Anthesis is the period during which a flower

Flower

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

 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

Banksia

Banksia is a genus of around 170 species in the plant family Proteaceae. These Australian wildflowers and popular garden plants are easily recognised by their characteristic flower spikes and fruiting "cones" and heads. When it comes to size, banksias range from prostrate woody shrubs to trees up...

species, for example, anthesis involves the extension of the style far beyond the upper perianth

Perianth

The term perianth has two similar but separate meanings in botany:* In flowering plants, the perianth are the outer, sterile whorls of a flower...

 parts. Anthesis of flowers is sequential within an inflorescence

Inflorescence

An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Strictly, it is the part of the shoot of seed plants where flowers are formed and which is accordingly modified...

, so when the style and perianth are different colours, the result is a striking colour change that gradually sweeps along the inflorescence.

Flowers with diurnal

Diurnal

Diurnal may refer to:* Diurnal cycle, any pattern that recurs daily** Diurnality, the behavior of animals and plants that are active in the daytime* Diurnal motion, the apparent motion of stars around the Earth...

 anthesis generally have brightly colored flowers to attract diurnal insects, such as butterflies.
Flowers with nocturnal anthesis generally have white or less colorful flowers, that contrast more strongly with the night. These flowers typically attract nocturnal insects including many moth species.