Bird's foot trefoil - an interestingly boring yellow flower

[Jim Lloyd]

I thought this flower would show an interesting UV signature. In my limited experience, yellow flowers are often highly UV reflective often showing false UV yellow with darker patterns. Also other similar flowers, such as gorse or ivy leaved toadflax had interesting UV appearance. But this is uniformly UV dark. I wonder why that is? I guess it has a particular colour for bees - what ever UV dark + yellow looks like ?

 

D3200 converted, UG1 2mm + BG40 2mm, EL Nikkor 80 mm f/5.6 at f/11 ISO 400 2 sec, indoor, sunlight

 

 

[Andy Perrin]

Well that's a very peculiar UV signature. I have not seen a dark yellow before, although given the experts around here, no doubt someone else has. It does look like that pea family, doesn't it?

[nfoto]

The actual false colour is quite dependent on how the file has been processed. Thus there is no 100% given answer. Besides this obvious fact, the lens itself influences the outcome. This is seen in particular in UV-dark areas which non-specialist lenses might struggle to render completely blackish dark.

 

My Lotus corniculatus looks like this in UV,

 

 

 

Panasonic GH-2 broad-spectrum, Coastal 60 mm f/4 APO, Baader U2.

 

Lotus belongs to the Pea Family.

 

Do note the marginal bands of brighter appearance. These are part of its UV floral signature.

[Andrea B.]

The members of the Pea Family we have posted so far show very diverse UV-signatures. If you page through the Fabaceae entries, you'll find UV-dark, UV-bright, false yellow, false blue, false bi-colours and UV-veining. Cool stuff!!

I even found a desert version which has a colour change in a small area after it has been pollinated.

 

The only bee visual receptor which would be stimulated by yellow reflections is (possibly) the green one if it extends far enough. So the bee probably sees this visible yellow whorl as green or lime-green.

[Andrea B.]

with apologies to Bjørn..................

 

Green contrast changes on the reflected yellow edges are probably used by the bee if the yellow is stimulating its green receptor.

Blue content of the flower's background (typically green bkg?) and of the flower itself are measured. The yellow flower has no blue content. The green grass has some blue content I think? Is there enough difference in blue content between the background and flower for the bee to detect it? Probably.

 

So trefoil might have an appearance somewhat like the following stylized colors.

 

Only beware! The bee does not "see" the flower. It measures blue content and detects green edge contrast changes. Its bee brain is too small to hold a visual of an entire flower with all its parts.

 

 

 

^{Edit: added some words to make reading smoother.}

[otoien]

Keep in mind that human vision is also to a very high degree based on edge contrast detection. It is still "seeing". Colors are detected both as color contrast edges, and colors over larger areas. We also have specific neurons for detection orientation and more complex shapes.

[Andrea B.]

I simply don't know how to word it yet. :D

Bees do not detect pattern or shapes (although they can detect symmetry.) They detect blue content and an edge contrast change while moving. Their brains are not large enough to hold an entire flower shape/pattern and all the flower parts as a visual image. The bee brain can only hold some landmarks like amount of blue content and location of green contrast edge modulation in its eye facets. So they really do not "see" the flower. This does not stop the bee from detecting the flower signals and finding the flower and enjoying the nectar or pollen.

[Andy Perrin]

Interesting coda to this -- I saw one yesterday and snapped a photo. Mine came out looking like Jim's even though I was using the quartz Resolve 60mm lens and 330WB80 filter. (There are two separate flowers in this photo. Ignore the purple one.) This is in sunshine.

 

However when I imaged it with 365nm Convoy S2+ torch, I got one like Birna's! (This second photo was with the Noflexar 35mm/3.5, but it shouldn't matter under a 365nm torch.) I was using 2mm UG11 and no IR blocker. The torch is filtered with U-340.

[Andrea B.]

It's difficult to make a judgement about false colours because if you white-click in just the right place, all that yellow disappears. So it is probably a combination of factors at work here. Taking a look at both versions in their raw colours (before white balance) would be the thing to do to determine whether the illumination is playing a big role or a small one.

 

Lotus corniculatus is very UV absorbing. Although there is a tiny bit of bumpery below 350 nm, it does not look like quite enough to produce all that false yellow.

 

But I'm certainly open to further experiments on this little flower.

 

Reference:

Ultraviolet as a Component of Flower Reflections, and the Colour Perception of Hymenoptera

by L. Chittka et al, 1993

 

From Figure 3,page 1492

The notation indicates that the flower absorbs UV and blue, reflects green and red.

[Andy Perrin]

I still have the flower, so if you have a suggestion...

By the way, it is white in SWIR 1500-1600nm:

[nfoto]

Firstly, UV-dark yellow flowers are not rare at all. In fact, they are common all over the Plant kingdom. Thus yellow-flowered members of the Daisy Family (Asteraceae) often are very UV-dark. In the Rose Family (Rosaceae), Potentilla fruticosa is a well-known example. Similar occurs in the Cruciferae, where members of the genera Brassica and Erysimum exhibit this rendition.

 

Secondly, Lotus corniculatus is a genetically diversified species with very variable phenotypical expression. Some forms are tall and elongated, others are prostrate. Some of the genetic diversity might well result in different UV appearance as well.

[Andy Perrin]

It could, certainly, but my photos are both of the same plant. It did not change its genes between photos.

[nfoto]

That wasn't implied -- however, the overall variation for widely distributed species should always be taken into consideration. The manifestation of a species is never set in stone.

[Andy Perrin]

I repeated the experiment in blue sky sunshine and got a rendition much closer to Birna's, using WB off PTFE. It may have been white balance issues in my other photo, as Andrea thought.

 

[ulf]

Andy, is the pale background the PTFE?

If so shouldn't it be grey after WB?

 

After WB on PTFE, my images look like this:

S8612 + U-360, 2+2mm stack

 

S8612 + BG3, 2+2mm stack

 

S8612 + U-330, 2+2mm stack

 

The two last images look makes me think that the reflectance graphs in Andrea's post above are correct.

 

Lens: EL-NIKKOR 80/5.6 Old metal type

Camera: Canon EOS 60D full spectrum modified

Light source: Modified Godox flashes with quartz flash tubes

 

The dye used for making the background cotton velvet VIS-black, unfortunately isn't wide-band black.

[Andy Perrin]

No, in neither case was the background PTFE. I own only a tiny piece of PTFE. The white balance was set ahead of time. The last photo is correct for sure— I was very careful. The background was plastic in the new photo.

[ulf]

No, in neither case was the background PTFE. I own only a tiny piece of PTFE. The white balance was set ahead of time. The last photo is correct for sure— I was very careful. The background was plastic in the new photo.

Glad to hear that.

The flowers in both our images look reasonably similar in colour and the pale purple in the background just made me wonder.