THE PSILOTALES 



laxity of the spiral structure, traces of which are nevertheless detectable at anaphase in 

 the former species (Fig. 236). These appearances, which will be met with again in 

 Lycopodium and Isoetes, may be of little phyletic significance; on the other hand, they may 

 perhaps add one slight contribution to the other reasons for believing these various 

 genera to be related. 



Secondly, with regard to the details of chromosome number, it is much to be desired 

 that the demonstration of the basic haploid for the group should be made clear with 

 complete finality, since in the present state of knowledge comparisons are suggested 





Zorjr 



Psi/otum n = 52-4 



Fig. 237. Explanatory^ diagram to Fig. 236a 

 from Manton (1942). x 1500. 



Fig. 238. Small shoot of Tmesipteris tannensis (Spreng.) 

 Bernh. of unknown wild origin from a live specimen 

 grown in the Botanic Garden at Glasnevin, Dublin. 

 Natural size. 



which, if based on erroneous information, may be very misleading. If the lowest 

 gametic number were indeed to be 54, it is tempting to draw a comparison not only with 

 the 108 of Equisetum but also with n = g o{ Selaginella which will be seen in the next 

 chapter. It is conceivable that all these high numbers are the end-products of very 

 ancient polyploid series which relate back to simple beginnings even in those groups such 

 as Equisetum, in which no direct traces of polyploidy at present remain. Further discus- 

 sion of this is not possible without facts of irreproachable accvu"acy to go upon; the 

 matter is, however, worth mentioning here in order to act perhaps as a spur to a favoured 

 observer who, with wild material accessible to him, may perhaps be tempted thereby to 

 make the very considerable effort required to remove our present doubts. 



SUMMARY 



Summing up the facts for the Psilotaceae, even in their present imperfect state, it has 

 now been shown that polyploidy exists in each of the two living genera of Psilotum and 

 Tmesipteris, the lowest number now known at the base of the series being of the order of 

 50 odd in Psilotum and 100 odd in Tmesipteris. The fullest information at present 



16 



MPC 



241 



