THE POLYPLOID SERIES IN OSMUNDA 



While the presence of both trivalents and univalents thus entails an element of random 

 variation in chromosome numbers at the close of meiosis, this is of only limited 

 extent in the tetraploid. In the triploid, on the other hand, variation will be extreme, 

 since random segregation here concerns not one or two chromosomes only but the 

 whole of the third haploid set. The probability that these will distribute themselves 

 to the poles to give exactly balanced spores with n or 2n chromosomes is only one in 

 about two million. All other types of spore will be unbalanced and possess chromo- 

 some numbers ranging from haploid to diploid but with the majority midway 

 between. 



Fig. 25. The behaviour of univalents at the two meiotic divisions in triploid Osmunda. a. Anaphase 

 of the first meiotic division showing lagging univalents in the act of splitting longitudinally. 

 b. Anaphase of the second meiotic division showing the half chromosomes produced from univalents 

 at the previous division lagging again on the spindle in both dividing cells since they are unable to 

 split longitudinally a second time. Lagging chromosomes at either of these divisions are therefore 

 very clear indication that unpaired chromosomes have been present earlier. From a section 

 stained in gentian violet, x 2000. 



That the expected types of spore are, indeed, produced is readily ascertained either 

 by counting chromosome numbers at the close of meiosis or by examining the early 

 mitoses in the germinating spores. The latter is the less laborious task, for observations 

 can be made within a week of sowing the spores. Two sample chromosome counts 

 showing 33 and 27 chromosomes respectively in spores from the triploid are given in 

 Fig. 2'ja and b. 



Table 5 summarizes chromosome counts obtained in several successive years among 

 spore sowings from the triploid and the preponderance of grossly unbalanced types is 

 exactly as expected. Comparable figures for spores from the tetraploid are given in 

 Table 6 and again expectation is reaUzed. Gross unbalance is this time absent, but 

 a high proportion (approximately two-thirds) of the spores which begin to germinate 

 have one, or a few, chromosomes too few or too many. This proportion, it may be noted, 

 resembles very closely the relative proportion of mother cells containing trivalents 

 (Table 4). 



39 



