Seeds of woodland species range in size from tiny microscopic orchid seeds e.g. Pterostylis curta to the seed of Hardenbergia violacea which is 9 mm long, and seed of the exotic *Olea europaea subsp. cuspidata which is up to 12 mm. Fern spores of Cheilanthes are also microscopic (40-80 um). Size and shape varies and is important for dispersal of the seed.
The soil seedbank
Unlike the other life stages of plants, which occur mainly above ground, the seed is dispersed from the parent plant with adaptions to assist it to enter the below-ground soil seedbank. The woodland at the Australian Botanic Garden has no species that have plant-stored seedbanks (bradysporous or serotinous) like many of the sclerophyllous Banksia and Hakea species in the sandstone woodlands.
The enclosed protective nature of the seed, and its limited metabolic requirements, allows species to survive in the soil seedbank, avoiding unsuitable conditions such as drought, until moisture and temperature conditions are suitable for germination. How long seeds remain in the soil seedbank depends on moisture and temperature conditions, and the capacity of the seed to remain dormant. Persistence in the soil seedbank varies for different species.
Seed longevity ranges from about a year for some annual species to more than 20 years for some Acacia species. Some seed may be kept alive for much longer periods in artificial seed storage in the Australian PlantBank at the Australian Botanic Garden Mount Annan. However, such periods are unlikely to occur in natural conditions, where moisture and temperature conditions vary and soil seed predators are present.
In most woodland species the adult stage is probably longer-lived than the seed in the soil, but populations of short-lived ephemerals such as Ranunculus sessiliflorus and Daucus glochidiatus may appear as adults for 2-4 months followed by 1-3 years in the seed bank.
Asterisk * indicates exotic species naturalised at the Australian Botanic Garden.