38 Transactions. 



and that of Tmesipteris as constituting a valid argument against the 

 possibility of the former belonging to the Psilotaceae. 



I must enter more into detail in comparing Lawson's observations on 

 the prothallus of Tmesipteris with my own, because although it will be 

 clear that they correspond in many particulars, yet it will be just as 

 obvious that the two accounts differ in many other respects. 



First of all, then, with regard to the similarities in the two accounts. 

 The prothallus is shown in both to be subterranean and saprophytic in 

 habit, of a characteristic brown colour, and covered with numerous lonu 

 rhizoids. It is cylindrical in form, is not differentiated into reproductive 

 and vegetative regions, and can branch. There is an endophytic fungus 

 which is found in any part of the prothallus-body and is not localized in 

 definite zones. The antheridia and archegonia are intermixed, and are 

 distributed in large numbers over practically all parts of the surface of 

 the prothallus. The two accounts of the structure of the mature sexual 

 organs are closely similar. The embryo is carried on a distinct pro- 

 tuberance of the prothallial tissues, the result of localized meristematic 

 activity in the cells of the latter keeping pace with the development of 

 the embryo. The embryo shows a hypobasal and an epibasal portion, the 

 latter being characterized by a peculiar development from its surface of 

 lobes or protuberances. This general similarity in the two sets of prothalli 

 and their essential organs might be sufficient to show that they both belong- 

 to the same order, Psilotaceae, or even also to the same genus, Tmesipteris. 



But there are also some very striking differences between them which 

 must be considered. To begin with, Lawson states that, " compared with 

 the Lycopodiales and other Pteridophytes, the prothallus of Tmesipteris is 

 small." His largest specimen measured only I in. in length. My prothalli, 

 except the very youngest, were very large compared with this, several of 

 the largest being up to f in. in length. The tissue of Lawson's prothalli 

 " is extremely soft t,nd fragile," and easily destroyed in the process of 

 cleaning with a camel's-hair brush, whereas my prothalli are firm and 

 solid and thick, and are very favourable objects for hand-sectioning in 

 elder-pith. A small but striking point of difference lies in the fact that 

 Lawson describes the rhizoids as characterictically twisted, but in my 

 figures they are shown as perfectly straight. Lawson speaks of the endo- 

 phytic fungus as being " more conspicuous in the surface cells and those 

 near the surface,'" although it may extend into the very interior of the 

 prothallus. I found that it was only in the oldest and lowest regions of 

 the prothallus that the fungus inhabited the epidermal cells and those of 

 the cortex immediately underlying it, but that it was uniformly present 

 throughout the prothallus-body (except, of course, at the growing apices) 

 in the more centrally placed cells. A comparison of figs. 1, 2, and 3 in 

 Lawson's paper with any of those in mine which show the complete 

 prothallus will reveal a noticeable difference in the fact that in the latter 

 cases there is always a bluntly rounded apex to each branch of the 

 prothallus, the growing apices usually taking the form of a swollen head, 

 whereas in the former the ends of the branches are shown (if not broken) 

 as pointed structures. It will be noticed that these differences between 

 the two accounts relate entirely to the external form of the prothallus and 

 the disposition of the fungal element. The appearance and structure of 

 the mature sexual organs is identical in both accounts. I must here point 

 out that the archegonia as seen and figured by Lawson, and described by 

 him as being very simple and peculiar, are only the old organs which, as has 

 been shown in the present paper, have lost the upper tiers of ceck -cells. 



