THE POLYPLOID SERIES IN OSMUNDA 



_ ■<-■/ 



^ 



c:-aMinp -W^^^w*-** 





!8v. 



Fig. 26. Comparison of germination in diploid, triploid and tetraploid Osmunda. x 40. a. Diploid 

 prothalli from the tetraploid. b. Prothalli derived from the triploid, almost all abnormal and 

 many non-viable, c. Normal haploid prothalli from the diploid. 





Fig. 27. Chromosomes in descendants of triploid Osmunda. Permanent acetocarmine. x 500. 

 a, b. Mitosis in two different germinating spores showing 33 chromosomes [a) and 27 chromosomes 

 {b). c. Meiosis in a sporophyte produced from a spore sowing from the triploid showing presence 

 of one extra chromosome which makes a single trivalent among the otherwise normal pairs. For 

 further description see text. 



The subsequent fate of these young prothalli is a matter of considerable interest. 

 The progeny from the tetraploid have not been closely followed, though it would 

 be expected that some unbalanced sporophytes must certainly be represented in the 

 next generation. The situation in the triploid has, however, been studied as closely as 

 circumstances would permit, because it might be expected that unbalance on the 

 scale shown might lead to the production of fundamentally new types of plant if a 

 prothallial culture of the type obtained could be self-fertihzed. That this expectation 

 was not fulfilled provides a highly instructive example of the power and mode of opera- 

 tion of natural selection. 



The uneven appearance of a culture of spores from the triploid at the age when 

 chromosome counts can most easily be made is shown in Fig. 26^, and this uneven- 

 ness is never effaced. Many of the spores which start to grow proceed no further, 



40 



