CUTICLES or ctcadean fronds. 



195 



character to those of Anomozamites and the Eennettitalean group. Nathorst compares 

 the structure of these forms with those of Otozamites, Fterophyllvm, &c., and it does 



w 



indeed seem prohahle that these Pseudocycas fronds should be included in the Eennetti- 

 talean group, though in the disposition of the stomata in a furrow they are distinct 

 from any of the known Jurassic forms. 



Berry, in his account of the Lower Cretaceous flora of Maryland (1911, p. 334), gives 

 a diagrammatic figure of the cuticle of Bioonites (= Zamites) BucJdanuSy Ett. The 

 cells as in Tseudocycas are arranged in regular rows. They are rectangular and 

 had somewhat undulating walls. The stomata are all orientated in the same direction 

 (cf. Ftilophyllum, &c.), presumahly with their long axes parallel with the long axis of 

 the pinna. If the analogy with JPtilophyllum held, however, the stomata should he 

 orientated at right angles to the long axis of the pinna. The structure of the guard- 

 cells is shown diagrammatically and cannot therefore be strictly compared with our 

 types ; there is considerable probability, how^ever, that they are similar in foim to those 

 of Otozcmiites and TcBuiopteris. Schenck has figured Dioonites Bunkericmus with 

 sinuous walls and stomata irregularly scattered. We can probably, therefore, include 

 Zamites (Bioonites) in the Eennettitalean group. 



The cuticular structure of various species of Thinnfeldia has been described by Sclienk, 

 Seward, Gothan, Zeiller, and other authors. It is very doubtful, however, whether 

 this genus should be included among the Cycadophyta. The cuticular structure seems 

 to be distinct from that described above, and is apparently more closely related to that 

 of the Ginkgoales, especially so far as the stomatal form is concerned. There is an 

 approach to the structure shown by Nilssonia medicma, but on the whole Thinnfeldia 

 seems to be quite distinct. 



(d) Summary of the Structure of the Fossil Cuticles. 



■ 



The comparison of the structures just described brings out very clearly one point, 

 viz., that the Cycad-like fronds from the Jurassic rocks of Yorkshire fall into two 

 sharply divided groups according to their epidermal structure. The fronds in the first 

 group have epidermal cells with sinuous walls and comparatively thin cuticles ; the cells 

 are in general rectangular in shape, but in most cases their shape is not very apparent. 

 The stomata occur only on the lower side of the leaf and are usually absent below the 

 vein-courses; they exhibit a strong tendency towards arrangement with tlicir axes at 

 right angles to the veins. Though there is some variation in the details of stomatal 

 structure, the general plan is as follows. The guard-cells were level with the surface or 

 only slightly sunk ; the outlines of the guard-cells themselves are never seen but only 

 those of the thickenings which they possessed. The thickenings of the guard-cells 

 usually appear as lamellae of definite shape and give the stomata a characteristic 

 appearance; on either side of the pore is a pair of smaller patches, probably representing 

 the lamella from the lower side of the guard-cells, and outside there is another pair of 

 lamellae, usually somewhat hemispherical in shape, representing the thickenings from 

 the upper side of the guard-cells. At the poles there are usually two clear spaces, 



SECOND SERIES. — BOTANY, VOL. VIII. 



2 



