Annals of Botany

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Great walls of fire

For large numbers of wild plant species, dormancy is an essential part of their mode of life. It is usually seen as a mechanism for avoiding germination in situations in which the seedling would be exposed to unfavourable conditions. Further, dormant seeds may remain in the soil forming seed banks whose germination can be delayed for several years. These features are illustrated in the Australian genus Grevillea by Briggs et al. (UWS and UNSW, Sydney, Australia, pp. 965–980). The seeds of certain Grevillea species contain, external to the embryo, an oil-filled structure, the elaiosome. This attracts ants that store the seeds in their nests, thus forming seed banks. The seeds are dormant but this may be broken by heat and/or smoke, thus causing the seeds to germinate and the seedlings to colonize the bare ground left by fire. In some species, scarification increases the rate of germination, even sometimes in the absence of fire-related factors, thus suggesting that the seed coat is involved in dormancy. However, it does not impose dormancy by preventing uptake of water. The authors have thus undertaken a detailed and elegant microscopic and histochemical investigation showing clearly that these ‘classical’ techniques still have much to tell us. Their results are presented in an extensive and beautiful set of micrographs; here we just focus on their conclusions. The seed coat is complex and multi-layered, and together with the hypostase (an incomplete layer of cells derived from the nucellus) acts both as a barrier to diffusion of high-molecular-weight compounds and as a mechanical restraint to the embryo; it is also possible that phenolic compounds in the exotesta hinder gas exchange. These are classic features of coat-imposed dormancy and the challenge now is to discover how these properties are modified by heat and/or smoke.