THE POLYPLOID SERIES IN OSMUNDA 

 Table 2. Frequency of trivalents in 109 cells of autotriploid Osmunda (^2 = 22) 



No. of trivalents per cell 



7 8 9 10 II 12 13 14 15 16 17 18 19 20 21 22 

 No. of cells .11 I 5 7 9 II 13 12 12 17 12 8 . 



Comparable information for the tetraploid is contained in Figs. 20c, 22^, 24 and in 

 Table 3. Where four homologous sets of chromosomes are seeking partners at meiosis, 

 some quadrivalents will inevitably result, though in a proportion of cases, the exact 

 numbers varying from cell to cell, a quadrivalent will be represented by two pairs or, 

 in a smaller proportion of cases, by a trivalent and a single. In the figured cell (Figs. 

 22^, 24) only pairs and quadrivalents are contained; this is a fairly common condition, 

 but examples with one or two trivalents and singles in addition to quadrivalents are 

 almost equally so. Table 4 gives the actual frequency of trivalents in the analysed 

 cells of Table 3. 



Table 3. Analysis of multivalent pairing in 101 cells of autotetraploid Osmunda 



No. per cell 



Quadrivalents only 



Total multivalents (quadri- + trivalents) 



Table 4. Frequency of trivalents in 101 cells of autotetraploid Osmunda 



No. per cell 



, * ■ . 



01234567 



1 01 cells of Table 3 34 36 24 4 i 2 



These numerical facts have been given, in what at first sight may seem to be rather 

 pedantic detail, for two reasons. In the first place they are needed to illustrate the 

 type of observation which, when it can be carried out, is more informative than any 

 other for cytogenetic analysis. Secondly, they are required to explain the breeding 

 behaviour of the polyploids, consideration of which will conclude this chapter. 



The fertility and subsequent behaviour of the spores produced by any plant are 

 very closely connected with their nuclear content which, in its turn, is largely controlled 

 by the details of chromosome pairing at meiosis. Quadrivalents and bivalents can 

 disjoin regularly with equal ease, and if only these were produced, meiosis in a tetra- 

 ploid would be as uniform and effective as in a diploid. Trivalents and univalents 

 cannot, however, disjoin equally. From a trivalent two chromosomes will generally go 

 to one pole and one to the other at anaphase of the first meiotic division, so that the 

 resulting nuclei will at once be dissimilar. A univalent cannot disjoin at all, but instead 

 it lags on the spindle until it splits longitudinally. The half-chromosomes pass to each 

 pole at the close of the first meiotic division, but they lag again at the second, being 

 unable to split a second time. Finally, they either pass at random to one pole or the 

 other or are lost. Two views of the behaviour of univalents are given in Fig. 25. 



38 



