The photo shows both flowers and fruits. Each flower has 4 sepals, no petals, a single stamen and a single carpel with one ovule. The fruit is a single seed, enclosed in the ovary, enclosed in the calyx; there's one lying on its side just above and to the right of the middle of the picture. The mature fruiting calyx is 1.5–2 mm long, and about 1 mm wide; at flowering it's a bit smaller.
|Scleranthus uniflorus, Mt Robert, Nelson Lakes National Park.|
This plant is Scleranthus uniflorus, and you might have trouble believing that it's in the carnation family (Caryophyllaceae), though it's more closely related to the chickweeds than the carnations. Scleranthus is a small genus found in Europe, West Asia, Australia, and New Zealand. In all three New Zealand species, S. biflorus, S. brockiei, and S. uniflorus, the plants form cushions, but in the other two the flowers are paired at the top of each stalk.
There's an Australian Scleranthus naturalized in the Wither Hills in Marlborough, one of the driest parts of the country (Garnock-Jones 1988). That one is interesting because for years it was thought to be the same as S. biflorus. It was first described as Mniarum fasciculatum by Robert Brown, but pretty soon another great British botanist Joseph Dalton Hooker had opined that it was no different from S. biflorus, even though he had earlier transferred it to Scleranthus. It seems nobody doubted Hooker's word for over 100 years; such is the nature of authority. By the time botanists in Australia started noting that this was different, its original name had faded into obscurity, and we came pretty close to describing it again as a new species, before I happened to see some of the early collections in the Kew herbarium in England). Interestingly, at least three Australian dry-land grasses are also naturalized in the Wither Hills, so perhaps they all came together with sheep and/or grass seed from Australia.
My first PhD student, Rob Smissen, did his dissertation on Scleranthus. Using DNA sequence data to discover the evolutionary history of the genus, he showed it was likely the genus has crossed the Tasman Sea twice by long-distance dispersal (Smissen et al., 2003). However, these little 1-seeded fruits have no obvious means of getting around. They join a number of other plants that seem to have dispersed, but for which we have no idea how.
Garnock-Jones, P.J. 1988: Caryophyllaceae. In Webb, C.J.;. Sykes, W.R; Garnock‑Jones, P.J. Flora of New Zealand Vol. 4 Pteridophytes, Gymnosperms, Dicotyledons. Botany Division, DSIR, lxviii + 1365 pp.
Smissen, R.D.; Garnock-Jones, P.J.; Chambers, G.K. 2003: Phylogenetic analysis of ITS sequences suggests a Pliocene origin for the bipolar distribution of Scleranthus (Caryophyllaceae). Australian Systematic Botany 16: 301 - 315.