Are these red petals bract? And in the future, how do I know whenever these flower petals are a bract or not?
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Hi, i am no expert on the family Asteraceae, but i do expect that the genus Zinnia got red, assymmetric flowers at the outer = lower side of the congested inflorescence, while the inner = âhigherâ flowers are radially symmetric. So the red, showy parts do consist of (i think 2) true petals, while true bracts are farther downwards, at the outer part of the whole inflorescence, and shall look like scales. This image tells what i meant.
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Hello erwin, thank you very much for your response.
This is a very interesting found for me as I did not know bract can also look like that. I understand now, thank you!
Zinnias and other plants in this family have kind of weird flowers. What you have photographed looks like one flower, but itâs actually a whole cluster of very small flowers. The flowers with big petals, that you asked about, are called ray flowers. The others, toward the center are called disk flowers. In your photo, the red ones in the center are disk flowers in bud. Next is a ring of open yellow disk flowers. Then a narrow ring of red, closed, old disk flowers. Then the ring of red ray flowers. Outside/below them are the bracts, not visible in this photo.
I like how these plants have evolved to get advantages that come from big flowers and from small ones. In this case, each flower makes just one seed. Itâs small like its flower and can travel easily. However, small flowers donât attract big, far-flying pollinators because theyâre hard to see, have little nectar, and arenât strong enough to support a big bumblebee. These small flowers cluster together to make what looks like a big flower. Itâs easy to see. It presents lots of nectar (a little in each flower). And itâs strong. Big insects can land there and walk around. So cool!
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Thanks, Barbara, for the well done explanation of flowers (in wide sense) in the family Asteraceae! I had needed to look for the English terms of the single flowers and the whole inflorescence of the Compositae. In German we got the terms âKorbblĂźtengewächseâ for Asteraceae, while the congested flower was a âBlĂźtenkorbâ but i donât know the exact terms in English.
True bracts may well be showy to attract pollinators in other plant families, but i donât remember to have seen such with Asteraceae, though i couldnât exclude the possibility.
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Iâm certainly not an authority on evolutionary phylogenetics so donât take my word for it, but its likely that the flashy red âpetalsâ we see on this species (and in most of the Genus) originated as parts of the inner sepal whorl (the appendages between the bracts and the petals). Over a long cycle, the hereditary success for the plant possibly selected for those sepals which grew out at an angle and became red and flashy so as to attract the desired pollinator
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Hereâs a couple of labeled diagrams focusing on the flowers of Asteraceae:
https://www.pottconservation.com/blog/plant_family_focus_asteraceae/
https://little-ship-of-horrors.tumblr.com/image/183869364992
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Hi Anthony, i need to ask about your explanation, which part of the ray flowers produced the showy, asymmetric crowns, as i am not sure that i got the right meaning.
I do start with the tiny, invisible bracts, which are likely present at the very base of the ray flowers, sitting on the axis, respectively the bottom of the basket-shaped compound flower. Those small bracts, in whose axils the ray flowers and disk flowers do arise, are homologous to the way larger bracts which do cover the lower, outer side of the compound flower.
The lower whorl of flower leaves, so named sepal whorl or calyx is for usual not showy in case of Asteraceae, syn. Compositae, but serves other purposes, as e.g. to turn into a propagational flying organ, when the fruit is mature. I never got aware that the showy, asymmetric organ of the ray flowers was grown by (parts of) the sepals, although such seems possible to me. I think to have learned (surely long ago), that a part of the crown leaves, the petals built the showy part of ray flowers. However, both ways could have been taken, i donât know the situation with the genus Zinnia. Perhaps you could tell us more about.
Kind regards, Erwin
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Welcome to the Forum!
Other than that, I can be of no help. Iâm not a plant person, and donât even understand what a âbractâ is 
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There is a sentence in the Flora of North America saying: " The lack of articulation of the corolla tubes in the ray florets of Zinnia verticillata Andrews (= Z. peruviana ) and the bilateral disposition of vascular bundles (continuous with vasculature of the ovary walls) in the ray florets led D. Don (1830) to conclude that true ray âcorollasâ in Zinnia are lacking, being replaced instead by de novo petaloid structures that mimic ray corollas of other Compositae."
So this seems to be an exceptional structure.
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Hi Erwin
It certainly sounds like you know far more about the bio-evolutionary history of Zinnia than I do at the moment and for many years to come.
My comment was for the greater part a conjecture predicated on the knowledge that plasticity in floral organ form as a function of the purpose it has to adapt or re-adapt to (over a very long timespan) is known to occur in several plant families, including Aizoaceae and Asteraceae. I know that in Zinnia peruviana for example, the gradation between the outer epicalyx and the inner âshowy petalsâ may seem smooth enough, but that the base of the latter is fused only tenuously to the rim around the gynophore, such that the top âhalfâ of the flower can be actually be removed in its entirety with the right degree of pulling force - Hence, I took an academic leap of faith to conclude that there may be a link
If you look at the photo in the OP, you will notice that the base of the showy part of the petals is pinched in much the same way as the cordifoliate connection between the petiolar ingress at the leaf base and the site of attachment to the stem. Thus, what you have concluded in your last comment about the petals being formed from restructured leaves may very well be the case, even more so when you consider the possibility that this taxon may have had to adapt its pollination strategy in order to provide a sturdier platform and altered metabolic pathways for colour display and volatile accumulation. Given that the diversity and maleability of metabolic pathways is greatest in the photosynthetic parts of the plant (i.e green leaves), it would be easier for the plant to adapt these structures to suit its needs under most conditions
Iâm sure there are alot more papers on the subject, but needless to say I have already learned a great deal from your input. Thank you for this food for thought! 
Best wishes
Yeah, Asteraceae are also known as composites because they are basically bunches of tiny flowers merged into a big, single ââfake flowerââ, instead of gathering them in traditional flower clusters, so those red thing arenât bracts, but they arenât true petals either. They are the petals of the ray flowers.
More information here: https://www.backyardnature.net/fl_comps.htm
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Well, thatâs cool! Do we know what structures the petaloid structures are derived from?
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How weird!! Interesting to learn.
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Botanical terms are many and complicated. Illustrated botanical glossaries are an extremely useful reference.
Hereâs a free PDF of one thatâs worth downloading and keeping around (Plate 11 shows the generalized form of the flower in question):
- Wilhelm & Rericha, 2017 ILLUSTRATED GLOSSARY OF BOTANICAL TERMS
A more dictionary-like approach is also a useful resource, hereâs another free PDF resource:
- Seiberling 2005 Glossary of Botanical Terms Used in the OpenKey Project
If you want to get a physical copy, the Harris and Harris 2001 Plant Identification Terminology: An Illustrated Glossary is what my botany instructors recommended in our classes.
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