Holloway. — Tht Prothallus and Young Plant of Tmesipteris 37 



In view of the fact that Boodle and others have found secondary 

 xvlem in the transition region of the stem of Psilotum, I closely examined 

 the stems of Tmesipteris from this point of view, but found there no traces 

 of it. Also, it may be mentioned that I did not find any evidence of 

 vegetative propagation in Tmesipteris corresponding to the formation of 

 bulbils (Brutknospen) described by Solms-Laubach (summarized in Engler 

 and Prantl, 1900, pp. 612-14) for Psilotum. The long aerial stems of 

 T. lanceolata are sometimes branched, but I did not examine the branch- 

 ing of the stele. It is interesting to note that on the fertile stems the 

 sporophylls occur in clearly defined regions corresponding to the habit so 

 well known in Lycopodium Selago, and that on the longest stems as many 

 as five or six such fertile regions may sometimes be observed separated 

 from one another by sterile regions. 



Comparative Remarks. 



It now remains for me to compare the prothallus and young plant of 

 Tmesipteris as described in this paper with what has already been brought 

 forward by other writers w T ith regard to the gametophyte generation in 

 the Psilotaceae, and also to include in this comparative survey certain 

 other pteridophytic types of prothallus. 



Lang's prothallus (1904), which he has provisionally assigned to 

 Psilotum, conforms to a type which certainly differs markedly from that 

 of Tmesipteris as described by Lawson and in the present paper. The 

 differentiation of the prothallus into vegetative and reproductive regions 

 with the meristem located between them, the organization of fungal zones 

 and their evident influence upon the form and structure of the prothallus, 

 is in striking contrast to what has been described for Tmesipteris. This 

 we would probably not have expected, considering the strong morpho- 

 logical and anatomical resemblances between the two genera with respect 

 to the adult plant. And yet, after all, there is not much greater 

 difference between Lang's prothallus and that of Tmesipteris than 

 what there is between, for example, the subterranean and the epiphytic- 

 types of Lycopodium prothalli ; and we have come to look upon 

 the latter as being but different modifications of a common funda- 

 mental structure of Lycopodium prothallus. Lang notes that the 

 prothallus described by him is ' practically identical with [that of] 

 Lycopodium comphuiatum " (1904, p. 576), and goes on to show that 

 it would not lie surprising if the prothallus of Psilotum were of the 

 subterranean type, for it commonly grows as a terrestrial plant as well 

 as an epiphyte. Apparently he did not obtain from this single prothallus 

 any information with regard to the archegonium or embryo ; but as re- 

 gards the structure of the antheridium there is certainly a great difference 

 between what he has described and what is now known in the case of 

 the antheridium of Tmesipteris. However, there is nothing to be gained 

 by drawing out any further this comparison, for Lawson (1917a, p. 786) 

 states that he has discovered '" a single specimen of a structure that he 

 believes to be the prothallus of Psilotum . . . [and that this] bears 

 no resemblance to the supposed prothallus described by Lang." In a 

 later paper he has described the prothallus of Psilotum, but this account 

 I have not yet seen. The point that I wish to emphasize here is that in 

 view of the remarkable diversities in form and structure known amongst 

 the prothalli of the various species of Lycopodium we cannot regard the 

 fact of the great difference in these respects between Lang's prothallus 



