THE CYTOGENETICS OF CAREX FLAVA AND ITS ALLIES 133 



Dolomites. Here, in North Italy, the habitat and the topography seemed suitable for 

 both species and they were hybridising freely. 



(2) C. lepidocarpa 



Subsp. lepidocarpa frequently hybridises with C. demissa, and a hybrid swarm often 

 predominates, while the parent species are comparatively rare, and sometimes difficult 

 to detect. This certainly is the case on the lower slopes of Widdybank Fell in Upper 

 Teesdale (Plate 13, fig. 17) and at Hokatorp (Plate 13, fig. 18), a calcareous fen by the 

 sea in S.E. Skane, Sweden. Likewise, on base-rich Scottish and Welsh mountains hybrid 

 swarms between C. demissa and subsp. scotica are common at about 1,000 m. where 

 the distribution areas of these two species overlap, whereas below and above this altitude 

 the two species remain quite distinct. 



In the same way hybrids might be expected between C. lepidocarpa and C. serotina 

 in the few calcareous localities where these species are known to occur together. However, 

 this hybrid is rare, apparently because C. serotina flowers and fruits at least one month 

 later than the other four species. At Fulbourn Pond (Cambridgeshire), where C. lepi- 

 docarpa and C. serotina grow completely intermixed, there is no sign of hybridisation, 

 and, in late June, the former species is in ripe fruit while the latter is still in flower. 



Consequently, it was rather unexpected to find a plant at Wicken Fen, Cambridge- 

 shire, which appeared to be a hybrid of this parentage, growing with C. serotina and 

 C. lepidocarpa. This plant had an irregular meiosis and the pollen showed 80 per cent 

 sterility. The only other locality examined where these two species were found growing 

 together is a disused gravel pit on the Cotswolds near S. Cerney, and, as C. demissa also 

 grows at the edge of the pit, the situation is complicated further. A complex hybrid 

 swarm involving all three species occurs, and the range of forms is very great. Although 

 the majority of plants seem to be intermediate between C. lepidocarpa and C. serotina, 

 and some forms contain C. demissa also, there is a large stand of pure C. serotina at one 

 side of the gravel pit. This can almost certainly be correlated with the late flowering 

 period of this species, for specimens of pure C. lepidocarpa and C. demissa were not seen 

 and these species had clearly been ousted by hybridisation. 



(3) C. demissa 



This species, which is tolerant of a wide range of habitats, is frequently found growing 

 with, or near, the other species, and hybrids are fairly common. 



The hybrids with C. flava and C. lepidocarpa have already been mentioned, but 

 on Shapwick peat moor in Somerset, this species forms a hybrid swarm with C, serotina 

 var. cyperoides. The latter is rather uncommon, and it seems likely that it may, in time, 

 be lost completely, if hybridisation with C. demissa continues, for an enormous range of 

 intermediate forms is now abundant on the moor, and C. serotina var. cyperoides is be- 

 coming increasingly rare. The cytology of this hybrid has been examined, and reveals 

 a highly irregular meiosis (Plate 13, fig. 19), with a conglomerate ring of chromosomes 

 surrounded by univalents. 



(4) C. serotina 



The few hybrids that are known between C. serotina and the other members of the 

 aggregate have already been discussed, and it undoubtedly seems to be the late flowering 

 of this species that acts as the main barrier, and keeps it distinct from the other members 

 of the group. 



(5) C. scandinavica 



C. scandinavica, in the British Isles confined to northern and western Scotland, has 



