Oxalis – the Plant That Moves
| Polish version is here |
The following article was originally published in the journal for educators Biologia w Szkole (eng. Biology in School) (5/2021):
Those of you who have read my earlier articles already know that plant movements are one of my favorite topics. So why do I find this topic so fascinating, especially in an educational setting?
A key element in effective learning is student engagement. When we explore life by dividing it into kingdoms — Animalia and Plantae (setting aside fungi Fungi and protists Protista for now) — students’ attention tends to focus on animals. After all, animals move energetically, display intriguing behaviors, and can interact directly with people. Plus, as fellow members of Animalia, they feel more relatable.
Plants, however — even though their anatomy and molecular metabolism are equally captivating — appear stationary, and their reactions to stimuli seem slow compared to animals. For many learners, that makes plants seem rather dull, which poses a real challenge for educators.
To spark curiosity about plants, we can demonstrate that they’re far from inert. For example, the carnivorous Venus flytrap Dionaea muscipula snaps shut in a fraction of a second, and the sensitive plant Mimosa pudica folds its leaflets at a touch [1]. While these species aren’t overly difficult to grow, they require specific conditions that may be hard to provide in a standard school lab.
I believe the most engaging approach is to highlight fast-moving plants that thrive locally. Sundews Drosera offer great examples: native species include round‑leaved sundew Drosera rotundifolia, English sundew Drosera anglica, oblong‑leaved sundew Drosera intermedia, and the hybrid Drosera × obovata. All are protected, so cultivation without permits is prohibited. Fortunately, ornamental sundews like Cape sundew Drosera capensis (Photo.1) are readily available and easy to grow [2].
Sundew trap leaves are covered in glandular hairs that secrete a sticky, insect‑attracting substance, forming dew‑like droplets. When an insect lands, it becomes stuck, and the leaf margins curl around the prey for digestion (Photo.2).
Another impressive rapid movement occurs when the stamens of barberry flowers — common barberry Berberis vulgaris or Japanese barberry Berberis thunbergii — are touched [3]. Barberries are common in our parks and hedges, making this an accessible demonstration.
Oxalis
Today’s focus is on wood sorrel, a member of the Oxalidaceae family, which includes five genera Averrhoa, Oxalis, Biophytum, Dapania, Sarcotheca and about 570 species, most in Oxalis [4]. It’s the only Oxalidaceae genus native to Poland. Oxalidaceae are cosmopolitan, absent only from the poles and extreme deserts. While Europe hosts few species, eastern Asia, South America, and southern Africa are hotspots. Some have culinary uses: oca Oxalis tuberosa tubers in the Andean highlands and starfruit from Averrhoa carambola. Several Oxalis are also grown ornamentally.
The common name wood sorrel Oxalis refers to the high oxalic acid H2C2O4 content in its tissues. Despite the similarity of their names, wood sorrels aren’t closely related to true sorrels Rumex, which also contain oxalic acid.
Fun fact: Polish makes the same distinction — “szczawik” refers to Oxalis, while “szczaw” denotes Rumex.
Of about 20 European Oxalis species, only two are native to the continent, and just one — wood sorrel Oxalis acetosella — occurs naturally in Poland. Other recorded species include:
- Oxalis tetraphylla – ephemeral;
- Oxalis deppei – cultivated;
- Oxalis dillenii – naturalized;
- Oxalis corniculata – naturalized;
- Oxalis stricta – naturalized [5].
In my garden, I spotted a plant clearly belonging to Oxalis (Photo.3).
With its trifoliate leaves, wood sorrel is often mistaken for clover Trifolium, such as white clover Trifolium repens (Photo.4).
A closer look at the leaflets shows distinct differences (Photo.5).
Wood sorrel leaflets are obcordate, while clover’s are obovate, slightly toothed, and bear a pale, horseshoe‑shaped mark.
To identify the species, I examined the small yellow flower (Photo.6). It has radial symmetry with five petals in cymes of two to five. Petals are oblanceolate to obovate; sepals are half their length, linear, erect, and persistent. Flower stalks measure 5–10 mm (0.2–0.4 in) with a tiny stipule at the base. Stamens form two whorls, and the ovary has five styles ending in stigmas.
The yellow blooms ruled out O. acetosella, which has white or pink petals. Of the Polish species, only O. corniculata, O. dillenii, and O. stricta flower yellow. Examining the pedicel bases revealed prominent pulvini in O. stricta (Photo.7) and no leaf stipules at the petiole base.
These traits confirmed my specimen as common yellow woodsorrel O. stricta, native to eastern Asia and naturalized here. It colonizes lawns, fields, gardens, parks, railways, roadsides, and wall crevices, thriving in sun or shade on various soils, especially moist ones. Seeds germinate in May, shoots emerge from rhizomes soon after, and flowering occurs from June to October [6]. Flowers open between 8 AM and 9 AM and close at night or on cloudy days. Insect‑pollinated, seeds are ballochorously dispersed up to several meters (~10 ft) when the capsules burst upon touch [7].
The fruit is an elongated, pentagonal capsule up to 1.5 cm (0.6 in) long (Photo.8). Each of its five chambers contains 4–10 ovoid seeds about 1.5 mm (0.06 in) long, brown, with transverse ridges and one longitudinal groove.
Like other Oxalis, its leaves fold together at night or when it cools. Most fascinatingly, it exhibits seismonasty: its leaflets respond to touch. On a warm, sunny day, they lie spread and nearly parallel to the ground (Photo.9, top).
A gentle brush of the leaflets causes them to sag toward the stem and fold together within seconds (Photo.9, bottom).
This response is easiest to see on an isolated leaf against a contrasting background (Photo.10). In its natural state, the leaflets are open (Photo.10A), while after contact they clearly sag (Photo.10B).
After several minutes, the leaflets return to their initial position.
Explanation
This movement is a nastic response, driven by turgor changes in specialized motor cells. Unlike tropisms, which grow toward or away from stimuli, nastic movements depend on internal pressure shifts. Seismonasty, a reaction to mechanical stimulation, involves the rapid export of K+ ions from motor cells, followed by water efflux. The resulting loss of turgor causes cells to shrink, producing the visible folding. Recovery requires water re-entry, which takes a few minutes. This swift folding may help regulate transpiration or deter herbivores.
References:
- [1] Ples M., Wstydliwa roślina (eng. Sensitive plant), Biologia w Szkole (eng. Biology in School), 6 (2015), Forum Media Polska Sp. z o.o., pp. 52-56 back
- [2] Ples M., O rosiczce słów kilka, czyli wyhoduj żywą muchołapkę! (eng. A Few Words About Sundews: Grow Your Own Living Flytrap!), Biologia w Szkole (eng. Biology in School), 1 (2016), Forum Media Polska Sp. z o.o., pp. 51-56 back
- [3] Ples M., Roślinny bokser? Szybkie ruchy pręcików berberysu (eng. A Plant Boxer? The Rapid Stamen Movements of Barberry), Biologia w Szkole (eng. Biology in School), 3 (2020), Forum Media Polska Sp. z o.o., pp. 81-85 back
- [4] Christenhusz Maarten J. M., Fay M. F. , Chase M. W., Plants of the World: An Illustrated Encyclopedia of Vascular Plants, Kew Publishing, The University of Chicago Press, Chicago. 2017, pp. 297 back
- [5] Mirek Z., Piękoś-Mirkowa H., Zając A., Zając M., Flowering plants and pteridophytes of Poland, Krytyczna lista roślin naczyniowych Polski, Instytut Botaniki PAN im. Władysława Szafera w Krakowie, 2002 back
- [6] Rutkowski L., Klucz do oznaczania roślin naczyniowych Polski niżowej, Wydawnictwo Naukowe PWN, Warszawa, 2006, pp. 282 back
- [7] Lollar M., Marble Ch., Biology and Management of Oxalis (Oxalis stricta) in Ornamental Crop Production, University of Florida, online: https://edis.ifas.ufl.edu/pdf/EP/EP51400.pdf [05.08.2021] back
All photographs and illustrations were created by the author.
Marek Ples