Erodium cicutarium - a plant catapult and moving seeds
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The following article was originally published in the journal for educators Biologia w Szkole (eng. Biology in School) (5/2020):
In this article, I would like to continue exploring movement within the plant kingdom. In our earlier research, we discovered that plants can move certain parts of their bodies relative to others. We also know that the causes and goals of these movements, as well as their mechanisms, are completely different from those employed by animals. Such differences are due to the different physiology of the Animalia and Plantae kingdoms, which have resulted from distinct evolutionary paths.
The major difference between plants and animals is easy to spot at first glance: plants are not capable of locomotion (i.e. the autonomous movement of the entire organism). If plants are unable to move from one place to another like animals, then how do they conquer new areas?
The spread of diaspores is the plant kingdom's most significant mechanism for colonizing new habitats, while the movement of individuals remains almost negligible. This term (Greek: dia – through, sporos – seed) refers to any plant part used for reproduction and spreading. Similarly, diaspores are also produced by fungi [1]. Therefore, a diaspore can represent the whole organism or a part of the organism capable of producing a new individual. Diaspores are divided into generative (seeds) and vegetative (fragments of thallus, tubers, rhizomes, turions, seedlings, spores) [2].
Plants are capable of many different types of motion. Examples include catching small animals as food (Venus flytrap Dionea muscipula, cape sundew Drosera capensis), for pollination (European barberry Berberis vulgaris) and protecting flowers against environmental conditions (strawflower Xerochrysum bracteatum). These movements are generally quite slow [3] [4] [5]. To observe plant movement, we usually have to use time-lapse photography.
As it turns out, there are plant organs capable of moving at such high speeds that special filming techniques are necessary to slow down the video for proper observation. The seeds of the plant I want to tell you about have the ability to independently… bury themselves in the ground. Interestingly, this plant is not exotic at all; it can be found right here in Poland.
Erodium
Erodium cicutarium was the first species belonging to the Geraniaceae family that I studied in my laboratory. Depending on the source, the genus Erodium includes between 60 and 130 confirmed species [6] [7]. Erodium sp. usually thrive in inhospitable places, such as sandy and rocky terrain. Southern Europe has the greatest species diversity in these plants, but they can also be found on other continents. Erodium cicutarium, which we are interested in, is found in the wild in Poland (most likely as an archaeophyte), although some related species are occasionally cultivated as ornamental plants. Additionally, it can be found everywhere except Antarctica.
For some reasons, which will become clear shortly, Erodium is commonly referred to as "stork’s nose"; other names related to a bird's beak are also often used.
Erodium cicutarium clearly prefers acidic, light, sandy or sandy-loam soils, which are also rich in nutrients; it is definitely a nitrogen-loving plant. It can be found in agricultural areas, which is why it is considered a weed in the cultivation of root crops and cereals, including maize. It can also be seen in clover Trifolium fields. The presence of Erodium in crops can lead to excessive drying of the soil and depletion of its nutrients. Additionally, its rapid spread often causes stunted growth or even death of seedlings of other plants. Thanks to its adaptations, Erodium can also reproduce very efficiently and spread its seeds over a relatively large area [8] [9].
In our climate (Poland), Erodium is a relatively small annual plant. Despite its wide distribution in nature, I had to spend some time finding specimens for observation. Interestingly, I discovered them in my garden, where they were most likely sown by accident (Photo 1).
Erodium cicutarium is not protected by law, so for more precise observation, we can collect it from the environment in parts or as a whole (Photo 2). Of course, a specimen of this interesting plant can also serve as a decoration for our herbarium.
The stem of this plant is usually erect and covered with long, soft hairs, and in the upper part, also with glands. The leaves growing close to the soil form a rosette, while the stem leaves grow alternately and get smaller as they ascend (Photo 3).
Small flowers on long stalks are gathered in umbels, with 3 to 10 flowers per cluster. They are delicately zygomorphic, purple, sometimes white or with lighter spots, with five egg-shaped petals that are clearly longer than the sepals. There are five stamens and the same number of staminodes (Photo 4).
The sepals are glandularly hairy, lanceolate or oblong, with membranous edges (Photo 5).
Erodium cicutarium, like other Geraniaceae, is pollinated by insects. Its flowers have small nectaries. After pollination, a fruit is formed, displaying interesting structural features and surprising behavior.
Natural Machine
The fruits of Erodium cicutarium (also of other species in this genus) are very distinctive. Their shape indeed evokes the image of a needle or a stork's beak, which gave rise to both botanical and common names of this plant (Photo 6).
This fruit should be classified as a schizocarp with an elongated tail-shaped structure (Photo 7). It is easy to see the remains of sepals, as well as mature mericarps.
The mature and dry schizocarp easily breaks down into 5 mericarps, which separate from the central elongated tail-shaped element (Photo 8). Each of them has a long, thin awn.
Inside each mericarp, one smooth brown seed can be found (Photo 9). The plant produces several hundred seeds, usually ranging from 200 to 600.
The way plants of the genus Erodium have adapted to living conditions and developed suitable dispersal methods is remarkable.
We know that plants show a variety of seed dispersal strategies. First, we can mention allochory, characterized by the use of various external factors to spread their seeds. Within this category, we can distinguish:
- hydrochory; by water (especially flowing),
- anemochory; by wind,
- zoochory; with the help of animals, for example ornithochory (via birds), myrmecochory (via ants) and others,
- anthropochory; due to human activity.
Nevertheless, there are mechanisms that do not involve external factors in spreading diaspores. This is called autochory (self-seeding), which can be divided into:
- blastochory; by growing the shoot lengthwise and leaving seeds some distance from the mother plant,
- barochory; gravitational, by direct drop of seeds to the ground,
- ballochory; by explosive mechanisms,
- herpochory; due to independent movements [10].
Interestingly, plants of the genus Erodium, and thus also Erodium cicutarium, use two types of autochory simultaneously: ballochory and herpochory… Yes! This plant can catapult its seeds (or rather mericarps) into the air by itself, and they have the ability to move autonomously. Let’s take a closer look at them (Photo 10).
As we can see, the two mericarps presented above differ in the shape of their awns. This is related to the level of water saturation: the hydrated awn (placed in humid air) is relatively straight, but as it dries, it bends significantly. This process is reversible, and it is possible to repeat the wet-dry cycle (straight-bent awn) multiple times. The plant uses this movement to spread its seeds.
Before the schizocarp breaks down, the mericarps form a structural unity with the other tissues. In the mature fruit, the connection of mericarps with the rest of the fruit is quite delicate and acts as a trigger. If the mericarp were not immobilized, its awn would simply bend as it dried. However, in this case, it is impossible due to the connection of the awns along their entire length with the remaining tissues of the schizocarp. Stress is generated in the structures responsible for the flexion movement. As a result, the plant accumulates potential energy that can be released by even a gentle touch of a ripe, dry fruit or by movements caused by the wind. The amount of stored energy is so great that the movement can happen even spontaneously when the threshold stress value is exceeded. This phenomenon is so fast that we cannot observe any details of it with the naked eye. Fortunately, with access to a high-speed digital camera, I was able to record this (Photo 11). I used almost 4,000 frames per second (4000 fps), while under normal conditions, the recording rate is typically between 25 and 60 frames per second (25–60 fps). I conducted the experiment by placing the mature fruit of Erodium cicutarium in a holder made of metal tweezers attached to a stand and then gently touching it with a needle. If the fruit is dry enough, even slight contact will initiate the reaction.
As we can see, the result of a delicate mechanical stimulus was the literal ejection of the seed due to the stresses accumulated in the awn, which, in consequence, took an arcuate shape.
In my experiments, the initial ejection speed of the seed, measured over the first 10 cm (3.9 in), was approximately 3 m/s (about 9.8 ft/s ±3.3 ft/s, with the high standard deviation likely due to humidity differences). This seems like quite a good result for such a small plant. The available literature shows that the initial velocity can even exceed 4 m/s (13.1 ft/s), and the seeds are dispersed within a radius of up to 0.5 m (1.6 ft) from the mother plant. This was also confirmed by my observations [11]. It is worth emphasizing once again that a single plant can densely cover an area of 1 m (3.3 ft) in diameter around itself with its seeds. This fact alone can explain the considerable evolutionary success of Erodium species, but this is not the only fascinating adaptation. As I mentioned earlier, they use not only ballochory but also herpochory.
Not only does the awn enable ejection, but it can also twist multiple times as it dries out. To perform the observations, immobilize the moistened (e.g., by storing in a chamber with a wet paper towel placed at the bottom) mericarp so that its awn remains free to move. During drying, we can notice quite quickly, in just a few minutes, that the awn begins to twist (Photo 12). The number of twists can reach up to nine.
The details of this movement are even easier to observe from the top (Photo 13). In this case, for clarity, the mericarp was immobilized by inserting an injection needle attached to a syringe placed on a stand.
The awn always twists in such a way that its tip rotates counterclockwise, and it is particularly easy to trace in the montage made from photographs of subsequent movement stages (Photo 14).
The described mechanism of hygroscopic movements allows Erodium seeds to bury themselves in the ground. As humidity changes (e.g., day-night cycle), the awn of the mericarp alternately straightens and twists. Its free, non-bending tip can be fixed, for example, by catching it on the shoots of neighboring plants. In such cases, the opposite, sharpened end of the mericarp containing the seed begins to rotate, thus screwing itself into the soil. Thanks to successive cycles of humidity changes, the seed penetrates the soil to a depth of several millimeters, which sufficiently protects it and provides favorable conditions for survival and germination.
Explanation
Such structures are typically formed from double-layered systems, where each layer responds differently to moisture. This difference leads to uneven expansion or contraction, causing the entire structure to bend. Similar adaptations can be seen in other plants as well, such as in the scales of female spruce cones (Picea) or the elaters of horsetail spores (Equisetum arvense) [12].
Hygroscopic movements primarily involve specialized dead tissues, which means they do not require any energy input and instead rely on changes in ambient humidity. However, forming such complex structures still demands a significant energy investment. The ability to spread seeds over a wide area through ballochory, combined with the capacity to bury them in suitable soil via herpochory, gives Erodium a notable evolutionary advantage.
References:
- [1] Szweykowska A., Szweykowski J., Słownik botaniczny, Wiedza Powszechna, Warszawa, 2003, pp. 241
- [2] Kuta E., diaspora, in: Otałęga Z., Encyklopedia biologiczna, Agencja Publicystyczno-Wydawnicza Opres, Kraków, 1998
- [3] Ples M., A jednak się porusza! Ruchy higroskopowe roślin (eng. And Yet It Moves! Hygroscopic Movements of Plants), Biologia w Szkole (Biology in School), 3 (2016), Forum Media Polska Sp. z o.o., pp. 52-56
- [4] Ples M., O rosiczce słów kilka, czyli wyhoduj żywą muchołapkę! (eng. A Few Words About Sundews: Grow Your Own Living Flytrap!), Biologia w Szkole (Biology in School), 1 (2016), Forum Media Polska Sp. z o.o., pp. 51-56
- [5] Ples M., Roślinny bokser? Szybkie ruchy pręcików berberysu (eng. A Plant Boxer? The Rapid Stamen Movements of Barberry), Biologia w Szkole (Biology in School), 3 (2020), Forum Media Polska Sp. z o.o., pp. 81-85
- [6] Philips R., Rix M., The Botanical Garden (vol. 2), Macmillan, Londyn, 2011, pp. 115
- [7] Erodium, w serwsie: http://www.theplantlist.org/, online: http://www.theplantlist.org/1.1/browse/A/Geraniaceae/Erodium/ [10.08.2020]
- [8] Paradowski A., Atlas chwastów, Plantpress, Kraków, 2013
- [9] Tymrakiewicz W., Atlas chwastów, Państwowe Wydawnictwa Rolnicze i Leśne, Warszawa, 1976
- [10] Vittoz P., Engler R., Seed dispersal distances: a typology based on dispersal modes and plant traits, Botanica Helvetica, 117 (2), 2008, pp. 109-124
- [11] Evangelista D., Hotton S., Dumais J., The mechanics of explosive dispersal and self-burial in the seeds of the filaree, Erodium cicutarium (Geraniaceae), Journal of Experimental Biology, 214 (4), 2011, pp. 521-529
- [12] Ples M., Skrzyp - roślina z przeszłości (eng. Horsetail – A Plant from the Past), Biologia w Szkole (Biology in School), 4 (2016), Forum Media Polska Sp. z o.o., pp. 56-60
All photographs and illustrations were created by the author.
Addendum
The flight of Erodium cicutarium seeds, or more precisely their mericarps, looks spectacular in footage captured with a high-speed camera, as shown in the video below:
Marek Ples