Devoid of light increases the chromoplasts: Chloroplasts ratio  

It could be an adaptation to attract insects for pollination.

I would say in the most part they are related to pollinator attraction.  I don't think this has been well-tested, though, or even the nature of the dark spots thoroughly described.

in most cases dark spots are used as nectar guides for pollinators (http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0055914). Thunbergia alata, Rudbeckia, Vicia faba,  have dark UV-absorbing spots as nectar guides for honeybees. Color patterns on lower tepals of Iris species also used likewise.

Aristolochia grandiflora is slightly different from other mentioned species. This plant pollinated by Diptera (sapromyiophily), so the center of the flower looks like rotted meat with the specific smell. (http://facstaff.columbusstate.edu/burgess_kevin/website/Publications_files/mobt%202004.pdf)

Dear Andrii,

I agree that these black spots are floral guides. But the term floral guide is just a description and not an explanation. The black spots in Red Poppy flowers and Gorteria difusa flowers might also be termed floral guides. However, in these cases it has been shown that they mimic potential flower-visiting mates and by this means attract beetles and bombylid flies.

Of couse the black spot in Vicia faber and other flowers might just be a visual cues that bees can learn and help the bees to reduce handling time.  In Vicia faber it is obvious that the black spots are covering the anthers and thus might (also) protect the pollen against UV radiation or regulate the temperature of the pollen. I would only agree with you if it had been experimentally tested that the black spot in Vicia faber reduces handling time in experienced bees or increases the probability the the bees handle the flowers in a legetimate way.

And of course it cannot be excluded that the black spot in Vicia faber has more than one function.

Best,

Klaus

Intense chromoplasts in relation to other plastics to make contrast for insect attraction 

Dear Klaus,

there are some experiments with different flower models (with and without nectar guides) and pollinators were conducted in 1970: http://www.jstor.org/stable/4533356?seq=1#page_scan_tab_contents

The most realistic explanation for me - all these adaptations were formed in the process of long coevolution between plants and their pollinators. Such adaptations can be very different in different cases, and can be fixed in the case of providing benefits in plant reproduction.

Some more recent information you can find here:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3572167/

In some cases is important to know how flowers looks like in near UV and IR, because pollinators and humans have a different vision. Some examples of such photos you can find here http://www.naturfotograf.com/UV_flowers_list.html#top/

All mentioned not exclude the possibility that these structures can have more than 1 function.

All the best,

Andrii

Sir, is it not pollination syndrome???

Regards

I agree and acknowledge. It is a long process of adaptation and co-evolution between plants and pollinators. 

Greetings

Dear Klaus, are there evidences that insect pollinators prefer 3D shapes rather than 2D shapes? deep flowers with hidden nectar (which is not accessible to all) compared to flat flowers whose nectar is exposed and therefore can be taken by all? in other words deep flowers may seem to insects more promising for rewards....if that is so the dark center mimic deep corolla tube (to human vision) ....every artist knows that.......I have another question - why humans like spiral shape and small leaves and tendrils make them feel happy and relax? like these on the pictures I attached

Dear Ekaterina,

The suggested 3D effect is a good idea. Many flowers are darker in the center than in the periphery.

I don't know whether humans like spiral shape. If so it may have to do with the so-called golden ratio of picture composition. But this is just an idea.

yes and then it goes to the esoteric unscientific field...but I am glad that you mention the golden ratio  about the spiral because then I am not that crazy when the idea came to me some months ago., just wanted to see if someone else would think in a similar way..may be this is something in our minds at sub-conciseness  level ...Thank you!

Dear Ekaterina,

when talking about mimicry I am often asked whether the animals possess a sense for beauty of feeling for art, because the mimicked signals often appear beautiful to human beholders. I think cause and effect are vice versa: Our feeling for art has developed along with natural objects.

http://www.mainlesson.com/display.php?author=seton&book=wild&story=silverspot

maybe this has something to do with the sense of beauty of the Corvidae? who knows

...and I am agree with you. Our feeling for art has developed along with natural objects. And also  this is one of the reasons for the stress in the urban humans, who live in a pile of concrete and do not see the fresh green leavelets 9even if there were any). Many of these people even do not realize why they feel anxious.

Dear Prof. Lunau,

I think that not some, but generally most flowers that display patterned pigmentation on corolla have patterns that are DARK compared to the background pigmentation.The pigment pattern is of the same or contrasting colour but intensely pigmented and are present at the ~central region of the corolla--whether it is the central 'bulls-eye' pattern (UV-pigmentation), or other patterns of the stripes, the spots, and the blotches on corolla.

And the pattern's central convergence in the corolla has more to do with the functional aspects of pollination, that you have already mentioned.

Regards,

Eapsa

In Serapias orchids and some Irises this fact has been associated with a certain shelter syndrome for bees. The dark central colour and some substances produced by the flowers rise the internal flower temperature compared with the surounding enviroments.