The featured photo for Houstonia caerulea shows mostly 4-petaled flowers. However, there are two flowers shown that are clearly 5-petaled!
https://www.inaturalist.org/photos/28229
Is this common with flowers? (At least I did not see any notes in Flora of Virginia or the NYBG Manual of Vascular Plants about petal number variation in this species. But perhaps this is common enough in many species to be left out of a botanical description?)
Yes, I have quite a few five-petaled Houstonia observations myself. While rare, it does occur, and I always liken it to finding a four-leaf clover. It seems to be some natural variation, probably just a developmental oddity that occurs by random chance every now and then. I’m sure someone would have already started cultivating five-petaled varieties if it was genetically stable.
This likely happens in all/most species, I’ve found a Denhamia silvestris with a single 6-petalled flower vs the normal 5 flowers
Not quite the same, but plants can vary the number of flower parts somewhat often, the variation will pretty consisently be not consistent across the plant. I’ve also see Trillium with 4 petals, and sometimes plants which normally have paired leaves leaves (like red maple) have 3 leaves per node
I wonder if this works similarly to those occasional monstrous animals with two faces? I read that it happens when the face is accidentally too wide; the genetic algorithm for facial features doesn’t compensate by making the features bigger, but by making more of them to fill the extra space. It makes sense that a flower with a calyx accidentally too big would compensate by filling the extra space with an extra petal.
As others have said, it’s not too uncommon in plants. A lot of animals couldn’t function too well with an extra eye or bones in the wrong places but plants are fairly plastic. I think it’s a case where having an extra leaf or a missing petal has a neutral effect on survival, so it happens sometimes and there no advantage to removing the slight mutants from the gene pool. Like being left-handed I guess.
I suspect that is probably how this works. Each organ primordium inhibits the formation of another primordium nearby, but if you increase the diameter there is enough space between inhibition zones to initiate another one. This is known as the inhibitory field hypothesis in leaf phyllotaxy, and since floral organs are essentially modified leaves I would expect this should apply to floral patterning as well.
Fascinating. Do you have a good reference to share for learning about phyllotaxis?
I wonder if electrical potential is involved. I’m reminded of the work of Michael Levin https://ase.tufts.edu/biology/labs/levin/
I think it’s controlled by hormone gradients, specifically auxin and cytokinin. Unfortunately it seems a lot of the papers are behind paywalls, but ScienceDirect has a summary that includes enough text and figures I think to give some idea even if you can’t get the individual pubs:
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/phyllotaxis
There’s a project for recording flowers with unusual petal counts: https://www.inaturalist.org/projects/peculiar-petal-counts
Unfortunately, it doesn’t seem to let me add an observation I already have up; it wants me to upload a new one. Unless there is something i can’t find?
You have to join the project. Then just go to the observation page of the one you want added, click “Add to a Project” and type part of the project name. Click on it and the observation will be added. You can also batch add observations using “Add from Your Observations” on the project page, but that seems unlikely to be what you want given the type of project.
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