The hyperarid climate of the Rub' al-Khali strongly conditions plant life in this subtropical hot desert. There is fossil evidence of a more humid climate here in late Tertiary and Pleistocene times and of a lakeforming phase as recent as 9000–6000 BP. Today rain may not occur for years in some parts, with annual means less than 25 mm, yet the extensive sands provide more scope for plant growth than do some rock and gravel tracts in less arid northern Arabia. The sand vegetation is characterised by very diffuse shrub communities led respectively by an endemic subspecies of Calligonum crinitum, the endemic saltbush Cornulaca arabica and, locally, by Haloxylon persicum. Cyperus conglomeratus, Dipterygium glaucum, Limeum arabicum, and Zygophyllum mandavillei are widespread associates. Sand-free gravel floors or salt flats, relatively small in extent, are nearly or entirely sterile. Tree forms are absent except for rare individuals on some borderlands. Floristic diversity is very restricted; only about 37 species (listed), of which hardly 10 are important in the vegetation, are recorded from an area of 500,000 km2. Annual plants are virtually absent, a situation contrasting sharply with that in Arabia to the N where more than half the desert species are therophytes.