Hollow ay. — The Prothallus <t>ul Young Plant of Tmesipteris. 43 



rather than reduced. The account given in the present paper of the 

 life-history of Tmesipteris lends weight to Kidston and Lang's suggestion 

 that the Psilotaceae, on account of their remarkable resemblance to Rhynia, 

 are to be regarded as possessing j)rimitive characters. The structure of 

 the sexual organs, of the embryo, and of the young plant of Tmesipteris 

 confirm the idea that the Psilotaceae should be removed from all other 

 existing classes of Pteridophytes. The structure and form of the prothallus 

 is also peculiar, but probably the gametophyte generation is always too 

 adaptive to form the basis for much generalization. The simple stele 

 found throughout the young plant of Tmesipteris in both rhizome and 

 aerial stem resembles that of the Psilophytales. The theory that the 

 mature plant of the Psilotaceae, as regards both its more complete vascular 

 anatomy and also the nature of its sporophylls, finds in the Sphenophyllales 

 its nearest resemblances is quite compatible with the belief that in other 

 respects the Psilotaceae have preserved the same primitive characters as 

 are exemplified in Rhynia. 



Just what is the degree of relationship between the Psilotaceae and 

 these groups of fossil Pteridophytes is still, of course, far from clear. But 

 this much, at any rate, may be said : that we have learned to look for the 

 nearest relationships of this peculiar modern class of plants in the fossil 

 record, just as has been done in the case of the Lycopodiums and Equi- 

 setums ; and that while undoubtedly certain outstanding characters in the 

 case of each of these modern remnants of once flourishing and important 

 groups are best interpreted as reduced or even as adaptive, others, again, 

 must be regarded as primitive, for they may be directly compared with 

 corresponding characters in fossil plants. 



Postscript. 



At the same time that the proofs of this paper were returned to me from 

 the printer for a second revision Professor A. A. Lawson's second account of 

 the prothallus of Tmesipteris (Lawson, 1917b) was kindly sent to me 

 by its autl or, so that I am able to give in the form of an appendix a 

 short comparison of his corrected results with mine. 



My own account of the prothallus of Tmesipteris as given above 

 corresponds more closely with that given by Lawson in his second paper 

 than in his first. Since writing his preliminary account Lawson found 

 a large number of prothalli, a certain proportion of which would be more 

 or less complete, at any rate as regards their growing apices. One of 

 these is figured by him (fig. 1). This prothallus shows a close resemblance 

 to those figured in the present paper. Certain differences are due to the 

 fact that Lawson's prothalli occurred terrestrially in a sandy soil, whereas 

 mine were found amongst the tangle of aerial rootlets on tree-fern stems 

 where the humus was scanty. More important differences to be noted 

 are that Lawson does not describe or figure the first-formed tapering 

 region of the prothallus : he describes the branching as irregular, whereas 

 I have shown that it takes place normally according to a regular sequence 

 of dichotomies ; and the growing apices of his prothalli are not swollen, 

 as were most of mine ; also, my prothalli are stouter and more strongly 

 grown. Otherwise, it seems to be clear from our two accounts that our 

 prothalli are identical in nature. My account of the mature archegonia 

 and antheridia corresponds also with that given by Lawson in his second 

 paper. He there corrects his previous account of the mature archegonium, 

 and shows, as I also have pointed out above, that there is a straight 



