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ELIZABETH W. DAVIES 



like Carex which has an aneuploid series with many alhed species with consecutive chromo- 

 some numbers. 



Finally young male spikes were fixed and meiosis was investigated using the modified 

 aceto-carmine method. This produced well stained and spread chromosomes and, in 

 contrast, a light background, as the cytoplasm absorbed little stain. 



3. Chromosome Numbers 



The chromosome numbers have perhaps not proved as valuable a criterion in the 

 taxonomy of these species as had formerly been hoped. However, the chromosomes 

 of the British members of the Extensae and of C. mairii Coss. & Germ., a closely allied 

 species from southern Europe, have been examined from several localities, and are 

 remarkably similar and uniform in number and morphology (Plates 12 and 13, figs. 1-14). 

 This is further evidence that they are a naturally closely united group distinct from 

 the other sections, such as Distantes, Acutae, Montanae, etc., which differ from them 

 in size and morphology of their chromosomes. 



The chromosome numbers of these species together with their subspecies and 

 varieties are given in Table 1. The number of plants of each species and their localities 

 is stated in each case. 



Thus this aggregate of species forms a short aneuploid series with chromosome 

 numbers ranging from 30 to 35. If this series is ascending, the low numbered members 

 are assumed to be primitive, while the species with more numerous chromosomes are 

 secondary and advanced. If this is the case, C. flava is the oldest and original member 

 of this aggregate, and the other species are derived and more recent in origin. This 

 aspect of the problem will be referred to, and elaborated, later, when the curious abnor- 

 malities, found at meiosis in hybrid sedges, have been discussed in the next section. 



4. Hybrids and Hybridisation 



The study of the interspecific hybrids of a critical group such as the C. flava aggregate 

 is of great importance, as it frequently yields information about the relationships of the 

 species, their age and evolution. The natural and artificial hybrids will now be discussed 

 separately. 



(a) Natural Hybrids 



Natural hybrids are found between members of this aggregate, whenever two or 

 more of the species grow together and their flowering periods overlap. However this is 

 rather infrequent in this country, except in the case of the widespread C. demissa, and 

 it appears that ecological barriers prevent much introgression between the species. The 

 hybrids found in the field are listed in Table 2, and these will be discussed later, after 

 some of the abnormalities seen at meiosis have been briefly outlined. 



The meiosis of hybrid Carices is always highly irregular, and cytology produces 

 conclusive evidence of hybridity in a suspected plant. However, the arrangement of 

 the chromosomes and their behaviour at metaphase and anaphase is very different from 

 what is usually seen in hybrids of most genera. First, and perhaps of most fundamental 

 importance, there are no lagging chromosomes at anaphase I, but these can usually be 

 seen to a small extent at anaphase II. Thus the normal sequence of events generally 

 accepted for hybrids in most genera, with the possible exception of Luzula, is reversed. 

 Secondly, at metaphase I, instead of the occurrence of tri- and bi-, or univalents, depending 

 on the nature of the hybrids, there are frequently chains or even rings formed of from 

 3 to 6 or even 8 chromosomes (Plate 13, fig. 16) and round the periphery of the metaphase 

 plate univalents are often seen. 



