760 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 



whether the sporangia terminated the main axes or some of the lateral branches. 

 It is, however, certain from detached specimens found in the silicified peat that some 

 aerial axes ended in large, elongate-pointed sporangia. 



That the rhizomes, aerial stems, and sporangia were portions of the same plant 

 might have been inferred from their association in a bed composed of one type of 

 plant only. Their continuity has, however, been directly traced, and is established 

 by anatomical evidence. 



Quite apart from any question of affinity, the build of Rhynia, with its rootless 

 rhizomes of delicate structure sending up xerophytic aerial stems, finds its closest 

 parallel among existing plants in the general morphology of Psilotum. 



Anatomy of the Vegetative Organs. 



The plant consists throughout of cylindrical axes with an epidermis, a relatively 

 wide cortex, and a simple central cylinder. The latter has a solid strand of tracheides 

 surrounded by a zone of phloem. It will be convenient to describe the rhizomes in 

 the first place and then to deal with the aerial stems. 



Rhizome. 



The rhizomes seem to have been formed of more delicate tissue than the stems 

 arising from them, so that the peat often consists entirely of the latter, the rhizomes 

 having decayed. In other portions of the peat, however, well-preserved rhizomes 

 have been found; thus PI. IV, fig. 13, shows a small portion of a section through 

 the peat in which rhizomes are seen in their natural position. The section 

 passes through three pieces of the rhizome. The uppermost is cut transversely, 

 and shows the broad cortex and the single stele. The lowest section is also 

 transverse, but goes through a rhizome where dichotomous branching is about to 

 take place, the stele having already divided. The middle section passes longi- 

 tudinally through a rhizome, just missing the xylem of the stele except on the 

 extreme right. The lower side of this rhizome shows two large hemispherical bulges 

 of the outer cortical tissues. Portions of these bulges, more highly magnified, are 

 represented in figs. 11 and 12 on PI. III. The greater development of the cortex 

 on the lower side of the two transverse sections in fig. 13 is due to the plane of 

 section passing through similar bulges. From the downwardly directed surfaces 

 of all three rhizomes, and especially from the hemispherical bulges of the middle 

 piece, numerous rhizoids extend into the peat.* 



Sections through other rhizomes are shown on PI. IV. In fig. 17 two rhizomes 

 of different sizes are seen in transverse section. The stele of the larger one has 

 divided preparatory to dichotomous branching. From the epidermal cells of its 



* The peculiar oval bodies in the cortex of the two lowest rhizomes in fig. 13 are the reproductive organs of 

 saprophytic fungi. The detailed description of the numerous fungi which occur in the deposit is reserved for a 

 future comnfunication, but the reader must allow for their presence in many of the illustrations to this paper. 



