On strict and facultative biennials

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Abstract
Most so-called biennial plants may flower in their second growing season, but usually take longer in the field. Work reported here and in four recent papers on six different species has identified natural populations of strictly biennial plants, which flower only in their second year. The strict biennials and facultative biennials seem to form two distinct groups. All six strict biennial species were growing in infertile sites, and at least four of the six have annual variants at other sites, so they may be regarded as ‘extended annuals’ growing in sites too infertile for the life cycle to be completed in one year. In contrast most facultative biennials are pauciennials (short-lived monocarpic perennials) of more fertile, disturbed habitats. Moreover, the rigid two-year life cycle of the strict biennials creates at each site two temporally separated sub-populations, which flower in odd and even years.