Portraits of Wildflowers

Perspectives on Nature Photography

Another sensitive plant

with 41 comments

Neptunia pubescens Flower Head 9010

This column has brought you several pictures of sensitive-briars, whose compound leaves perform the neat trick of folding up within seconds after something touches them. A relative of those plants that has learned the same trick is Neptunia pubescens, called tropical neptunia, which makes its debut here today. Where the color of the sensitive-briar’s flowers varies from pink to violet, tropical neptunia wears bright yellow. Where the sensitive-briar’s tiny flowers are dispersed rather uniformly around the globes they form, a flower globe of tropical neptunia is asymmetric.

Neptunia pubescens grows in all and only the American states that border the Gulf of Mexico, as you can confirm on the USDA’s map. Although the Texas map there doesn’t show this species in Travis County, which is where Austin is, tropical neptunia has spread widely here over the last decade and I’ve see it in various places. The one that provided today’s picture was on Burnet Rd. near the old Merrilltown Cemetery; the date was May 28.

© 2014 Steven Schwartzman

Written by Steve Schwartzman

July 16, 2014 at 5:55 AM

41 Responses

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  1. Very asymmetric. Looks only half finished!


    July 16, 2014 at 6:17 AM

  2. As a biologist I cannot resist the topic of explosive dehiscence …. here’s a cut-paste from The mechanics of explosive seed dispersal in orange jewelweed (Impatiens capensis), Hayashi, Feilich, and Ellerby. J Exp Bot. May 2009; 60(7): 2045–2053.

    Explosive dehiscence ballistically disperses seeds in a number of plant species. During dehiscence, mechanical energy stored in specialized tissues is transferred to the seeds to increase their kinetic and potential energies. The resulting seed dispersal patterns have been investigated in some ballistic dispersers, but the mechanical performance of a launch mechanism of this type has not been measured. The properties of the energy storage tissue and the energy transfer efficiency of the launch mechanism were quantified in Impatiens capensis. In this species the valves forming the seed pod wall store mechanical energy. Their mass specific energy storage capacity (124 J kg−1) was comparable with that of elastin and spring steel. The energy storage capacity of the pod tissues was determined by their level of hydration, suggesting a role for turgor pressure in the energy storage mechanism. During dehiscence the valves coiled inwards, collapsing the pod and ejecting the seeds. Dehiscence took 4.2±0.4 ms (mean ±SEM, n=13). The estimated efficiency with which energy was transferred to the seeds was low (0.51±0.26%, mean ±SEM, n=13). The mean seed launch angle (17.4±5.2, mean ±SEM, n=45) fell within the range predicted by a ballistic model to maximize dispersal distance. Low ballistic dispersal efficiency or effectiveness may be characteristic of species that also utilize secondary seed dispersal mechanisms.

    Did you catch the reference to ‘spring steel?’


    Pairodox Farm

    July 16, 2014 at 6:28 AM

    • On a hike last week in eastern IA, we saw a very large number of jewel weed plants, only one flower so far. Next time we are out there should be a lot. Interesting mechanics for them. I’m a former physics teacher.

      Jim in IA

      July 16, 2014 at 7:20 AM

      • When I saw the species name capensis I figured jewelweed must have come from South Africa, but this turns out to be another case of misattributed identity. According to Wikipedia: “The species name ‘capensis’, meaning ‘of the cape’, is actually a misnomer, as Nicolaas Meerburgh was under the mistaken impression that it was native to the Cape of Good Hope, in southern Africa.” The first such case I think I became aware of is Asclepias syriaca, which is native to the United States and doesn’t grow in Syria. I discussed that mistaken identity at:


        Steve Schwartzman

        July 16, 2014 at 7:37 AM

      • Hey there Jim … as a physics-type, perhaps you’d be interested in my post on spore dispersal (http://wp.me/p1yRFa-3Da) and one concerning leaping in fleas as well (http://wp.me/p1yRFa-3EX) … I’m a zoologist by training (molluscs especially), but I’ve always had a thing for mechanics. D

        Pairodox Farm

        July 16, 2014 at 9:48 AM

    • But doesn’t all that hissing and de-hissing tire the plants out? (Sorry, I couldn’t resist.) I’d never thought about measuring the force involved in a phenomenon like dehiscence—yes, WordPress, there is such a word, so you can stop putting little red dots under it—but I shouldn’t be surprised that scientists have. With such an explosive subject, I’m sure grants will be forthcoming.

      Yes, I did catch the reference to spring steel. I’ve occasionally read that this or that native species in my area disperses its seeds explosively, but I’ve never been lucky enough to be looking at one when that happened.

      Steve Schwartzman

      July 16, 2014 at 7:29 AM

    • The science is just a bit above my pay grade, but this still was quite interesting. I’ve tried to remember, and can’t, which plant it was that I dragged home and put in a vase some years ago. It was past its bloom and had some lovely seed pods. I thought they were attractive enough to add to my decor.

      Imagine my surprise, about a week later, when I walked into the living room and found seeds covering the table and carpet — some of them, as much as six feet away from the plant stems. It took me a while to believe what I was seeing. That energetic little plant was doing its best to reproduce in my living room. It was an amazing experience.

      Spring steel, indeed.


      July 18, 2014 at 9:22 AM

      • You’re fortunate to have had the experience. In the wild it’s hard to recognize seeds that have been catapulted some distance from their source, but I still hope someday I’ll be looking at one of those plants at just the right moment.

        Steve Schwartzman

        July 18, 2014 at 9:31 AM

      • Funny story … thanks for taking the time to relay it. D

        Pairodox Farm

        July 18, 2014 at 2:24 PM

  3. You need to set your camera on a tripod and then mechanically release the seeds with one hand while tripping the shutter with the other … you can do it! D

    Pairodox Farm

    July 16, 2014 at 9:49 AM

  4. Intéressant Steve. J’ai déjà rencontré des plantes sensitives dans certaines régions de France et c’est très impressionnant.


    July 16, 2014 at 11:42 AM

  5. ok. i’m going right now to examine some of the primos growing near the house! thanks for the nudge to take a good close look at the flowers!


  6. Lovely little flower, I like the asymmetric aspect of it, most flowers are pretty even and symmetric and this one is out of proportion which adds a different kind of beauty to it! Love the name “Neptunia pubescens”, most of us know plants by everyday names so it’s really nice to get to know the official botanical definition. So today I learned something new, got acquainted with Neptunia and I thank you for that!


    July 16, 2014 at 12:06 PM

    • You’re welcome for the introduction to this little plant. I wondered about the name neptunia, which of course suggests Neptune, the Roman god of the sea. so I looked in my most thorough and authoritative reference, Shinners and Mahler’s Illustrated Flora of North Central Texas. There I learned that Neptune was also a god having to do with perpetuity of springs and streams. More to the point for botany, though, the genus was apparently given its name from the fact that some Neptunia species are aquatic. Live and learn, as I’ve said so often here.

      Steve Schwartzman

      July 16, 2014 at 12:29 PM

  7. i returned from ‘the field’ with much more awareness of the plant that grows near the river.   i removed the one in my yard last year, as it was prickly and often ‘bit’ me… these didn’t have any spines, though i base that comment on two or three tests but not on every plant.

    yes, the flowers are very asymmetrical, though from certain angles one sees only the globe of yellow.

    internet speed is too slow to send/upload any images.. for now they’re pending with several others!



    • Some species of sensitive-briar have prickles, while others don’t. The most common one in Austin does, as my skin can attest. Even the pods of that species have prickles on them.

      Sorry about your slow Internet speed, Z. Maybe you can incorporate your pictures into a future post. We’ll look forward to it.

      Steve Schwartzman

      July 16, 2014 at 12:54 PM

  8. Wow, that Mahler guy was multi-talented.
    Nice explosive discussion going on up there. I found a partridge pea (Cassia fasciculata the other day that has some sensitivity issues…don’t we all?

    Steve Gingold

    July 16, 2014 at 3:50 PM

    • Shinners and Mahler did foundational work, but three more-recent authors are responsible for the current book:


      I’ve read that partridge pea’s leaves are slightly sensitive to being touched, but I think I’ve tried to get a reaction and haven’t been able to. I didn’t realize you have that species way up in Massachusetts.

      Steve Schwartzman

      July 16, 2014 at 5:57 PM

      • That is a very cool looking book. I wish we had something like that for Massachusetts.
        I was hinting at it being Gustav with my remark.

        Steve Gingold

        July 16, 2014 at 6:12 PM

        • I thought it might be an interesting book despite being for TX. But then I saw the price when I scrolled down. Then I scrolled down and saw 1640 pages, so worth the price I would say. Do you own a copy?

          Steve Gingold

          July 16, 2014 at 6:15 PM

          • Sheesh, I am slow-witted tonight. Yes, it is supposed to be sensitive to the touch and I tried it but no response. Maybe I just don’t have the right touch. That was always a problem for me.

            Steve Gingold

            July 16, 2014 at 6:17 PM

          • I did spring for a printed copy of the book when it came out more than a decade ago. You’re in luck, however, because it’s now available for free online:


            Steve Schwartzman

            July 16, 2014 at 7:31 PM

            • This is why I never miss reading through the comments on your posts. There are treasures to be found.


              July 18, 2014 at 9:25 AM

              • Agreed. I’ve learned plenty from the comments as well. Volume 1 of the book for east Texas has already appeared,


                and volumes 2 and 3 are in progress. My guess is that for several years each new book will be for sale only, then made available for free on the Internet.

                The large majority of the species in Travis County appear in the north-central book, so I’ve found it quite helpful. Many of the species in your coastal region appear in the north-central book too.

                Steve Schwartzman

                July 18, 2014 at 9:42 AM

        • Actually my first thought when I read your comment was of Gustav Mahler.

          Steve Schwartzman

          July 16, 2014 at 7:28 PM

  9. I love your posts, they are so rich in information and the photos are so exceptional.

    • Thanks, Charlie. I’m sorry for the late reply, but WordPress mysteriously thought your comment was spam. Silly WordPress.

      Steve Schwartzman

      July 21, 2014 at 8:46 AM

  10. I had to go back to the post about Cephalanthus occidentalis to find the word that seems to apply here, too: exserted. The asymmetry reminds me of some of the grasses I’ve learned to recognize, like sideoats grama, our Texas state grass.


    July 18, 2014 at 9:32 AM

    • You’ve made me wonder, with respect to bellybuttons, whether anyone has ever referred to an outie as exserted.

      I’m thankful for the asymmetry of sideoats grama, because in general I find grasses hard to identify.

      Steve Schwartzman

      July 18, 2014 at 9:53 AM

  11. […] flower globe, Desmanthus illinoensis, and the plant’s shadowed leaves. Unlike its relatives tropical neptunia and sensitive briar, the prairie mimosa has compound leaves that do not close up when […]

  12. […] The hazy “sun” behind the rain-lily was a conveniently out-of-focus flower globe of Neptunia pubescens, known as tropical neptunia or tropical puff. The pink in the upper background may have come from […]

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