|Figure1. A photograph showing part ofmy own carnivorous plant collection taken in September 2017. From left to right:Sarracenia sp. (likely a hybrid of S. leucophylla), S. flava, S. purpureapurpurea, S. minor, Sarracenia x ‘Maroon’, S. psittacina, Sarracenia x ‘Maroon’, Dionaea muscipula. Photograph by JulianKiely.|
Welcome toPalaeoflora, and to my first blog post! I already have many ideas of articles I’dlike to write, however, to begin with I wanted to cover a topic which I have aparticular interest in, and is very dear to me; Carnivorous Plants! Many peopleare aware of the existence of carnivorous plants, but this knowledge oftenextends only to the famous Venus Fly-Trap (Dionaeamuscipula), with the Pitcher Plants of Asia and America (Nepenthes and Sarracenia respectively) coming a close second. But the diversityof modern carnivorous plants in both morphology and taxonomy is trulyastounding, and even within the course of researching for this article, Idiscovered many more lineages of carnivorous, or possibly carnivorous plantsthan I even suspected. In this first article I will provide an introduction tocarnivorous plants, their habitats and their various trapping mechanisms. Inthe second article I would like to take you on a guided tour of modern carnivorousplant diversity, and describe to you the biology of the various carnivorousplants which exist today. And in my third article I will be focusing on theevolution, phylogeny and fossil record of carnivorous plants, and will alsotalk about speculations which can be made about their inclusion withinprehistoric environments and palaeoart. So, without further ado, I’ll begin ourjourney, through the world of Carnivorous Plants!
So, what arethese so called ‘Carnivorous Plants’?
There are twomain terms used to describe carnivorous plants; the first is of course‘carnivorous’ and the second is ‘insectivorous’. Throughout this blog I havechosen to refer to them by the former name, as while it is true that mostcarnivorous plants prey primarily upon insects, many will also trap otheranimals (including vertebrates on occasion). Next, when we say ‘carnivorousplant’ what do we actually mean? Well, The Kew Plant Glossary (Second Edition,Beentje 2016) provides the following definition:
‘carnivorous, plants that trap animalsand derive some or most of their minerals from digesting them’
While thisprovides us with a basic definition for vegetable carnivory, I would like tosuggest a few more criteria which most carnivorous plants adhere to:
- ·Theyactively implement methods of attracting their prey to their traps.
- ·Theymust permanently trap their prey, causing their eventual death.
- ·Theyutilise methods of breaking down their prey to extract minerals from them.
- ·Andfinally, they must be able to absorb those minerals either directly orindirectly from their prey.
However,even these criteria are not present in all ‘carnivores’ (in these cases I maybe inclined to use the term ‘pseudocarnivory’), and even within true carnivoresthere are some which are active and some which are passive. Active and passivecan also have several meanings; in the case of water filled pitfall traps,‘active carnivory’ indicates that the plant produces its own digestive enzymes,and ‘passive carnivory’ indicates the presence of symbiotic bacteria, whichaids in digestion. However, ‘active carnivory’ can also be used to mean a trapwhich responds to stimuli from its prey with movement, and a ‘passivecarnivore’ is one which does not. So, asyou can see, the division between what is and what isn’t carnivorous is not as finelycut as most people think, but instead more of a blur. All levels and stages ofcarnivory are present in modern floras, from plants which display perhaps onlyone carnivorous feature, to those who are fully carnivorous.
Figure2. A Simplified evolutionary treeof carnivorous plants, showing all modern genera which are considered to be‘true carnivores’ (however, both Roridulaand Byblis are not technically truecarnivores, I have included them here as they are often included in discussionsabout carnivorous plants). The thumbnails show the basic trap design of eachgenus, and the darker green indicates the evolution of carnivory within aclade, and the light green indicates their non-carnivorous ancestors. As youcan see, carnivory has evolved independently at least eight times within plants(although some authors may include up to ten, Ellison and Adamec, 2018, whileothers identify only six, Ellison and Gotelli, 2008). This treeis based primarily on Ellison and Gotelli, 2008, with updatesfrom Ellison and Adamec, 2018. Artwork by Julian Kiely.
FantasticCarnivores and Where to Find Them
Within moderncarnivorous plants, of which there are over 600 extant species (Ellison andGotelli, 2008; Christenhusz, Fayand Chase 2017), there are many different forms of trapping mechanisms, whichoften have evolved independently in multiple lineages; it is thought that full,true carnivory has evolved independently in at least six modern lineages(Ellison and Gotelli, 2008),all of which are angiosperms (so, unfortunately no known carnivorous ferns – yet…)(see Figure 2.). The convergent evolution of carnivory multiple times withinplants indicates that there must be some big advantages to having it, but underwhat sort of conditions does it occur? Well, in modern carnivorous plants,carnivory is a way for the plants to acquire minerals vital for their growth,and this is often most useful when they grow in environments with very lowmineral content. These habitats are often very wet and sometimes permanentlywaterlogged, as in these habitats the presence of water leaches minerals out ofthe substrate (soil), creating low concentrations of the minerals which plantsrequire to grow. For this reason, many carnivorous plants can be found growingin peat bogs and semi-aquatic environments, around the world, and even here in theBritish Isles (where I live) there are ten species of carnivorous plants acrossthree genera (Rose and O’Reillyn 2006) (see Figure 3.).
Thismay also be a good time to dispel the myth that carnivorous plants are alltropical and require warm temperatures; while it is true that some carnivorousplants do live in the tropics, many are also found in temperate regions acrossthe globe. Sarracenia, the NorthAmerican Trumpet Pitchers are found as far north as Canada and Alaska, and cansurvive frost and being frozen for short periods of time (USD Plant Database;D’Amato 2013; Christenhusz, Fay and Chase 2017). There is even a species of Pinguicula (Pinguicula alpina, the Alpine Butterwort) which is found in Siberiaand Iceland, and in mountain ranges across Eurasia, were it thrives in the coldconditions (Slack 1979).
Figure3. A photograph showing two ofour native carnivorous plant species, Pinguiculavulgaris and Drosera rotundifolia.I found these growing primarily in sphagnum moss or on bare rocks, onwell-watered valley slopes in Snowdonia. In cultivation, it is generallyadvised, particularly for Pinguicula,not get too much water on the leaves, as this impairs their trapping ability;however, seeing them in the wild was interesting, as often they would begrowing in the algae and slime covered rocks in pools of waters and slowflowing streams, often partially submerged. This indicates that in at leastsome species, wild individuals are able to grow quite happily in conditionswhich would generally be considered sub-optimum in cultivation. Photographtaken by Julian Kiely, July 2017.
Rainy,wet and humid environments are often common places for carnivorous plants toevolve, because these conditions make the production of aqueous substances, suchas the nectar-like ‘glue’ of many flypaper traps, a worthwhile investment; inthese conditions, the virtually unlimited supply of water allows the productionof large quantities of aqueous substances, and the humidity prevents thesesubstances from evaporating quickly (Ellison and Adamec, 2018). An exception tothis association of carnivorous plants with water is Drosophyllum (the Dewy Pine), which instead grows in the dry, nutrientpoor, fire-prone sandstone soils of the Western Mediterranean (Ellison andAdamec, 2018). Epiphytic carnivorous plants are also known, however this isuncommon; among angiosperms as a whole, ~ 9% are epiphytes, whereas amongcarnivorous species, only 2% are, with many of these not representing trueepiphytism, as they often inhabit wet microsites such as the water filled tanksof bromeliads.
Tricks and Trapsof Carnivorous Plants
Modern carnivorous Plantsare incredibly diverse, and show many different methods of catching their prey,but despite their often independent origins the same forms of traps haveevolved time and time again. In the past, this has led some scientists toclassify certain species together, which have now been shown to be unrelated. Aprime example of this is Roridulaceae, a monogeneric family containing thegenus Roridula, which in the past hasbeen associated with both Droseraceae and Byblidaceae (all three families aresimilar for containing genera which produce sticky flypaper traps), but is infact a sister clade to Sarraceniaceae (the American Pitcher Plants)(Christenhusz, Fay and Chase 2017).
Figure4. A basic diagram of the 17modern genera of carnivorous plants, grouped by the type of traps they possess.Overlaps indicate where the genus contains species which display multiple formsof traps. Genera depicted, clockwise from top left: Utricularia, Aldrovanda, Dionaea, Drosera, Drosophyllum, Pinguicula, Byblis, Roridula, Triphyophyllum, Genlisea, Darlingtonia, Sarracenia, Nepenthes, Cephalotus, Heliamphora, Catopsis, Brocchinia. Artwork by Julian Kiely.
Ihave found that the various trapping mechanisms seen in modern carnivorousplants can be broadly classified into four different types; Flypaper Traps,Pitfall Traps, Motile Traps and Lobster-pot Traps (see Figure 4.).
Themost common form of trapping mechanism (the form which is present in the mostmodern genera) is the Pitfall Trap; these traps have evolved independently on 5occasions (twice within Bromeliaceae, and once in the Nepenthales, Rosids andEricales), and are generally composed of a single leaf which has developed intoa funnel or pitcher-like shape which holds water (hence the common name pitcherplant). An exception to this are the carnivorous bromeliads, whose traps arecomposed of a rosette of tightly packed leaves, which holds water in themiddle. As the bromeliad carnivores are very different in morphology from otherpitfall traps, for the moment we will ignore them in this general description.In the eudicot pitfall traps (Figure 5.), most species possess a lid;these are flat extensions of the leaf,which extend over the opening of the pitfall trap, and generally prevent thetraps from flooding with too much rainwater, however many individual specieshave adapted the lid to perform a variety of other functions. The peristome(rim of the opening) varies greatly between different species, being not muchmore than a rolled edge in Sarraceniaand often highly developed in Cephalotusand Nepenthes; it is often smoothwith a slippery surface. The lid, neck and occasionally peristome of pitfalltraps produce nectar, which is used to attract insect prey with the sugaryliquid. Ultraviolet patterns which resemble those flowers use to directpollinators are also present in some species (Ellison and Adamec, 2018). Whenthe prey lands, they will often slip on the smooth, waxy surface of theperistome, and fall into the pitcher chamber of the trap, which is often waterfilled (this can be rain collected water as in Heliamphora, or self-secreted, as in Darlingtonia). The slippery inner walls and (often present)downward pointing hairs prevent the prey from climbing up to escape, andinstead the prey drowns in the water. This water may contain digestive enzymesproduced by the plant, or may rely upon symbiotic bacteria to digest their prey.Minerals are then absorbed by the plant via glands at the base of the pitcherchamber. Carnivorous bromeliads similarly utilise ultraviolet patterns and aslippery surface, but instead have adventitious roots growing into the watertank, which absorb minerals (Christenhusz, Fay and Chase 2017).
Figure5. Pitcher Plant cross-section, morphologicalcomparison. From left to right, Nepenthes,Sarracenia, Cephalotus (not to scale). Ax, Leaf Apex; DZ, Digestive (glandular)Zone; Li, Lid; PC, Pitcher Chamber; Ps, Peristome; Te, Tendril; Wa, Water; Wi,Wing. By Julian Kiely.
Thesecond most common form of trap is the Flypaper Trap. These traps are oftencomposed of leaves with glandular hairs which secrete a sticky substance attheir tip. In addition to these secretory glands, many (but not all) flypapertraps also possess glands which produce digestive enzymes and absorb mineralsfrom the digested prey. Flypaper traps are often designed to catch small flyinginsects, which are attracted to the leaves by the sparkling dew (sticky substance),which in some instances produce ultraviolet patterns similar to the markings ofsome flowers (Ellison and Adamec, 2018). Once the insect lands, the sticky‘glue’ prevents them from escaping and, in most cases, suffocates the insectsas the ‘glue’ clogs up their spiracles and trachea (tiny holes and tubes, alongthe sides of their body, which they use to breathe). At this point, someflypaper traps (such as those in Drosera)also act as motile traps, as their hairs and leaves fold inwards to wrap aroundthe insect (Ellison and Adamec, 2018). The prey is then digested and theminerals adsorbed. Despite its seeming complexity, this is the simplest form oftrap (with the possible exception of the pitfall traps of certain Poales).
|Figure6. Drosera leaf life drawing and cross section. Ab, Abaxial leafsurface; DG, Digestive Glands; GH, Glandular Hair; Gu, ‘Glue’ (mucilage); TS,Trapping Surface. By Julian Kiely.|
Motiletraps are perhaps the most enigmatic of the carnivorous plants, and include thefamous Venus Fly-Trap (Dionaea muscipula).The morphology of motile traps varies greatly between the handful of generawhich have developed them, but the significant element in all of them it theuse of movement to trap their prey. In some, such as Dionaea and Aldrovanda,they use two lobes of a leaf to snap shut around their prey when stimulated. Assuch these traps are sometimes referred to as ‘snap-traps’ (Ellison and Adamec,2018). Another form of highly advanced motile trap is the bladder, or suctiontraps of Utricularia species, whichgrow on subterranean or aquatic stems where they are surrounded by water.Bladder traps are highly derived leaves which take the form of a hollow bladderwith a trap door at one end. They work by pumping water out of the trap, creatinga partial vacuum within them; when small prey stimulate hairs around thetrapdoor, the trapdoor opens and the prey are sucked in. Once inside the trap,enzymes are released to break down the prey (Ellison and Adamec, 2018,Christenhusz, Fay and Chase 2017). As aforementioned, some Drosera and Pinguiculaspecies also move in response to prey; in Pinguiculathis movement is slow and minimal, but in Droserait is often more rapid and noticeable (in D.glanduligera, the edges of their leaves possess snap-tentacles, which hingeinwards when stimulated, to further entrap prey; Ellison and Adamec, 2018). Thelid Nepenthes gracilis also acts as apassive motile trap, utilising raindrops which hit the lid to throw prey intothe pitcher (Ellison and Adamec, 2018).
Figure7. Dionaea muscipula life drawing and morphology diagram. DG,Digestive Glands; Lo, Lobes; MR, Midrib (hinge region); NG, Nectar Glands; Te,Teeth (cilia); TH, Trigger Hairs. By Julian Kiely.
Thefinal form of trap is the lobster-pot trap (sometimes called a pigeon trap inGenlisea and Sarraceniapsittacina). The basic mechanism of these traps is to have a one wayentrance, so that the prey, once inside are unable to back out. In Darlingtonia (which is generallyconsidered a pitfall trap) and S.psittacina, these traps are modified pitfall traps, where the lid andperistrome have fused and curved inwards (Figure 8.), to produce a smallopening which funnels prey inwards, but prevents them from escaping back out ofthat opening. The pigeon trap, while similar in its trapping mechanism, isdifferent in that it composes of a hollow tube with an opening at one end (in S. psittacina, the lobster-pot opening,and in Genlisea, small holes atintervals along the corkscrew like trap), and many stiff hairs within the tubepointing away from the opening. These hairs prevent any prey which enters thetrap from moving backwards, so the prey moves slowly deeper and deeper into thetrap before dying. Pigeon traps are generally submerged within water where,particularly in the case of S. psittacina,a standard pitfall trap would not work.
Figure8. Sarracenia psittacina cross section, morphology diagram. BPH,Backward Pointing Hairs; Li, Lid; LpT, Lobster-pot Trap; Ps, Peristome; PT,Pigeon Trap Region; Wi, Wing. By Julian Kiely.
So, as you cansee the diversity of trapping methods utilised by modern carnivorous plants istruly astounding, and reflects the many different prey items which they catch. Thereare many different deviations from the basic design of each trap and thus theabove descriptions provide only the briefest of overviews. Carnivorous plantsare so incredible and diverse that, even at the end of this series of blogs, Iwill have only scratched the surface of their biology and diversity. Next timeI will introduce all the different genera of carnivorous (and possiblycarnivorous) plants, talking more about the specific adaptations of each genus,and the species within each genus. I will also delve a little more into theirbasic phylogeny (although, this will be covered in more detail within Part 3).
Butfor now, I hope you have all enjoyed this blog and found it interesting andinformative. As it is my first proper article I’m still getting used to thewriting style, so I apologise if I have rambled a little bit. Please let me knowwhat you have thought about it and whether there is anything I can improve on.I know some of you may feel cheated that in the first post on a palaeobotanyblog there hasn’t been a single sign of a fossil, but trust me, I will begetting to that soon enough; so, if it helps, you could always think of this asjust the preamble before the main show.
And with that,all that’s left to say is Thank-you Very Much for reading!
All the Best,
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D’Amato, P. (2013). The Savage Garden Revised. SecondEdition. Ten Speed Press, Berkeley.
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USD Plant Database. Sarracenia purpurea L. purple pitcherplant. https://plants.usda.gov/core/profile?symbol=sapu4