APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES 



In. addition, the general appearance of the cells in question, especially the relatively 

 large number of unpaired chromosomes, is very strong confirmation of a necessary 

 concomitant of triploidy (whether this be auto- or alio-), namely, hybridity. For the 

 attainment of the triploid state wherever found would seem to necessitate an immediate 

 or antecedent cross between two dissimilar plants, at least one of which must have 

 been sexual. 



Since all three of the wild species and the one horticultural variety, var. Rockfordii, 

 have the same chromosome number, it must be further assumed that the morphological 



i9l«# 





^^^^ 



Cc/r/^om/am 



3n = c. /20 



Fig. 182. Explanatory diagrams to Fig. 181. All x 3000. a. The Wisconsin plant. Chromosomes in 

 focus in Fig. 181 a shown in black, those in focus in Fig. 181 c in dots, and the remainder in outline. 

 Approximate count is 1 19-123. b. The cell of Fig. 181^. Approximate count c. 121. c. The 

 cell of Fig. 181 «. 



differences which distinguish them are secondary to their apogamous habit and have 

 been developed later than this by a process presumably of genie mutation. It is obvious 

 that in any apogamous species non-injurious mutations would tend to be transmitted 

 indefinitely without disturbance or segregation. 



The next species to claim our attention can be Dryopteris atrata (Wallich) Ching (Fig. 

 186), formerly known as Nephrodium hirtipes and partially investigated by Steil (1915, 

 1919) in work which has been already quoted (p. 160 above). The species in the wild 

 state occurs from China to the Himalayas and is available only in botanic gardens, 

 since it is not a fern of commerce. My material was obtained from Kew and since 



180 



