The plant boxer - rapid movements of a barberry stamens
The following article was originally published in journal for teachers and lecturers Biologia w Szkole (5/2020):
Movements in nature
I have always wondered why the vast majority of people - not just laymen - consider plants to be more primitive than animals. It is the latter (with few and relatively recently discovered exceptions) that are not equipped with a wonderful mechanism that allows direct drawing of energy from easily available sunlight. The belief that plants are more primitive than animals certainly results from their significant difference from ourselves. As humans, we belong to Animalia, and it is easier for us to understand - or at least we usually think so - other members of this kingdom. A plant organism seems less advanced only because it does not defend itself against manipulation, while every animal driven by a self-preservation instinct will try to protect itself as much as possible. This of course is not true, because plants also have defense mechanisms appropriate to their needs . However, often they are more difficult for us to observe (and less obvious than simple escape of an animal) due to different evolutionary strategies developed by these groups of organisms.
One of significant differences between organisms belonging to kingdoms of Animalia and Plantae is the inability of plants to move in terms of locomotion, i.e. autonomous movement of the entire organism. Nevertheless, plant organisms are perfectly capable of whole range of other kinds of movements, fundamentally different in their mechanism to animal motility based primarily on use of muscles.
Movements in the world of plants - if we take into account living structures, because there also are motion mechanisms based on use of dead tissues, e.g. hygroscopic movements of bracts (modified leaves forming involucrum) in representatives of the genus Xerochrysum, as well as elements of spruce Picea sp. cones  - are mainly caused by cell growth (growth movements) or changes in pressure inside them (turgor movements).
The most frequently cited division of plant movements includes:
- nastic movements.
Tropisms are reactions of an organism - usually of a plant - in form of growth or turning movement in response to an environmental stimulus. In tropisms, this responses depend on direction of the stimulus. They are caused by uneven distribution of specific phytohormones (auxins) within the tissues. Tropism can be positive (+) when the organ is directed to the site with the stronger stimulus, or negative (-) in the case of opposite reaction. Depending on the type of stimulus causing specific tropism, we can say about phototropism, geotropism, hydrotropism, thermotropism and others.
Taxes (singular taxis) are movements of an organism in response to many different kinds of stimuli e.g. light or presence of food. In this case the whole organism is motile and demonstrates guided movement towards (+) or away (-) from the stimulus source. These movements are used to seek for the best environmental conditions, e.g. temperature (thermotaxis), concentration of specific chemical compounds (chemotaxis), light intensity (phototaxis) and so on.
Nastic movements are non-directional responses to stimulation. The movement can result from changes in turgor (more often) or take form of specific tissue growth. Decrease in turgor pressure causes shrinkage while increasing results in swelling of cells and thus whole tissues. Direction of nastic movements is independent from stimulus's position in space. Taking into account the type of stimulus it is possible to distinguish, among others, chemonasty, thermonasty, seismonasty and photonasty .
Movement of plants is usually imperceptible to naked eye and its effects can only be noticed after a long time span. However, if we want to observe the phenomenon of plant movement, we can use the technique of time-lapse photography but it is quite a laborious process.
There are plants movement of which can be observed without use of specialized devices. Examples include some carnivorous plants, such as sundews Drosera or the Venus Flytrap Dionaea muscipula, trapping structure of which (formed by the terminal element of each of the leaves) usually closes in seconds.
The representative of exceptionally active non-carnivorous plants is the Mimosa pudica also known as touch-me-not plant . It exhibits very distinct seismonastic movements: leaflets of its compound leaves (Photo 1A) fold inward and droop within seconds after even a gentle touch (Photo 1B).
It may seem that plants showing fast movements are representatives only of exotic species. However, as It turns out, among species that occur naturally in Poland, you can also find those whose movements can be easily observed with the naked eye.
I want to tell you, my Dear Reader, about a native to Poland plant which is the barberry Berberis vulgaris, also known here as kwaśnica (from polish "kwaśny" - "sour"). Said plant is a deciduous shrub belonging to the Berberidaceae family.
This species of barberry was once common throughout Poland - similarly to the entire continental part of Europe, and also (as naturalised) on the Scandinavian Peninsula and the British Isles. It grew on balks, slopes, thickets and on the edges of forests. However, farmers have noticed that presence of barberry can be harmful to local crops, because this shrub is an intermediate host of Puccinia graminis - fungus causing a dangerous cereal disease called stem rust . It was the reason why this plant was almost completely eradicated in nature. Therefore, if we find barberry somewhere, we should observe this plant, but not damage or harm it in any way.
Related species are currently often used as ornamental plants, for example Thunberg's barberry Berberis thunbergii from Japan (Photo 2).
Barberry has dimorphic shoots: long shoots which form the structure of the plant, and short shoots up to 2mm long. The leaves on long shoots aren't photosynthetically active, developed into one to three or more spines. The bud in the axil of each thorn-leaf then develops a short shoot with several normal, photosynthetic leaves. Leaf blade is undivided (lat. folia integrum) and serrated. My Thunberg's barberry is of the 'Atropurpurea' cultivar, one of the most common. Its leaves are purple-red rather than green. This plant has a characteristic, domed shape.
Barberry tissues contain large amounts of an interesting chemical compound called berberine C20H18NO4+. This substance shows strong biological activity: it has antibacterial, antiprotozoal, antidiarrheal, anti-canceral, antidiabetic, antihypertensive, antidepressant, and anti-inflammatory properties, while alsoreducing cholesterol levels in our organism. However, it should be mentioned that it has numerous side effects, e.g. related to release of bilirubin C33H36N4O6 from its insoluble complex with albumin in the blood . Berberine also has interesting optical properties: its yellow-green fluorescence under UV light is very strong .
From the point of view of topic of this article, the most interesting are the generative organs of plants of the Berberis genus, because it is them - or rather their specific elements - that show an interesting adaptation related to movement.
The flowers of barberry are small, yellow or yellow-orange in color (Photo 3). They are arranged in umbels, and they do not smell strongly, though pleasantly.
Stamens and pistils mature simultaneously. Flowers, due to their small dimensions, are not the most decorative elements of barberry, although it must be admitted that during the flowering period (in spring) the plant displays peculiar beauty - the contrast between colors of leaves and flowers can be a source of really nice aesthetic impressions.
Barberry is pollinated by insects.
The structure of the flower is shown in the floral diagram (Figure 1)  .
As you can see, both calyx and corolla contain two whorls of three elements each: sepals sepala and petals petala. The androecium is similar, consisting of six stamens stamina. The gynoecium consists of one pistil pistyllum which is made of a single carpel carpellae. At the base of the petals there are nectaries nectaria - structures that secrete nectar thus attracting insects which pollinate the plant. The described elements can be seen in Photo 4.
Photo 4A shows natural appearance of the flowers - we can easily see the beautifully colored sepals, petals and the stigma of the pistil. The petals, however, hide the stamens from our sight. Of course observations can be done even in such case, but it is easier to do it after preparing the flower by gently cutting or tearing off the petals at their base with tweezers (near nectaries), while not damaging the remaining elements of the flower (Photo 4B). This way the stamens are exposed. I admit that described operation requires a steady hand and a good eye (or a magnifying glass), but after some training it is possible to perform said operation not only at the laboratory table on a flower cut from the plant, but also outside directly on the living plant. Of course, when the flower gets cut-off from the plant, observations must be done relatively quickly so the tissues will not dehydrate and die.
The result of exemplary observations of barberry stamen movements is presented in Photo 5. Initially the stamens are evenly bent outwards and the imaginary line connecting the anthers thecae forms a fairly regular circle (Photo 5A). However, it is enough to touch stamen - even extremely delicately - with a preparation needle or other tool (Photo 5B) to be able to see an interesting phenomenon. Right after contact, the whole stamen makes a very quick movement (lasting only a fraction of a second) towards the pistill (Photo 5C). The photo shows effects for only two stamens before and after being touched to make the comparison easier.
It is worth mentioning that the described seismonastic movement is not a single-time reaction. After some time, the stamens - if the flower is not dried out - return to their original positions. Just after making the movement, as we have already seen, the stamens stick to the pistil (Photo 6A). After several minutes, stamens are straightened and then they become able to perform new cycle of movement (Photo 6B). Some bending of the sepals can also be noticed here, but this is most likely caused by their drying out resulting from lights necessary to take the photo.
The described observations are very interesting and, despite their small scale, truly spectacular. Decorative varieties of barberry are often found in our gardens, but also in city parks as small hedges, etc. This ensures high availability of material for research, which is invaluable from the point of view of any naturalist.
Movements of the stamens of barberry are undoubtedly caused by a change in turgor in groups of specific cells. Mechanical contact initiates the transport of certain ions across the cell membrane. In the case of changes in turgor of cells responsible for seismonastic movements, the most important factor seems to be the potassium cation K+. As a result of stimulation, they are intensively transported outside the cells, which also causes outflow of water from the protoplasts. The turgor of these cells is lowered, causing their dimensions to change. These changes are relatively small, but due to the large number of cells, they add up in the form of observed motion. The plant takes some time to regenerate by transporting water back into the cells.
Why has the plant developed such an interesting mechanism of described movement? I think that the answer to this question will not be too difficult for the Reader to find. This is, of course, an adaptation to pollination by insects. If we look at Photo 4A, we will easily notice that the insect, in order to get to the nectar produced at the base of the petals, must pass through a small window formed by converging petals, and then pass in-between the pistil and the stamens. After being touched they perform movement looking as if they are hitting the insect, which became inspiration for this article. In this way, much more pollen is deposited on the surface of the pollinator's body than if stamens remained stationary. On the other hand, pressing the insect against the pistil increases the chance of transferring the pollen to the stigma of the plant. As you can see, seismonastic movements can significantly increase the plant's chances of pollinating (and thus reproducing), which is a important thing from the point of view of every living organism.
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The author of the text and photos is Marek Ples.