Anthesis is the period during which a flower is fully open and functional. It may also refer to the onset of that period.
The onset of anthesis is spectacular in some species. In Banksia species, for example, anthesis involves the extension of the style far beyond the upper perianth parts. Anthesis of flowers is sequential within an inflorescence, so when the style and perianth are different colours, the result is a striking colour change that gradually sweeps along the inflorescence.
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Dehiscence is the splitting of a mature plant structure along a built-in line of weakness to release its contents. This is common among fruits, anthers and sporangia. Sometimes this involves the complete detachment of a part. Structures that open in this way are said to be dehiscent. Structures that do not open in this way are called indehiscent, and rely on other mechanisms such as decay, digestion by herbivores, or predation to release the contents.
A similar process to dehiscence occurs in some flower buds (e.g., Platycodon, Fuchsia), but this is rarely referred to as dehiscence unless circumscissile dehiscence is involved; anthesis is the usual term for the opening of flowers. Dehiscence may or may not involve the loss of a structure through the process of abscission. The lost structures are said to be caducous.