Seed cone structure in conifers in relation to development and pollination: a biological approach

Seed production in conifers involves an extended sequence of developmental events that may take as long as 3 years to complete. There is a conflict between two major processes: initial pollination of ovules and subsequent maturation of seeds after fertilization. The conflict arises because ovules must be exposed at first to receive pollen (the gymno-ovulate condition), and subsequently seeds must be protected during maturation (the angiospermous condition). The Florin model, which shows that the coniferous cone was ancestrally a compound structure, provides a constructional Bauplan whose modifications can be understood by developmental study. In the majority of conifers there is no consistent ovule-bearing structure, even though this is required by the model; much of cone ontogeny is determined by intercalary meristematic activity that results in cone closure after pollination. Two main constructional types can be recognized. In Cephalotaxaceae, Cupressaceae, Sciadopityaceae, Taxaceae, and Taxodiaceae (all without saccate pollen), pollen structure and function is uniform and not associated with special modification of the ovulate cone at the time of pollen capture. In the Pinaceae and Podocarpaceae (mostly with saccate pollen), the ovulate cone is usually highly specialized with regard to pollen capture. In the former group, developmental emphasis is on postpollination events that lead to seed protection or are associated with seed dispersal. In the latter group, there is a duality in ovulate cone morphology, initially associated with pollination, and subsequently with seed protection and dispersal. The general conclusion is that the coniferous cone cannot be treated as a static entity for comparative purposes. The functional attributes of cone structures need to be considered in a very broad context when the evolution of the coniferous cone is discussed.