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M. C. F. PROCTOR 



Fig. 2. Map showing the distribution of Helianthemum chamaecistus (from Meusel, 1943) and the localities 

 from which chromosome counts are given in the present paper. 



material. Cytological methods have advanced greatly in recent years, and for this reason 

 alone too much weight must not be laid on the early counts. With careful use of modern 

 methods there is no reason for serious mistakes to occur. Cytologists are however usually 

 dependent on outside sources, especially botanic gardens, for their material, and this 

 used uncritically can be a fertile source of error. I have received from two very reputable 

 sources seed of an annual species of the Brachypetalum section of Helianthemum as Tuheraria 

 guttata, and plants of Helianthemum chamaecistus as H. alpestre. Both these might have 

 given rise to incorrectly recorded chromosome counts. The remedy lies partly with the 

 botanic gardens and others distributing plants and seeds, but it is also a responsibility 

 of the cytologist to be sure that his plants are correctly named. It is particularly desirable 

 that seed should be grown on and checked very carefully, and specimens of the plants 

 used preserved. 



The distribution of chromosome numbers in the Cistaceae accords quite well with 

 existing classifications. It seems quite clear that the four genera within Helianthemum 

 s. L are natural groups, and that they are not particularly closely related one to another. 

 In particular Halimium seems in all respects to be much closer to Cistus than to the 

 restricted Helianthemum. This is borne out by the occurrence of bigeneric hybrids be- 

 tween Halimium and Cistus section Ledonia. The chromosomes are rather small, so it 

 has not been possible to get much help from their morphology, though a few generalisa- 

 tions can be made. Halimium has roughly equal sized chromosomes with median centro- 

 meres. The species of Helianthemum investigated show two or three pairs of chromosomes 

 with obviously unequal arms, and there is some variation in size. In Tuheraria a large 

 proportion of the chromosomes are asymmetrical. 



Two conspicuous characteristics of the Cistaceae are the absence of polyploid 

 chromosome races, and the variation in basic number between the genera. Genome 



