SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 767 



lower side rhizoids are given off without the cortex forming a very definite projection. 

 On the other side of this rhizome a distinction is evident between the narrow zone 

 of outer cortex and the broad inner cortex. The smaller section in this figure has 

 a single stele, and its outer tissues continue on the lower side into a marked bulge 

 which would doubtless later have borne rhizoids. 



The position and general appearance of the rhizoids in all the examples examined 

 show that the rhizomes were growing naturally in the peat when preserved. In 

 fig. 18 a portion of the rhizoid-bearing surface of another rhizome is more highly 

 magnified, and shows very clearly the relation of the rhizoids to the peaty soil. The 

 rhizoids are seen to be non-septate, and do not appear to have been divided off from 

 the epidermal cells bearing them. In fig. 14 the rather large rhizome seen in 

 transverse section is attached by numerous rhizoids below, while from the opposite 

 side an aerial branch ascends vertically. 



The small rhizome shown rather more highly magnified in fig. 15 is preparing 

 to branch. The slender, transversely extended strand of tracheides is cut some- 

 what obliquely. Another rhizome in transverse section is shown still more highly 

 magnified in fig. 19. Around the central strand of xylem (x.) comes the thin- 

 walled phloem (ph.), passing without a sharp limit into the inner cortex (i.e.). The 

 two or three outermost layers of the cortex (o.c.) contrast with the inner cortex 

 in their appearance. The epidermis (ep.) of the rhizome has its outer cell walls 

 thin as compared with the aerial stems to be described below, and the cuticle is less 

 developed. Stomata have not been observed in the epidermis of the rhizome, the 

 cells composing which are four- to six-sided in surface view and almost isodiametric 

 (fig. 16). On the vertical branch in fig. 14, which was presumably the base of an 

 aerial stem, the epidermal cells assume a more elongated form and a stoma occurred 

 near to the base of the branch. 



The histological details of the various regions will be described more fully in 

 connection with the aerial stems. It is sufficient to recognise here, as shown in 

 the sections of rhizomes on PL IV : — 1. The epidermis, the cells of which can grow 

 out as rhizoids ; 2. The narrow zone of outer cortex ; 3. The broad inner cortex ; 

 ■i. The phloem ; 5. The slender strand of tracheides forming the xylem. These 

 tissues are often less sharply defined than in the aerial stems, but, except for the 

 differences in the epidermal layer, there is no fundamental distinction between 

 the anatomy of the rhizome and that of the aerial stem. 



The epidermis, the outer cell walls of which are thin as compared with those 

 of the aerial stem, is seen to be fairly well defined in fig. 19 and on the upper side 

 of the rhizomes in figs. 15 and 17. On the lower side, where numerous rhizoids 

 are borne, no cuticle is recognisable, and the epidermis itself is less sharply dis- 

 tinguished from the cortex on account of periclinal divisions having taken place 

 in the superficial cells. In other cases, as is best seen in fig. 13, this cell division 

 in the epidermis and outer cortical layers has led to the formation of large hemi- 



