36 Transactions. 



stem is shown in fig. 91, in which it will be seen that the cell-walls of 

 the entire cortex are strongly thickened (taking both the safranin and 

 the haematoxylin stain) and that the brown deposit is also present. In 

 fig. 92 is shown the vascular cylinder of the same region of the stem in 

 longitudinal section, in which there is a good example illustrated of the 

 progressive method of deposit of the brown substance in the inner cortical 

 cells. The conclusion I would draw T is that whereas the general configura- 

 tion of the vascular tissues is the same for both forms, T. tannensis and 

 T. lanceolata, as regards both the rhizome and the aerial stem, yet there 

 are certain less important but constant histological differences between 

 them. The rhizome of T. tannensis does not attain as large a size as 

 that of the loose-humus-growing T. lanceolata, and hence does not show 

 the same extent of development of vascular tissues with the consequent 

 splitting-up of the xylem into constantly changing groups. Also, in the 

 drooping aerial stem of T. lanceolata there is an absence of the thickening 

 of the walls of the cortical cells and of the formation of the brown deposit, 

 both of which features are present in the more xerophytic stem of 

 T. tannensis. 



From the present study it would seem that there is no great difference 

 between the stele of the rhizome and that of the aerial stem, and this one 

 would expect, seeing that they are merely different regions of the plant- 

 shoot, differing only in function. Any of the rhizome-branches are able 

 to emerge from the surface of the humus and develop leaves. In the 

 youngest plantlets the shoot is all rhizome, and one or both ends of it 

 turn upwards and acquire the aerial habit. The rhizome portion functions 

 largely probably as a storage organ, bearing rhizoids, harbouring an 

 abundant mycorhiza, and showing the presence of starch in the cortical 

 cells. The aerial stem shows an absence of all these characters, but the 

 comparatively large leaves, with their strongly decurrent bases and the 

 fertile structures, constitute its dominant feature. In the youngest plants 

 the configuration of the vascular tissues is identical in both rhizome and 

 aerial region. In both, as the number of vascular elements increases, there 

 is manifested a disposition for the xylem to arrange itself in groups 

 surrounding a central " pith," this being more marked and definite a 

 feature in the aerial stems, probably on account of the influence of the 

 leaf-trace system. In the aerial stems the xylem strands are character- 

 istically mesarch, and Miss Sykes has shown that this is so also in those 

 parts of the rhizome where the xylem is arranged in separate strands. 

 In both there is a disposition for neighbouring xylem strands to coalesce 

 to form curving plates of tissue surrounding the central pith as by a broken 

 ring. Thus the nature of the full-grown stele throughout the Tmesipteris 

 plant, and the manner of its development both at the apex of the mature 

 rhizome and in the young plant, from the monarch or collateral condition, 

 through the stages of diarch, triarch, and quadrarch to the ring-like con- 

 dition, may be closely compared with the form and development of the 

 stele in the adult plant of Psilotum triquetrum such as Miss Ford (1904) 

 and Mr. Boodle (1904) have described it. In his paper Boodle traces the 

 similarity between Tmesipteris and Psilotum with regard to the stem- 

 anatomy, and shows that one great point of difference between them — 

 viz., the mesarch structure of the xylem strands in the aerial stems of the 

 former — to a certain extent breaks down owing to his discovery of isolated 

 instances of mesarch structure in the lower regions of the aerial stem of 

 Psilotum. 



