48 THE MORPHOLOGY OF PTERIDOPHYTES 



phylogenetic relationship. However, until more is known of 

 the factors which determine the polarity of developing 

 embryos, such suggestions should be received with consider- 

 able caution. 



For many years there has been speculation among 

 botanists as to the kind of life-cycle that might have been 

 exhibited by the earhest land plants. Some held the belief 

 that there was a regular alternation of sporophytes and 

 gametophytes that resembled each other in their vegetative 

 structure and that even their reproductive organs (sporangia 

 and gametangia, respectively) could be reconciled as having 

 a similar basic organization: on this basis, the generations 

 were regarded as 'homologous'. Others believed that the 

 sporophyte generation evolved after the colonization of the 

 land by gametophytic plants. From being initially very 

 simple, the sporophyte then evolved into something much 

 more complex, by reason of its possessing far greater poten- 

 tialities than the gametophyte. On this basis, the generations 

 were regarded as 'antithetic'. Until bona fide gametophytes 

 are described from the Devonian, or earlier, rocks, there is 

 little hope that this controversy will be resolved satis- 

 factorily. All that can be done is to examine the most 

 primitive living land plants and see whether, at this level 

 of evolution, the sporophyte appears to have fundamentally 

 different capabilities. 



The extremely close similarity in external appearance 

 between the gametophytes of the Psilotales and their rhi- 

 zomes is, therefore, of more than passing interest. Until 

 1939, however, it was believed that there was one important 

 anatomical distinction between them, in that gametophytes 

 were without vascular tissue. In that year, Holloway^* des- 

 cribed some abnormally large prothalli of Psilotum from 

 the volcanic island of Rangitoto, in Auckland harbour. New 

 Zealand. These were remarkable in having well-developed 

 xylem strands, of annular and scalariform tracheids, sur- 

 rounded by a region of phloem which, in turn, was enclosed 



