The involvement of four factors (life history, predation, possession of a persistent seed bank and seedling establishment) in the evolution of seed size in flowering plants is discussed. Among herbacaeous plants, biennials seem to have larger seeds than annuals and perennials, a difference which may explain the capacity of some biennials to establish in closed vegetation. In many species, small seeds may have evolved at least partly as a defence against predators. Seeds < 3 mg have some immunity from vertebrate predators but much smaller seeds are consumed by invertebrates. Burial reduces or eliminates most predation by invertebrates.
Possession of a persistent seed bank is associated with small, compact, smooth seeds with exacting requirements for germination. Seeds of species which lack seed banks are larger, frequently long or flat, and often have hairs or awns and lax requirements for germination. The evolution of the former group is often intimately connected with the preferences of earthworms. Burial facilitates anchorage of seedlings.
In closed herbaceous vegetation, establishment of seedlings seems to be dependent on a low rate of exhaustion of seed reserves achieved by a large seed, low relative growth rate of seedlings or both. In many data sets, seed size and seedling growth rate are negatively correlated, but the generality of the correlation is in doubt. The large seeds of many herbs, shrubs and trees of woodland are suspected to be of critical importance in seedling emergence through tree litter.
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