Weird Science

Erodium cicutarium - a plant catapult and moving seeds

The fol­lo­wing article was ori­gi­nally publi­shed in jour­nal for tea­chers and lec­tu­rers Bio­lo­gia w Szkole (5/2020):

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Ples M., Iglica pospo­lita - roślinna kata­pulta i ruch­liwe nasiona (eng. Ero­dium cicu­ta­rium - a plant cata­pult and moving seeds), Bio­lo­gia w Szkole, 5 (2020), Forum Media Pol­ska Sp. z o.o., str. 54-57

In this article I would like to con­ti­nue the topic of move­ment in the plant king­dom. In our ear­lier rese­arch we alre­ady lear­ned that plants are quite capa­ble of move­ment. We also know that cau­ses and goals of these move­ments, as well as their mecha­ni­sms, are com­ple­tely dif­fe­rent than for ani­mals. This is due to dif­fe­rent phy­sio­logy of the Ani­ma­lia and Plan­tae king­doms resul­ting from dif­fe­rent evo­lu­tio­nary path­ways.

The dif­fe­rence between plants and ani­mals that noti­ce­a­ble at first glance is the plants ina­bi­lity of loco­mo­tion (auto­no­mous move­ment of the whole orga­nism). If plants can­not move from place to place like ani­mals, how do they conquer new areas?

For plants the most impor­tant mecha­nism of colo­ni­zing new habi­tats is not the move­ment of indi­vi­du­als (loco­mo­tion), but the spread of dia­spo­ras. This term (greek: dia - thro­ugh, spo­ros - seed) refers to any plant part that is used for repro­duc­tion and spre­a­ding. In a simi­lar sense dia­spo­ras are also pro­du­ced by fungi [1]. The­re­fore, a dia­spora can be both the whole orga­nism and its part capa­ble of pro­du­cing a new indi­vi­dual. Dia­spo­ras are divi­ded into gene­ra­tive (seeds) and vege­ta­tive (frag­ments of thal­lus, tubers, rhi­zo­mes, turions, see­dlings, spo­res) [2].

Plants are capa­ble of many dif­fe­rent types of move­ments, for exam­ple for cat­ching small ani­mals as food (venus fly­trap Dio­nea musci­pula, cape sun­dew Dro­sera capen­sis), for pol­li­na­tion (euro­pean bar­berry Ber­be­ris vul­ga­ris) or pro­tec­ting flo­wers aga­inst envi­ron­men­tal con­di­tions (straw­flo­wer Xero­ch­ry­sum brac­te­a­tum). These move­ments are - with rare excep­tions - quite slow [3][4][5]. In order to obse­rve the phe­no­me­non of plant move­ment, we usu­ally have to use the tech­ni­que of time-lapse pho­to­gra­phy.

As it turns out, there are plants organs of which have the abi­lity to move at spe­eds so high that use of spe­cial fil­ming tech­ni­ques is neces­sary to slow down the recor­ded video for obse­rva­tion. The seeds of the plant I want to tell about have the abi­lity to inde­pen­den­tly… bury them­se­lves in the gro­und. Inte­re­stin­gly, the men­tio­ned orga­nism does not belong to any exo­tic spe­cies and can be found in our coun­try.

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Ero­dium

Ero­dium cicu­ta­rium belongs to the Gera­nia­ceae family rarely men­tio­ned in my works so far. The genus Ero­dium inc­lu­des - depen­ding on the source - from 60 to even 130 con­fir­med spe­cies [6][7]. Ero­dium sp. usu­ally live in not very hospi­ta­ble pla­ces, i.e. sandy and rocky ter­rain. Sou­thern Europe offers the gre­a­test spe­cies diver­sity of those 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), but some rela­ted spe­cies are some­ti­mes cul­ti­va­ted as orna­men­tal plants. Besi­des that, it lives eve­ry­where except Antarc­tica.

For some rea­sons, which will become clear in a moment, 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 and light, sandy or sandy-loam soils, and at the same time rich in nutrients - it is defi­ni­tely a nitro­gen-loving plant. It can be found in areas used for agri­cul­ture, which is why it is con­si­de­red a weed in cul­ti­va­tion of root crops and cere­als, inc­lu­ding maize. It can also be seen in clo­ver Tri­fo­lium fields. The pre­sence of Ero­dium in crops may lead to exces­sive dry­ing of the soil and its deple­tion of nutrients. Addi­tio­nally, its rapid expan­sion often cau­ses slow­down in 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 (in Poland), Ero­dium is a rela­ti­vely small annual plant. I admit that despite its wide distri­bu­tion in nature, I had to spend some time fin­ding spe­ci­mens for obse­rva­tion. Inte­re­stin­gly, I disco­ve­red them in my gar­den, where plants were appa­ren­tly sown by acci­dent (Photo.1).

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Photo 1 – Blo­o­ming Ero­dium cicu­ta­rium

Ero­dium cicu­ta­rium is not a pro­tec­ted by law, so for more pre­cise obse­rva­tion we can take it from the envi­ron­ment in parts or as whole (Photo 2). Of course, a spe­ci­men of this inte­re­sting plant can also be a deco­ra­tion of our her­ba­rium.

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Photo 2 – Ero­dium cicu­ta­rium spe­ci­men

Stem of this plant is usu­ally erect and cove­red with long, soft hairs, and in the upper part also with glands. Lea­ves gro­wing close to the soil form a rosette, while the stem lea­ves grow alter­na­tely and are get­ting smal­ler upwards (Photo 3).

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Photo 3 – Leaf

Small flo­wers on long stalks are gathe­red three to ten in umbel-like inflo­re­scen­ces. They are deli­ca­tely zygo­mor­phic, pur­ple, some­ti­mes white or with ligh­ter spots, with five egg-sha­ped petals, cle­arly lon­ger than the sepals. There are five sta­mens and the same num­ber of sta­mi­no­des (Photo 4).

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Phot.4 – Flo­wer, front view

The sepals are glan­du­larly hairy, lan­ce­o­late or oblong, with mem­bra­nous edges (Photo 5).

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Phot.5 – Flo­wer, side view

Ero­dium cicu­ta­rium, like other Gera­nia­ceae, is pol­li­na­ted by insects - its flo­wers have small nec­ta­ries. After pol­li­na­tion, a fruit is for­med and it shows inte­re­sting struc­tu­ral fea­tu­res and sur­pri­sing beha­vior.

Natu­ral machine

The fru­its of Ero­dium cicu­ta­rium (also of other spe­cies of this genus) are very cha­rac­te­ri­stic - their shape can indeed bring to mind a nee­dle or a stork's beak, which gave rise to both bota­ni­cal and com­mon names of this plant (Photo 6).

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Photo 6 – Fruit

Said fruit sho­uld be clas­si­fied as schi­zo­carp with elon­ga­ted tail-sha­ped struc­ture (Photo 7). It is easy to see the rema­ins of sepals, as well as mature meri­carps.

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Photo 7 – Mature schi­zo­carp

Mature and dry schi­zo­carp easily bre­aks down into 5 meri­carps sepa­ra­ting from the cen­tral elon­ga­ted tail-sha­ped ele­ment (Photo 8). Each of them has a long, thin awn.

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Photo 8 – Fruit struc­ture; top – meri­carps, bot­tom - other ele­ments

Inside each meri­carp, one brown smo­oth sur­fa­ced seed can be found (Photo 9). The plant pro­du­ces seve­ral hun­dreds of them, usu­ally 200-600.

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Photo 9 – Seed

The way plants of the genus Ero­dium have adap­ted to the living con­di­tions and deve­lo­ped sui­ta­ble disper­sal methods is stri­king.

We know that plants show variety of seed disper­sal stra­te­gies. First, we can men­tion an allo­chory cha­rac­te­ri­zed by use of various exter­nal fac­tors for spre­a­ding their seeds. Within this cate­gory, we can distin­gu­ish:

On the other hand, there is a set of mecha­ni­sms which do not invo­lve exter­nal fac­tors in spre­a­ding dia­spo­ras. It is cal­led auto­chory (self-see­ding), and it can be divi­ded into:

Inte­re­stin­gly, plants of the genus Ero­dium - and thus also Ero­dium cicu­ta­rium - use two types of auto­chory simul­ta­ne­o­u­sly, namely bal­lo­chory and her­po­chory… Yes! This plant can cata­pult its seeds (or rather meri­carps) into the air by itself, and they them­se­lves have the abi­lity to move auto­no­mo­u­sly. So let's take a clo­ser look at them (Photo 10).

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Photo 10 – Meri­carps; left – high humi­dity, right – low humi­dity

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 con­di­tion) 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 schi­zo­scarp bre­aks down, meri­carps form a struc­tu­ral unity with the other tis­sues. In the mature fruit, con­nec­tion of meri­carps with the rest of the fruit is quite deli­cate and acts as a trig­ger. Was the meri­carp not immo­bi­li­zed, its awn would have sim­ply bent as it dried. Howe­ver in the descri­bed case it is impos­si­ble because of the con­nec­tion of the awns along their entire length with the rema­i­ning tis­sues of the schi­zo­carp - stress is gene­ra­ted in struc­tu­res respon­si­ble for the fle­xion move­ment. Because of that, the plant accu­mu­la­tes poten­tial energy that can be rele­a­sed by even a gen­tle touch of a ripe and dry fruit or by move­ments cau­sed by the wind. Amo­unt of sto­red energy is so great that said move­ment can hap­pen even spon­ta­ne­o­u­sly, when the thre­shold stress value is exce­e­ded. This phe­no­me­non is so fast that we can­not see any deta­ils of it with the naked eye. For­tu­na­tely, having access to a high-speed digi­tal camera, I was able to record this (Photo 11). I would like to point out that I used almost four tho­u­sand fra­mes per second (4000fps), while under nor­mal con­di­tions recor­ding rate is usu­ally twenty-five to sixty fra­mes per second (25-60fps). 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 with a nee­dle. If the fruit is dry eno­ugh, even a sli­ght con­tact will ini­tiate a reac­tion.

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Photo 11 – Bal­lo­chory of Ero­dium cicu­ta­rium; A – 0ms (touch with nee­dle), B – 5ms, C – 10ms, D – 15ms, E – 20ms; aste­risk - nee­dle

As we can find out, the result of a deli­cate mecha­ni­cal sti­mu­lus was lite­ral ejec­tion of the seed due to the stres­ses accu­mu­la­ted in the awn, which in con­se­qu­ence took an arcu­ate shape.

In case of my expe­ri­ments, the ini­tial ejec­tion speed of seed mea­su­red at a distance of first 10cm was about 3m/s (±1m/s, the high value of the stan­dard devia­tion is most likely due to dif­fe­ren­ces in humi­dity), which seems to be quite a good result, as for such a small plant. The ava­i­la­ble lite­ra­ture shows that the ini­tial velo­city may even exceed 4m/s, and the seeds are spread within a radius of 0.5m from the mother plant - which was also con­fir­med by my obse­rva­tions [11]. It is worth empha­si­zing once again that a sin­gle plant is able to den­sely cover an area of 1m in dia­me­ter aro­und itself with its seeds. This fact alone can justify a con­si­de­ra­ble evo­lu­tio­nary suc­cess of Ero­dium sp., but this is not the only inte­re­sting adap­ta­tion. As I men­tio­ned ear­lier, they use not only bal­lo­chory, but also her­po­chory.

Awn not only allows ejec­tion of the seed, but is also capa­ble of mul­ti­ple twi­sts as it dries out. To per­form the obse­rva­tions, immo­bi­lize the moi­ste­ned (e.g. by sto­ring in a cham­ber with a wate­red paper towel pla­ced at the bot­tom) meri­carp so that its awn rema­ins free to move. During dry­ing, we can notice quite quic­kly - because in just few minu­tes - twi­sting of the awn(Photo 12). The num­ber of turns can reach up to nine.

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Photo 12 – Awn move­ments while dry­ing; A - 0s, B - 30s, C - 60s, D - 90s, E - 120s; side view
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The deta­ils of this move­ment are even easier to obse­rve from the top view (Photo 13). In this case, for the sake of cla­rity, the meri­carp was immo­bi­li­zed by inser­ting an injec­tion nee­dle atta­ched to a syringe pla­ced on a stand.

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Photo 13 – Awn move­ments while dry­ing; A - 0s, B - 30s, C - 60s, D - 90s, E - 120s; top view
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Twi­sting of the awn is always done in a way that makes the tip turn coun­terc­loc­kwise it is par­ti­cu­larly easily tra­ce­a­ble in the mon­tage made from pho­to­gra­phs of sub­se­qu­ent move­ment sta­ges (Photo 14).

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Photo 14 – Mon­tage sho­wing the posi­tions of awn in sub­se­qu­ent sta­ges of move­ment

The descri­bed mecha­nism of hygro­sco­pic move­ments allows Ero­dium seeds to bury them­se­lves in the gro­und. As the humi­dity chan­ges (e.g. day-night cycle) the awn of meri­carp stra­i­gh­tens and twi­sts alter­na­tely. Its free, non-ben­ding tip can be fixed, for exam­ple, by cat­ching it with the sho­ots of nei­gh­bo­u­ring plants. In such case, the oppo­site, shar­pe­ned end of meri­carp con­ta­i­ning the seed is set into rota­tion, and thus scre­wed into the soil. Thanks to suc­ces­sive cyc­les of chan­ges in humi­dity, the seed pene­tra­tes into the soil to a depth of seve­ral mil­li­me­ters, which pro­tects it suf­fi­cien­tly, and pro­vi­des favo­u­ra­ble con­di­tions for survi­val and ger­mi­na­tion.

Expla­na­tion

The shape chan­ges of the awn result from hygro­sco­pic move­ments. Struc­tu­res sho­wing such pro­per­ties are usu­ally built as dou­ble-lay­e­red sys­tems. These lay­ers react to pre­sence of water with an une­qual change in volume, which cau­ses the entire struc­ture to bend. Such tis­sue Descri­bed struc­ture sho­wing hygro­sco­pic move­ments is cha­rac­te­ri­stic and occurs also in other plants. An exam­ple may be sca­les of female spruce cones Picea, as well as ela­ters of hor­se­tail spo­res Equ­i­se­tum arvense [12]

Mainly spe­cia­li­zed dead tis­sues take part in hygro­sco­pic move­ments - the­re­fore they do not requ­ire energy and are sup­plied from the exter­nal envi­ron­ment thro­ugh chan­ges in humi­dity. Howe­ver, it must be kept in mind that cre­a­tion of such com­plex struc­tu­res is asso­cia­ted with large energy expen­di­ture. The pos­si­bi­lity of spre­a­ding over a rela­ti­vely large area allo­wed by bal­lo­chory or pro­vi­ding the seeds with safe and favo­ra­ble con­di­tions thro­ugh her­po­chory appa­ren­tly gives the plant such great advan­ta­ges that this evo­lu­tio­nary solu­tion made Ero­dium suc­cess­ful.

Lite­ra­ture:

The author of the text and pho­tos is Marek Ples.

In rela­tion to article publi­shed in the jour­nal, minor edi­to­rial chan­ges were intro­du­ced in order to sup­ple­ment and adapt this text to pre­sent it on web­site.

Marek Ples

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