Why do evening primroses bloom at dusk?

What environmental factors cause them to bloom only at dusk, if any? I was thinking maybe it could have been decrease in light or temperature.. Or maybe the biological clock of it happens to have it bloom at dusk?

Why do evening primroses bloom at dusk?
Oenothera is a genus of about 125 species of annual, biennial and perennial herbaceous flowering plants, native to North and South America.



It is the type genus of the family Onagraceae.



Common names include evening primrose, suncups, and sundrops.



The flowers open in the evening, hence the name "evening primrose", and are yellow in most species but white, purple, pink or red in a few; there are four petals.



One of the most distinctive features of the flower is the stigma with four branches, forming an X shape.



Pollination is by Lepidoptera (moths) and bees; like many members of the Onagraceae, however, the pollen grains are loosely held together by



viscin threads (see photo below),



meaning that only bees that are morphologically specialized to gather this pollen can effectively pollinate the flowers (it cannot be held effectively in a typical bee scoop).



Furthermore, the flowers are open at a time when most bee species are inactive, so the bees which visit Oenothera are also compelled to be specialists.



1 ) Photo of viscin thread connecting the pollen grains --

http://en.wikipedia.org/wiki/Image:Oenot...



2 ) Photo of Oenothera speciosa yellow flower with cross shaped stigma-

http://en.wikipedia.org/wiki/Image:Oenot...



3 ) Photo of Oenothera erythrosepala -

http://en.wikipedia.org/wiki/Image:Floar...



4) Photo of pink evening primrose --

http://en.wikipedia.org/wiki/Image:Pinkp...
Reply:Yes that is correct, Moths are pollinators of flowers at night.

These flowers are usually not brightly colored. They are often white or yellow but have a strong fragance.



In tropical countries there are bats that pollinate flowers at night.
Reply:Evening primroses open at dusk so they can be pollinated by night-flying insects. They respond to the absence of light by stopping photosynthesizing. The same signal tells their flowers to open.