SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 769 



adjacent stems. Its cells are smaller than the underlying cortical cells. Their outer 

 wall is thick, and in favourable cases exhibits a distinction into several layers, the 

 outermost layer being the strongly developed cuticle. More usually, as in this 

 figure, it is so preserved as to appear thick and uniform. The lateral and inner 

 walls are thin. The epidermal cells are longer than broad, and, viewed from the 

 outside, either on the surface of exposed stems (PL III, figs. 6-8) or in tangential 

 sections (PL VI, figs. 31-32) are seen to be broadly fusiform. They are often 

 characterised by a peculiar dark median line (fig. 3l). Suitable transverse sections 

 show that this line is the expression of a sharp cuticular ridge springing from the 

 middle of the slightly convex surface of each epidermal cell. This ridge is shown 

 in fig. 36, where, however, a slight obliquity exaggerates the thickness of the outer 

 cell wall. In other cases this ridge is wanting, sometimes at least as the result of 

 imperfect preservation, but in some regions of the stem the epidermal cells were 

 wider and normally lacked the central ridge (PL III, fig. 6 ; PL VI, fig. 32). This 

 lias been verified from the study of outer surfaces of stems by reflected light as 

 well as from sections. 



Stomata occurred in the epidermis, but in no region do they seem to have been 

 at all numerous. They have not been observed on the rhizome, but in the case of 

 the stem springing from a rhizome in PL IV, fig. 14, a stoma was present on the base 

 of the branch at St. They were thus present in the epidermis of the transition 

 region, though doubtless more abundant on the upper portions of the aerial stems. 



A stoma is shown in surface view T in PL VI, fig. 32, and figs. 33 and 34 are of 

 stomata in transverse section. Fig. 37 passes longitudinally through a stoma at st. 

 ks these figures show, there is nothing peculiar about the shape of the guard cells, and 

 they are not depressed below the general surface, though the sparseness of the stomata 

 and the thickness of the cuticle indicate the xerophytic construction of the plant. 



Cortex. — The outer cortex (PL VI, fig. 35, ox.) consists of one to four layers of 

 cells which are large relatively to the cells of the epidermis and of the inner cortex. 

 It is best marked in the lower and thicker portions of the aerial stems. The cells 

 of the outer cortex generally appear clear and empty-looking, and are slightly 

 longer than broad. 



The cells of the inner cortex are round in transverse section and were separated 

 by fairly large intercellular spaces (PL VI, figs. 38 and 40). They are longer than 

 broad, this being more marked on passing from above- downwards in the stems 

 (cf. PL VII, fig. 42, with fig. 43). As preserved, the inner cortex often has a darker 

 brown colour which is specially marked in the cells immediately below the outer cortex 

 (PL V, figs. 21, 22, and 24). In a few cases the cells of the inner cortex were filled 

 with closely crowded bodies suggestive of chloroplasts or possibly starch (PL VI, 

 fig. 39). The inner cortex was especially liable to decay, and stems are often found 

 in which it had wholly disappeared while the stele and. outer cortex remained. 



That the inner- cortex in the aerial stems probably constituted the assimilating 



TRANS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 24). 112 



