| Summary: | Biosystematic investigations have been carried out on members of the Hypnum cupressiforme aggregate which are variable morphologically and on Atrichum undulatum which is variable cytologically. Cultivation experiments have shown that variation in gametophyte plants of the Hypnum cupressiforme aggregate is partly genotypic and partly environmental and that there are four distinct species within the aggregate. H. cupressiforme sensu stricto, H. jutlandicum, H. mammillatum and H. vaucheri. Within H. cupressiforme s.s. there are three varieties, var. cupressiforme, var. lacunosum and var. resupinatum and within H. mammillatum are two varieties, var. mammillatum and var. filiforme. These conclusions are supported to some extent by physiological and electrophoretic studies but the aggregate is cytologically uniform. In Atrichum undulatum there are haploid, diploid and triploid cytotypes which are indistinguishable both in field material and after cultivation, morphological variation being both genetical and environmental. Only a single taxon can be recognized although there are measurable differences in DNA content between the three cytotypes. There are significant differences in the relation frequencies of the three cytotypes in different parts of Britain.
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