PSILOTOPSIDA 45 



plete accessory fertile appendage, while in Fig. 6M both 

 leaves are so replaced and instead of the sporangial region 

 there is a single leaf. There has for a long time been a widely 

 held belief that freaks are 'atavistic', i.e. they are a reversion 

 to an ancestral condition. However, it must be stressed that 

 this beUef rests on very insecure foundations. As apphed 

 here, the conclusion has been that the reproductive organs 

 of the Psilotales are reduced from something more complex, 

 at one time assumed to have been a fertile frond. It may well 

 be, however, that the only justifiable conclusion is that, at 

 this level of evolution, leaf and stem are not clearly distinct 

 as morphological categories, and that they are freely inter- 

 changeable — interchangeable on the fertile appendages of 

 abnormal plants, just as, on any normal shoot, fertile 

 appendages replace leaves in the phyllotaxy. 



Few botanists have had the good fortune to see living 

 specimens of the gametophyte (prothallus) stage of either 

 Psilotum or Tmesipteris, but all who have testify, not only to 

 their similarity to each other, but also to their remarkable 

 resemblance to portions of sporophytic rhizomes. So similar 

 are the prothalU and sex organs of Psilotum to those of 

 Tmesipteris that the same diagrams and descriptions will 

 suffice for both. Like the rhizomes the prothalli are irregu- 

 larly dichotomizing colourless cylindrical structures, covered 

 with rhizoids (Fig. 7A), and the similarity is further en- 

 hanced by the fact that they are also packed with mycor- 

 rhizal fungus hyphae. Both archegonia and antheridia are 

 borne together on the same prothallus (i.e. they are mon- 

 oecious), but because of their small size they cannot be used 

 in the field to distinguish prothaUi from bits of rhizomes. 

 Stages in their development are illustrated in Figs. 7B — H 

 (archegonia) and 7I — M (antheridia). ^^ 



The archegonium is initiated by a pericHnal division in a 

 superficial cell (Figs. 7B and 7C) which cuts off an outer 

 'cover cell' and an inner 'central cell'. The cover cell then 

 undergoes two anticUnal divisions, followed by a series of 



