Pollination - genus Symphyotrichum, Asteraceae family

Nature Talk
#1

Can you smart people help me? I have a basic understanding of pollination. I know about the birds and the bees. I am looking for more detail, though, and I’m hoping some of you kind people will be willing to help. I have read and looked and searched, but there are still gaps in my knowledge.

I’ve attached a close-up of a flower head of a member of the Asteraceae family, Symphyotrichum genus (specifically Symphyotrichum lateriflorum). The flower on the right which I mostly cropped out had likely been thoroughly pollinated. The one with the yellow disk florets is just beginning and perhaps hasn’t been visited yet (I think). I’m referring to the one on the left.

Are the arrows I drew pointing to the styles? Do they have sigmas at the ends, and if so, why can’t I see them? Is this pollen on the ends? Is this prior to pollination? Did the style elongate from the disk floret with the pollen on it, or pick it up on the way out? When insects pollinate composite flowers like this one, what part of the flower contains it and to what part of the flower does it need to go? Isn’t the ovary at the bottom of the disk floret, and it needs the pollen? Does the ray floret also touch the ovary? I believe I understand that the nectar is there to attract the pollinators, but where is it?

Why do we have both rays and disks? (You can maybe ignore that last question. I sound like a 3-year-old discovering the world and words at the same time.)

Also, would someone help me with the term “corolla”? It seems to be used interchangeably with other terms. I’ve looked it up, seen diagrams, but still when I read it in florals, journal articles, etc., I often don’t know what part(s) are being discussed.

Calico_Asters_25_September_2012
Calico_Asters_25_September_20121323×1397 287 KB

Image credits and license:
Author: Cathie Bird
License: CC-BY-SA-2.0
File: https://commons.wikimedia.org/wiki/File:Calico_Asters_25_September_2012.jpg
Originally from: https://www.flickr.com/photos/39993830@N05/8572439623
Modified by Elizabeth Ballard cropped and added text.
License link: https://creativecommons.org/licenses/by-sa/2.0/deed.en

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#2

There’s a stylized image in wikipedia that might help with terminology: https://en.wikipedia.org/wiki/File:Mature_flower_diagram.svg

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#3

@trscavo Thank you. Yes, that is a good diagram. I’ve seen it and then sometimes can’t find it, so thank you for the link. I’ve looked for a similar one for composite flowers but no luck on that yet.

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#4

Your arrows are pointing to clumps of pollen being pushed out of the connate anther tubes (reddish, below arrows) by the developing styles (not visible).

In general: The five anthers in a disk floret are fused with each other along their edges to form a cylinder. The anthers release pollen on the inside of that tube. The style grows up through the tube, picking up pollen on the nonreceptive outer surface and pushing it out into the open. This is when pollinators may pick it up. The style then splits into two branches, revealing the receptive stigmatic surface inside. This is what receives external pollen.

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#5

The corolla of these disk florets is the translucent white-green star shape. The corolla of the ray is the strap like white ‘petal’ and the tube at its base. Every floret has its own ovary. The nectar is at the bottom of the corolla tube. Rays serve the same purpose as petals in conventional flowers – attraction. Rays are often large and showy, and may contrast in color and have UV nectar guides. The reason composites have evolved to often have both rays and disks is because each floret can produce seed. So one visit by a pollinator to a flower head can pollinate multiple florets producing multiple seeds. It’s efficient!

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#6

@dgreenberger Oh! Enlightenment! Wonderful!

I had confused stigma with styles (among other confusions). Are the styles are not visible because they are covered in the pollen? I’ll find an image of what I think are the styles. I believe I have some.

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#7

@dgreenberger In this next photo, I don’t see any pollen on the ends of what you called the anther tubes (also stamens?). So I guess that means those were pollinated. At the ends of where my arrows are pointing, are those the styles? It looks like two fused, and you said “the style then splits into two branches.” I don’t see any that are split yet in this image. Or perhaps the ones that appear to have holes are actually splitting…

Calico_Aster_(Symphyotrichum_lateriflorum)_-_Kitchener,_Ontario_2018-09-29-cropped
Calico_Aster_(Symphyotrichum_lateriflorum)_-_Kitchener,_Ontario_2018-09-29-cropped554×559 66.5 KB

Image origination and attribution: Ryan Hodnett, CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0), via Wikimedia Commons
Original: https://commons.wikimedia.org/wiki/File:Calico_Aster_(Symphyotrichum_lateriflorum)_-_Kitchener,_Ontario_2018-09-29.jpg
Cropped and added text by Elizabeth Ballard

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#8

Yes, those appear to be mid-split.

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#9

One of the very best and most approachable books to assist with plant terminology is the 1994 Harris and Harris Plant Identification Terminology an Illustrated Glossary.

It’s well worth hunting down a copy.

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#10

@earthknight Yes! I have it! I purchased it after it was recommended to me on this forum a few months ago. It’s great. Still, for some reason, I need photographs, and the terminology book doesn’t tell me why and how. It tells me what, but I also don’t always know what I’m looking for. Having a place like this to ask questions, and then receive answers from you all like the paragraphs from @dgreenberger seemed to make things kick in. :)

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#11

And then to make it more confusing, I’m looking solely at composite flowers. I would never have been able to get that the five anthers that you see on a lily could evolve into something fused together to make a tube. Harris and Harris show me a lily-like (non-composite flower) anther diagram. I’ve read Wikipedia articles on the family, order, genus (all need work) and can’t find what I’m looking for, or if I do, I still have something blocking the understanding. I look on my photo and see nothing resembling that at all. Five anthers fused to make a tube. Who knew? I’d like to see a microscopic photo or two of that.

Which brings me to another question. @dgreenberger Where are the anthers of the ray floret?

This image from @ddennism (CC-BY 4.0 https://creativecommons.org/licenses/by/4.0, via Wikimedia Commons, also observation 32445482) shows an expanded ray floret on the left. I believe the “petal” actually is three pieces fused.

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#12

In Symphyotrichum the ray florets lack stamens.

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#13

Which means… ray and disk florets both have ovaries. Only disk florets produce pollen, but seeds can develop from both once cross-pollinated by our beautiful short and medium length tongued insects.

#14

@ddennism @dgreenberger and others

From FNA on eFloras, Symphyotrichum entry:

Receptacles flat to slightly convex, pitted, epaleate. Ray florets (8–)12–35(–75+); usually in 1 series, in 2–5 series, rarely in 4–5+ series in S. frondosum, pistillate, fertile; corollas white, pink, blue, or purple (rays 0, peripheral pistillate florets in 2–5+ series, corollas lacking laminae in sect. Conyzopsis). Disc florets (7–)15–50(–110), bisexual, fertile; corollas yellow to white, becoming purplish to reddish or pinkish at maturity, ± ampliate, tubes usually shorter than funnelform (cylindric in sect. Conyzopsis) throats, lobes 5, erect, spreading, or reflexed, deltate, triangular, or lanceolate; style-branch appendages lanceolate.

  1. What are the “receptacles”?
  2. Ray florets - “pistillate” is specified here to say that the ray florets are female only, correct?
  3. Disc florets - “bisexual” is what it means.
#15

I found this useful link.

https://cronodon.com/BioTech/asteraceae.html

#16

  1. The receptacle is the inside of the capitulum where the individual florets attach to the inflorescence. It is not part of the flower (read floret) and it remains on the plant after seeds have been shed. It’s the dimpled surface you see after all the dandelion seeds blow away. Contrast how the receptacle is described for Symphyotrichum with Rudbekia to get some feel for the variation in the family.

  2. Correct. Ray florets generally can be pistillate or sterile.

  3. Just means that it has male and female parts.

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