302 



MARTIN BORRILL 



pairs in G. flmtans (Figs. 1, 5). Metaphse plates of G. fluitans could generally be distin- 

 guished from those of the other species by the more compact appearance of the bivalents. 



G. plicata 



In this species, plants from eight populations were studied in detail. In two of these 

 (H.414 and H.423) pairing was regular with twenty bivalents at metaphase (Fig. 6). Ana- 

 phase stages were very clear and one bivalent was occasionally seen to disjoin later than 

 the others, probably associated with an interstitial chiasma; the same behaviour occurs 

 in G. fluitans (Figs. 2, 7). In the other six populations a variable number of pollen mother 

 cells had one, occasionally two quadrivalents, the rest of the chromosomes pairing as 

 bivalents (Fig. 8). The sporadic occurrence of quadrivalents is consistent with the view 

 that occasional pairing takes place between homologous chromosomes from all four 

 genomes in the tetraploid, and is not due to segmental interchange. 



Considering all the populations, the percentage of cells with one or two quadrivalents 

 ranged from 0 to 41 ; as follows : — 



H. 414 H.423 H.400 H.472 H.430 Race G H.444 H.418 



Nil Nil 8 11 15 19 32 41 



The majority of these quadrivalents were of alternate configuration and disjoined 

 regularly; the pollen fertility approached 100 per cent. 



In the plant represented by specimen H.418 from the population with the highest 

 quadrivalent frequency, giant pollen mother cells were observed with twice the somatic 

 chromosome number. These cells were scattered amongst normal cells and divided at 

 the same time, and all showed features characteristic of first meiotic metaphase. It would 

 appear therefore that these giant ceils arise during a division preceding meiosis as a con- 

 sequence of failure in cell wall formation. 



Sterile G. plicata (H.426) 



This sterile clone was discussed in a previous paper (Borrill, 1956), the conclusion 

 being drawn that on morphological grounds it could be classified as G. X pedicellata 

 or G. plicata forma. This material was therefore examined cytologically ; at first metaphase 

 most pollen mother cells displayed one or more multivalents, and the different types 

 present in five panicles are classified in Table 3. 



Table 3. 



First Metaphase pairing in 5 panicles of sterile G. plicata (H.426). 



Panicle 

 Number 



Number 

 of Cells 



Type of association per cell 



II 



III 



IV 



0-07 

 0-13 

 0-01 

 0-02 



15-7 



12- 1 



15- 2 



13- 5 



16- 3 



0-07 

 0-13 



0-01 



1-20 

 1-25 



0- 90 



1- 00 

 MO 



0-01 

 0-01 



Total 



Average per ceil 



52 



0-046 



14-6 



0-04 



1-09 



0-004 



