hand and the Dioons on the other. Bowenia spectahills has 35 stomata per sq. mm. on 

 the under side and 14 on the upper side of the pinnae. 



The stomata of the Cjcads as a group are larger than those of any other group 

 of Gymnosperms *. Their size seems to be fairly constant, being about '075 X '034 mm., 

 while their structure conforms to a general plan — the " Gymnosperm type " of Porsch. 

 The guard-cells in median and polar section show a constant form and position relative 

 to the general epidermal level. 



The ventral and dorsal walls of the guard-cells have in every case lignified lamellgef, 

 though these are slightly variable in extent and in clearness of definition J. They give 

 to those stomata, the guard-cells of which are only slightly sunk, a very characteristic 

 thickened appearance in surface view (PL 17. fig. 1, Stangeria ; figs. 7 & 8, Bowenia ; 

 text-fig. 26, Bowenia). In Bowenia^ for example^ the surface view of a stoma shows 

 the pore bounded on each side by a somewhat crescentic structure formed by the 

 lignified lamellae on the upper or dorsal surface of the guard-cell. This is covered by 

 a layer of cutin. The lamellae on the lower or ventral surface of tlie guard-cells are 

 only seen at the poles of the stoma, where the guard-cells are raised and the lamellae are 

 extended, appearing as thickened projections. Text-fig. 26 shows the correlation of 

 serial sections through a stoma with the surface view. The gradual raising of the 

 guard-cells towards the poles and extension of the ventral lamellae Avith accompanying 

 diminution of the dorsal lamellae are plainly seen. In the case of guard-cells which are 

 much sunk, the surrounding, and usually overarching, epidermal cells obscure the 

 characteristic appearance caused by the lignified lamellae. In intermediate conditions 

 such as exhibited by Zamia (PI. 18. fig. 13), the lamellae of the guard-cells may be seen 

 on focussing slightly below the epidermal cells. They are quite comparable with those 

 of Bowenia and Stangeria. 



The epidermal cells surrounding the guard-cells and stoma-opening—" subsidiary 

 cells " — are usually fairly constant in number and arrangement in a genus. In no case 

 was material available young enough to trace the entire development of the guard-cells 

 and subsidiary cells from an epidermal mother-cell. Four seems to be the original 

 number of subsidiary cells, the typical arrangement of these being one at each pole of 

 the stoma and one at each side. It seems possible from a comparison with Gnefum and 

 Welwitschia §, together with a consideration of fairly young examples in Cycas revohita 



Malhert (1885), p. 57. 

 t These are not figured in every case ; they are shown in text-figs. 10 & 26 and in PI. 18. figs. 4 & 6. 

 t Karzel (1908). § Takeda (1913'), p. 351 ; & (191 3'), pp. 365, 366, text-fig. 8. 



