THE PROTHALLUS OF TMESIPTERIS TANNENSIS. 793 



not curved back and overlapping the surface of the prothallus, but growing straight 

 forward in the form of a straight tube. 



It will be seen from this brief description that the structure of the archegonium 

 in Tmesipteris bears no very striking resemblance to either Equisetum or Lycopodium 

 or to any other Pteridophyte. In its general form and simplicity one is inclined to 

 regard it as reduced. In certain respects it recalls Selaginella, but the resemblance 

 here is a remote one. It should be remembered that we are dealing here with a 

 plant whose sporophyte and gametophyte are both reduced and highly specialised 

 and adapted to definite habitats. It is, therefore, not surprising to find a reduction 

 also in at least one of the sexual organs. 



The Embryo. 



The discovery of the gametophyte structure of Tmesipteris will no doubt prove 

 of very great interest, not only because it fills in an important gap in our know- 

 ledge of the Pteridophytes as a whole, but also because it helps to establish the 

 phylogenetic position of the PsilotaJes. But I fully realise that in respect to the 

 latter the structure of the embryo sporophyte will prove of more importance than 

 the prothallus itself. It should be remembered that in both Psilotum and Tmesipteris 

 no roots have ever been found as a structure of the sporophyte. It will be of very 

 great interest to know whether the very young sporophyte ever possessed a root, 

 and if so at what time such a root disappears. It will also be of interest to know 

 if a suspensor is present in the very young embryo. Added to this, the facts revealed 

 from a study of the vascular anatomy of the embryo will prove of great value f 

 especially in view of their supposed relationship to the fossil sphenophylls. 



Realising the importance of these matters, I believe it will be best to leave the 

 matter of the embryo for the present until more material is obtained. I found only 

 one specimen of the very young embryo, but this unfortunately was not sufficiently 

 advanced to show any vaseular tissue, and was too old to show the initial divisions 

 of the egg cell. The embryo found is shown in figs. 6 and 16, and is represented in 

 median section. The region of the prothallus occupied by the embryo bulges forward 

 as a distinct protuberance. Carried forward on the prothallial tissue one may see 

 two archegonia, one of which has been fertilised and had developed the embryo. The 

 general position of the embryo in relation to the prothallus is seen in fig. 6. The 

 details of the embryo at this stage and as seen in median section are shown in fig. 16. 

 There is clearly here an upper and lower portion of the embryo. The upper part forms 

 the main axis of the embryo. But it was impossible to distinguish the cotyledon 

 region from the initial stem. The lower half of the embryo shows three lobes in 

 section. One of these, marked R, I interpret to be the root. The two other lobes 

 constitute the foot. But the curious division of this latter region is significant, and 

 the smaller portion of it may prove to be a suspensor. But this is merely suggestive, 

 and must be confirmed by the study of additional material. 



TRANS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 25). 115 



