|Oenothera acaulis leaves (the left one with a seed capsule attached to the leaf axil)|
Oenothera acaulis sprawls across the ground; its weak stems can't seem to stand up straight. Its leaves are wrinkly and untidy, and look a bit like the leaves of many weedy plants. But when its huge white flowers open just as the sun is setting, it's a transformation.
|An Oenothera acaulis plant in flower.|
The flowers stay open for a day, then they turn pale pink and wither.
|Oenothera acaulis flower.|
It's not strictly a wild flower in New Zealand, but a few people cultivate it. I got some seeds from the Christchurch Botanical Gardens for research on seed dispersal. It has established in my garden and this year's plants have come up of their own accord from seeds in soil where I planted it last year. Some botanists would count that as wild, but it's not the definition we used in Flora of New Zealand Vol. 4, where crossing a boundary (such as into the neighbor's yard or onto the street reserve) was needed.
|Oenothera acaulis old flower withering and showing the 150 mm long floral tube.|
Anyway, these huge flowers open so quickly at sunset you can watch them moving. It's a beautiful sight. The style is very long and sits inside a long floral tube. The pollen has to grow all that way to reach the ovary.
|Oenothera acaulis: this old flower has turned pink as it withers|
My interest in O. acaulis began years ago when I grew some in Christchurch. I'd been working on the ice-plant family Aizoaceae and I noticed that the capsules of O. acaulis were stumpy and woody; they looked more like the capsules of Aizoaceae than the usual long and slender Oenothera capsules. So I wondered if they opened the same way.
|Oenothera acaulis unopened fruit.|
Most Aizoaceae have an unusual method of capsule opening; it's called hygrochasy. Hygrochastic capsules open when they're wet. Most ice plants live in the deserts of South Africa and Namibia, and wetting their capsules causes special tissues, called expanding keels, to swell. The expanding keels are attached to a non-swelling resistance tissue and their swelling generates curvature in the capsule walls that forces the capsules open. Other ice-plants have more complex add-ons that help to splash the seeds out of the capsules using the energy of falling rain-drops.
So I picked a couple of Oenothera capsules and dropped them into a glass of water. Within 15–20 minutes they opened fully. That's a fairly slow rate of opening, but it's about what the ice-plants do. It's thought to be a mechanism that ensures they won't open until enough rain has fallen to enable the seeds to germinate and establish.
|Oenothera acaulis cross section of a capsule|
Oenothera capsules don't have expanding keels, but they do have four spongy internal walls (bright and pale above), and running up each of their four sides is a rigid woody vein (brown above). The spongy walls swell when they take up water and the rigid vein constrains the swelling to force the capsule open. In the enlarged photo below, the rigid vein is on the right, characterised by rows of thick-walled cells; the cells of the capsule's internal walls are spongy and can expand when they're wetted.
|Oenothera acaulis cross section of a capsule, enlarged, and viewed under polarised light and filters to show cell arrangements and thickened walls.|
This is the only time I've been scooped in my research, because while we were writing the paper about this, German botanist Hans-Dieter Poppendieck published a thorough account of fruit opening in O. acaulis (Poppendieck 1995). He also came to this through noting the similarity with Aizoaceae.
Like the ice-plants of Africa, O. acaulis is also a desert plant, but it's from South America. It's a remarkable example of parallel evolution, where the same function has evolved twice in unrelated plants, and the two plants have achieved the same result in ways that are structurally very different.
Also, this is a plant that can move, both when its flowers open rapidly at dusk and when its fruits open on a rainy day. It's the "ugly duckling" of the plant world.
Poppendieck, H.-H. 1995. Hygrochastic capsules in Oenothera (Onagraceae). Mitteilungen aus dem Institut fuer Allgemeine Botanik Hamburg 25: 99–115.