Sepals and Petals and Lips Oh My!

All orchids have 3 sepals and 3 petals, with one petal modified to form the "lip" or labellum. The ancestral condition (the flower structure of the last shared common ancestor of all orchids) likely had 3 identical sepals, forming an outer whorl which encases the flower during bud growth, and 3 identical petals, forming an inner whorl only revealed when the sepals open or are cut away. You can see this simple structure in the iris family Iridaceae, which is a close relative of the orchid family Orchidaceae. Though irises have their own complicated elaborations, they retain the 3 identical sepals (apparently called "falls" in horticulture) and 3 identical petals (called "standards"). In what was probably a key event in evolution of orchids, one the three petals was drastically modified and became the labellum.

The special structure of the lip is nearly impossible to ignore if you look at most orchid flowers, or read even the briefest article on orchid flowers. In many orchids, the other two petals are nearly identical to each other, as are the three sepals (to each other). But some orchids have a dramatically altered one of the sepals as well. Look at the Scaphosepalum flower below and try to figure out what parts you are looking at (yeah, yeah, most people just gasp and say "beautiful!" or "weird!" or "phallic!", but c'mon lets dig beneath the skin, plumb the depths of nature, and generally get all geeky).

Even if you are familiar with a lot of orchid flowers, you will probably have trouble telling what is what, and if you guess you will probably be wrong. This is one weird flower. The first confusing thing is that it is non-resupinate (the flower is not twisted around, so its lip is at the top, not the bottom as usual). The second really confusing thing is that the "dorsal" sepal (bottom sepal in this flower) is barely even recognizable - it is the purple projection sticking out like... well ya know. I thought at first that this was the lip, but in fact the lip is tiny and buried in the middle of the flower and is barely visible here. The two other sepals end in those yellow spikes, which look to me like the glorious waxed moustache of a vain blond Victorian gentleman. They also include a fused base that is sheet-like and spotted and forms the entire back of the flower from this view. Except for being long and narrow, the two types of sepals look almost nothing alike. By the way, the understated petals are the two small tan-gold flanges near the base of the modified sepal.

This genus of orchids has TWO totally different lips (yes, that is wrong term to use) - one of the three petals and one of the three sepals. Asymmetry of this sort is quite difficult to evolve, as molecular genetic studies show - it requires additional genes whose expression breaks the usual radially symmetric pattern of flowers. 

By the way, I am not aware of any information on why Scaphosepalum flowers look like this - some of them are even more exaggerated than this one (for example Scaphosepalum gibberosum and Scaphosepalum swertiifolium). Surely not to induce Victorian ladies swoon at the glorious moustache. Scaphosepalum are reported to be pollinated by small flies of some sort but really, what about this looks like it is meant to attract flies? It doesn't look (or smell) like a fruit (fruit flies), or a fungus (fungus gnats), or dung (dung flies), or rotting meat (corpse flies). A fly that admires abstract sculptures? That must be it.