PSILOTOPSIDA 49 



by a clearly recognizable endodermis. There was, therefore, 

 almost no morphological feature distinguishing them from 

 the sporophytic rhizomes, except their archegonia and 

 antheridia. It was subsequently found^^ that the cells of these 

 prothalli contained twice as many chromosomes as those 

 from Ceylon (i.e. they were diploid), while the sporophytes 

 from this locality were tetraploid. To some botanists, this 

 appeared to be sufficient to explain the presence of vascular 

 tissue, and tended to diminish the importance of the similar- 

 ity of these gametophytes to the rhizomes. But it must be 

 emphasized that diploid prothalh are known elsewhere 

 among pteridophytes and that no morphological aberration 

 need necessarily accompany a simple doubling of the 

 chromosome number. This being so, then, whatever their 

 chromosome content, these abnormal vascularized prothalli 

 still provide strong support for the Homologous Theory of 

 Alternation of Generations. This topic is discussed further 

 in the final chapter. 



Concerning chromosome numbers generally in the group, 

 it now appears that all plants of Psilotum nudum from Aus- 

 traha and New Zealand have the same chromosome number 

 n= 100-105, while plants from Ceylon are like Psilotum 

 flaccidum in having about half this number (n=52-54). 

 Tmesipteris tannensis has a chromosome number n=200+, 

 while of the six new species (or subspecies) recognized by 

 Barber^^ five have n=204-2io and one has n= 102-105. It is 

 suggested that both Psilotum and Tmesipteris occur in poly- 

 ploid series, but that both have the same basic number. 



