Tried the Canon lens on petals of
Dasycarpa (
Potentilla)
fruticosa. This is a species with dark to very dark appearance in UV, and initially believed to "lack" a UV signature. Which of course is plainly wrong and caused by insufficient methods.
First, a quick shot of the
D. fruticosa pulled from my archive. Not the same flower as I shot with the Canon lens, but it is the very same bush (on my front porch, by the way, and it is now coming into full bloom).

Ultraviolet light: Nikon D40X (modified), Coastal Optics 60 mm f/4 APO lens, internal Baader U2" (Venus) filter, SB-140 flash.
With the high resolving power of the camera and lens in UV, one sees the petals aren't really featureless UV-dark, but has variegated areas due to the presence of conical cells.
Now, fast forward to this shot taken with D600 (modified), Canon 20 mm f/3.5 macro lens at f/5.6, Baader U2" (Venus) filter in filter box, Broncolor studio flashes (uncoated Xenon tube), m=10.5X. I selected to shoot at an incidence angle of 60 degrees in order to bring forth the 3D shape of the surface of the petals. Thus, focus stacking was required to gain adequate depth of field. I used 26 frames for this capture.
A small section of the petal shown in 100% (average
m=10.5X) . Some stacking jitter adversely impacts the final sharpness, plus I'm not entirely convinced the Canon lens performs very well in UV. However, the orderly lines of taller and narrow conical cells amongst the smaller papillate surface cells are very evident. It should be noted these cells are much smaller than what was documented for
Lysimachia punctata earlier in this thread. Also noteworthy is that either configuration of conical cell shape makes the cells to reflect UV brightly. The overall effect apparently is Nature's original version of optical brighteners: flower colours will gain clarity and appear more vivid.