Exotic structure of the Lesser Periwinkle

[rjlittlefield]

Some weeks ago, I posted Purple star with fibers, an image shot straight down the center of a Lesser Periwinkle (Vinca minor).

At the time, I wrote that "I'm pretty weak on the anatomy of these things. ... I presume that the cream-colored structures are part of the ovary, but what might be style, stigma, etc. is still a mystery to me."

It turns out, that was a serious understatement. "Clueless" would have been a better description than "pretty weak"!

For your viewing pleasure and edification, here's what I found when I investigated further. (One image, two panels.)



The "cream-colored structures" in the center of the star image are not part of the ovary at all. In fact, they are the outer faces of the anthers! The ovary is almost a full centimeter away, clear down at the bottom of the tube.

The stamens grow very high on the tube. The bases of the stamen filaments are inserted only slightly below a prominent ring near the top of the style. The filament itself forms a prominent "knee" that appears to rest against the side of the style, keeping it precisely centered in the tube. Above the knee, the filament juts back to the outside of the tube, then turns back in. The anthers arch farther in and over the stigma, completely hiding it from view outside the flower. The anthers discharge pollen grains by opening along vertical slits on their sides, the contents of two adjacent anthers merging into a single sticky glob of pollen that rests underneath the stigma. The stigma itself is composed of many hairs, a few of which grow down between the blobs of pollen. Details appear in the following two images, first with the style/stigma in its proper place, then moved to the side and viewpoint shifted so you can see what was behind it. (One stamen and one glob of pollen have been removed from all images.)





I am still puzzled about some aspects of these flowers. The literature says that these things are pollinated by bees. I wonder how that works? There's often a drop of slightly milky liquid that apparently exudes from the ring on the pistil as seen in these pictures. It seems likely to me that the liquid is some sort of nectar, but I really don't know for sure. For that matter, I can't recall ever seeing a bee visit my Vincas, or seeing any sort of fruit or seeds. Something to look for, I guess!

Hope you enjoy the pictures!

--Rik

PS. Before somebody asks... I already did the pollen through a microscope -- see Vinca minor pollen.

Technical: Canon 300D, 80 mm and 38mm Olympus bellows macro lenses, stacked at f/8, 0.005", and f/4, 0.002". Halogen dual fiber illuminator, no diffuser, custom white balance in camera.

Edit: Replace first image with annotated version. Text and lines added in PowerPoint, then SnagIt to Photoshop for finishing.

[Ken Ramos]

You did an outstanding job on this Rik Lots of info in the narrative and the included annotated image really helps to explain what you are presenting, I especially liked that since I am not really up to snuff on plant anatomy and many other things as well. Sharp and quite detailed, great color, and an excellent job on the cut away, what more could you ask for.

[DaveW]

Great images Rik. I think you are possibly correct in thinking the "ring" is a source of nectar and a lure for the bees. From how the anthers release their pollen it looks as if the plants may "self" themselves if not pollinated. Really you need a series of sections of different flowers over time to see if there is any differential growth of style/stigma and filaments/anthers.

Also not all insects are nectar feeders, some feed on pollen I believe, so the plant sacrifices some pollen in return for covering the insect with pollen which then fertilises the next plant it goes to.

In the cacti the stigma is often exerted before the flower is fully open and the filaments/anthers lengthen later and sometimes "self" a self fertile species if they have not already been pollinated.

Also in the cacti extra floral nectaries are often found (Coryphantha, Ferocactus etc) sometimes called gland spines. These exude nectar and are frequented by ants, one can only presume the ants keep the plant free of some pests in return?

Again great images.

DaveW

[rjlittlefield]

Thanks for the comments, guys!

Dave, I did some searching this morning. There are lots of references saying things like "The fruit is a pair of follicles 2.5 cm long, containing numerous seeds." (Wikipedia). But... There are also lots of references that say the plants seldom or never set seeds here in the U.S. "Spreads by vegetative means only...viability of seeds yet to be reported" [texasinvasives.org], "follicles rarely develop from flowers in Illinois" [illinoiswildflower.info], "apparently does not spread by seed" [Connecticut Botanical Society]

My own plants have been blooming for well over a month with no trace of pollination. The flowers just form, open up, and fall off. I have no idea how this plant behaves in its native habitat, but I guess now I'm not expecting to see much besides flowers, leaves, and stems on the ones under my trees here in Washington.

--Rik

[Bruce Williams]

Rik - Altogether a neatly constructed and well thought through posting. The images are photographically excellent and extremely well suited to their purpose. You've done a great job with the annotated pic too.

I enjoyed this posting, thanks.

Bruce

[rjlittlefield]

Thanks for the feedback, Bruce.

The annotation owes a lot to my wife.

Shortly after I posted the original non-annotated version, I showed her what I had been playing with.

As I stood behind her, watching her read the thing, I noticed her head bobbing up and down.

"How nice," I thought, "she's nodding in agreement."

Then I figured out that wasn't the cause at all.

In fact, her head was just going along for the ride. The driver was that her eyes were bobbing up and down: picture to words to picture to words to picture to words... You get the picture. It wasn't pretty.

She seemed much happier with the annotated version. I'm glad you folks liked it too.

--Rik

[beetleman]

WOW, what an incredible set of photos. Sharp, crisp, and packed full of information. Well done and very professional.

[rjlittlefield]

Thanks for the compliments, Doug.

Well, here we are, six weeks later, and I never saw a hint of a seed pod. So I guess they don't self-pollinate, and I guess nothing here pollinates them either. Curious.

Handy though -- makes it so much easier to keep them confined where we want them...very unlike the neighbor's Virginia Creeper that used to be two houses down the street...

--Rik

[karl-m]

These are amazing......i would never have thought of cutting up a flower head for this type of detailed work.....

My eldest Daughter will like seeing these because they've been learning about plant reproduction.....

[Planapo]

Rik - The six weeks ago I did already see this outstanding contribution of yours, enjoyed it and had in mind to post an answer but, as it goes, got distracted and then this post went out of sight and I forgot about it.
So, now that it has been brought up again I don´t wanna miss to drop a few lines.

For a botany course I had to take some time ago it was Vinca minor´s flower that I chose as object of study. Accordingly I had to dig a bit into the literature and thus can provide some more information on this flower´s peculiar functional morphology. I think if I had had your photos, I wouldn´t have felt the need to use a stereoscope for my work at that time.
Besides and interestingly, the honourable Charles Darwin had a close look at this plant already.

Accordingly to the literature consulted it is mainly a bee flower and around our house where it grows I was able to see Anthophora and bumble bees visiting it and the papers mention these bees too. The colour of the flower is sometimes referred to as bee blue and can be perceived by bees especially well. The pollination is the key to the special morphology: The outer broad parts of the petals provide the insect a place to land and sit. The nectar is secreted at the bottom of the flower, the nectaries are the greenish structures that can be seen below and around the ovaries in your annotated photo. Hence the nectar can normally only be reached by an insect with a longer proboscis.
The structures are partly interpreted as shelter against water/rain that otherwise, if it got into the flower, would dilute the nectar and thus render it less or not attractive for the insects and to take up the own pollen and prevent it from going down into the flower The hairy tuft on top of the style that you labelled as stigma, is not interpreted as such. Depending on the paper consulted it´s either the undersurface of the discoidal structure or its rim that you named "ring" that is regarded as the stigma.

Though the different papers are a bit contradicting, the process of pollination can be imagined roughly as follows : An insect comes from another Vinca flower and thus foreign pollen clings to her proboscis. She inserts her proboscis into the next Vinca flower to reach the nectar. And while the proboscis passes the sticky rim of the disc, foreign pollen is transferred to the stigma or after having sucked up the nectar and while pulling back her proboscis the foreign pollen is scraped off by the slightly protruding lower edge of the discoidal ring and in this way transferred onto the stigma (undersurface of the disk).
Then, when the thus cleaned proboscis is pulled back further, it is covered with the sticky fluid from the disk and then, being pulled back further again, the flower´s own pollen, which itself is dry and not sticky, now clings to the proboscis when passing the anthers´ and tuft´s pollen loaded hairs.
In this way the special morphology is thought to help to prevent self pollination and foster foreign pollination. This morphology of stamina and carpels is referred to as herkogamy.

Sorry for my longish text, but I think maybe you find some of the information useful.

Anyway Rik, your fotos here and this post are excellent as are many of your other contributions that I´ve read. The way you thoroughly investigate things is great. Honestly, you are a true naturalist! And moreover, thanks for teaching and sharing your sound knowledge about photography.

Best wishes,
Betty

[rjlittlefield]

Betty,

Check out the new picture!

I am delighted to read your post about the structure and function of this plant. There is so much to learn, so little time to find it out, and so easy to get misled by what we think we know! I would never have guessed from appearance that the stigma is on the "ring", and I can barely imagine how one would determine that experimentally. Artificially pollinate and see what works? Microscopically examine cross sections to track the pollen tubes? Do you happen to know how they did it?

Thank you for the compliments. I always did like "show-and-tell", and I was lucky enough to have parents and teachers who indulged and encouraged my curiosity. If I can pass along some enthusiasm or insight, that will be great! I think maybe it is the same for you?

--Rik

PS. I'm not quite sure what "dry and not sticky" means, with respect to the pollen. See this post for a description that I wrote right after wrestling with the pollen for a micrograph. Maybe it varies from flower to flower?

[Planapo]

Rik - at that time I consulted handbooks of botany (secondary literature) which only give descriptions of the function but unfortunately don´t explain how the results were gained experimentally. Some of them list the primary literature which their descriptions are based upon. Of the primary literature I only had a look into one of Darwin´s papers (Darwin, 1861) which deals with V. major and sheds some light on experimental evidence that was used then: Yes, he figured out that the "concave tire of the wheel [discoidal structure of the style] is the stigmatic surface, as was very evident when pollen was placed on it, by the penetration of the pollen-tubes". And yes, he was able to obtain seed pods (fruits) by artificial cross-pollinating with a bristle he used to mimic an insect´s proboscis.

(Now, in the near future I imagine a Mr. Littlefield on his knees in his garden tickling his periwinkles with a bristle. )

And as it comes to "dry and not sticky" pollen, I remember how that got me thinking when I had read it in one of the books, because what we see when looking closely at V. minor are pollen grains sticking together in lumps at the plant´s hairs. So the pollen grains certainly stick to each other and to the hairs and at least in V. major the pollen seems to have stuck to Darwin´s birstle, but the author states that it would not stick to an insect´s proboscis. Maybe the chemical/physical properties of the various surfaces are different.
And variation can be an issue, variation in time (plant ripening) and, as you said, variation in the plant-line is an issue, i.e. it should be paid attention to what plant material is actually investigated (The investigations were even made with different species). Vinca easily reproduces asexually by runners and I think most Vincas we see are garden varieties that are reproduced asexually by the gardener. Hence nectar or pollen quality isn´t paid attention to. Around our house I ´ve seen bees visiting Vinca only very very scarcely.

Cheers,
Betty.

Darwin, C. R. 1861. Fertilisation of Vincas. Gardeners´ Chronicle and Agricultural Gazette. (15 June): 552

[rjlittlefield]

(Now, in the near future I imagine a Mr. Littlefield on his knees in his garden tickling his periwinkles with a bristle. )

A task for next spring! How could I resist?!

Thanks again for the further great info.

--Rik

[PaulFurman]

I was able to see Anthophora and bumble bees visiting it and the papers mention these bees too

Betty,
Did they say what the bumble bees were doing? Perhaps just bumbling in and getting frustrated? It sounds like they can't reach the nectar or maybe they can but don't have a probiscus texture suitable for sticking pollen to carry on and pollinate.

[adamandrea]

beuteful