Walton, David Winston Harris (1975)
Ph.D. thesis, University of Birmingham.
The species of Acaena (Rosaceae) growing on the sub-Antarctic island of South Georgia have been examined from two different aspects - the world level and the insular level. A revision of the taxonomy and synonymy of the two sections of the genus containing A.tenera and A.magellanica has resulted in the reduction to synonymy of many species, and a complete synonymyn for Sect. Acrobyssinoideae Bitt. and for A.magellanica (Lam.) Vahl (which comprises most of Sect. Ancistrum Bitt.) is provided. Morphological descriptions are given for A.magellanica and A.tenera, and for plants from South Georgia which are judged to be the hybrid A.magellanica x tenera. An examination of aspects of generic morphology together with consideration of relevant biogeographical literature has resulted in the conclusion that the genus probably arose from tropical ancestral stock which may have been common to Poterium. Speciation within South America has resulted in a spectrum of types, from primitive woody species with racemic inflorescences and multispined fruits to more highly evolved species with less woody stems, few spined fruits and compact globular heads. Dispersion of the genus from South America has taken place at various times, resulting in different stages of generic evolution. The majority of the Australasian species are found in New Zealand and appear to have a common ancestor. The relationships of other disjunct species were traced and relatively recent long distance dispersal was found to be a satisfactory explanation for most of their distributions. The taxa on South Georgia were shown to be highly evolved. The inter-relationships between them are given in terms of breeding patterns and it is suggested that F1, hybrids are normally formed with A.tenera as the female, whilst F2 and any subsequent generations are probably due to backcrossing to A.magellanica as the male. A.magellanica appears to be generally outcrossing whilst the reverse is true of A.tenera. Examination of floral development showed no evidence of preformation of flowers in the previous season, initiation occurring simultaneously with snow melt at most sites. The rate of floral development appeared to be linked to site aspect. Seed germination studies showed a warm day/cold night regime to be the most effective. A.magellanica seedlings grew at a much higher rate than those of A.tenera, but seedling production on a per head basis was similar for both species. Under a given light regime there was a linear rate of leaf production for both taxa. Rates for seedling establishment in various soils at different sites were seen to correspond to a pattern predictable from the general ecological data for the species. Measurement of changes in standing crop of an A.magellanica community showed it to be highly productive, although the tnojof part of dry matter production was for vegetative rather than flowering tissue. Initial measurements of photosynthetic rates demonstrated marked differences between geographically isolated populations of A.magellanica. A significant rate of photosynthesis was found to occur at and below 0°C. in all the South Georgian taxa. These data were discussed in terms of adaptation of the taxa to South Georgia and the other sub-Antarctic islands, whilst the information gathered on these and other species was used to propose a hypothesis for the origin, evolution and distribution of the genus.
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