Carnivorous Plant Diversity and Evolution: Part 1 – An introduction to Killer Plants
Hello all,
Welcome to
Palaeoflora, and to my first blog post! I already have many ideas of articles I’d
like to write, however, to begin with I wanted to cover a topic which I have a
particular interest in, and is very dear to me; Carnivorous Plants! Many people
are aware of the existence of carnivorous plants, but this knowledge often
extends only to the famous Venus Fly-Trap (Dionaea
muscipula), with the Pitcher Plants of Asia and America (Nepenthes and Sarracenia respectively) coming a close second. But the diversity
of modern carnivorous plants in both morphology and taxonomy is truly
astounding, and even within the course of researching for this article, I
discovered many more lineages of carnivorous, or possibly carnivorous plants
than I even suspected. In this first article I will provide an introduction to
carnivorous plants, their habitats and their various trapping mechanisms. In
the second article I would like to take you on a guided tour of modern carnivorous
plant diversity, and describe to you the biology of the various carnivorous
plants which exist today. And in my third article I will be focusing on the
evolution, phylogeny and fossil record of carnivorous plants, and will also
talk about speculations which can be made about their inclusion within
prehistoric environments and palaeoart. So, without further ado, I’ll begin our
journey, through the world of Carnivorous Plants!
So, what are these so called ‘Carnivorous Plants’?
There are two
main terms used to describe carnivorous plants; the first is of course
‘carnivorous’ and the second is ‘insectivorous’. Throughout this blog I have
chosen to refer to them by the former name, as while it is true that most
carnivorous plants prey primarily upon insects, many will also trap other
animals (including vertebrates on occasion). Next, when we say ‘carnivorous
plant’ what do we actually mean? Well, The Kew Plant Glossary (Second Edition,
Beentje 2016) provides the following definition:
‘carnivorous, plants that trap animals
and derive some or most of their minerals from digesting them’
While this
provides us with a basic definition for vegetable carnivory, I would like to
suggest a few more criteria which most carnivorous plants adhere to:
- · They actively implement methods of attracting their prey to their traps.
- · They must permanently trap their prey, causing their eventual death.
- · They utilise methods of breaking down their prey to extract minerals from them.
- · And finally, they must be able to absorb those minerals either directly or indirectly from their prey.
However,
even these criteria are not present in all ‘carnivores’ (in these cases I may
be inclined to use the term ‘pseudocarnivory’), and even within true carnivores
there are some which are active and some which are passive. Active and passive
can 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, which
aids in digestion. However, ‘active carnivory’ can also be used to mean a trap
which responds to stimuli from its prey with movement, and a ‘passive
carnivore’ is one which does not. So, as
you can see, the division between what is and what isn’t carnivorous is not as finely
cut as most people think, but instead more of a blur. All levels and stages of
carnivory are present in modern floras, from plants which display perhaps only
one carnivorous feature, to those who are fully carnivorous.
Fantastic Carnivores and Where to Find Them
Within modern
carnivorous plants, of which there are over 600 extant species (Ellison and
Gotelli, 2008; Christenhusz, Fay
and Chase 2017), there are many different forms of trapping mechanisms, which
often 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 within
plants indicates that there must be some big advantages to having it, but under
what 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 low
mineral content. These habitats are often very wet and sometimes permanently
waterlogged, as in these habitats the presence of water leaches minerals out of
the substrate (soil), creating low concentrations of the minerals which plants
require to grow. For this reason, many carnivorous plants can be found growing
in peat bogs and semi-aquatic environments, around the world, and even here in the
British Isles (where I live) there are ten species of carnivorous plants across
three genera (Rose and O’Reillyn 2006) (see Figure 3.).
This
may also be a good time to dispel the myth that carnivorous plants are all
tropical and require warm temperatures; while it is true that some carnivorous
plants do live in the tropics, many are also found in temperate regions across
the globe. Sarracenia, the North
American Trumpet Pitchers are found as far north as Canada and Alaska, and can
survive 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 Siberia
and Iceland, and in mountain ranges across Eurasia, were it thrives in the cold
conditions (Slack 1979).
Rainy,
wet and humid environments are often common places for carnivorous plants to
evolve, because these conditions make the production of aqueous substances, such
as the nectar-like ‘glue’ of many flypaper traps, a worthwhile investment; in
these conditions, the virtually unlimited supply of water allows the production
of large quantities of aqueous substances, and the humidity prevents these
substances from evaporating quickly (Ellison and Adamec, 2018). An exception to
this association of carnivorous plants with water is Drosophyllum (the Dewy Pine), which instead grows in the dry, nutrient
poor, fire-prone sandstone soils of the Western Mediterranean (Ellison and
Adamec, 2018). Epiphytic carnivorous plants are also known, however this is
uncommon; among angiosperms as a whole, ~ 9% are epiphytes, whereas among
carnivorous species, only 2% are, with many of these not representing true
epiphytism, as they often inhabit wet microsites such as the water filled tanks
of bromeliads.
Tricks and Traps of Carnivorous Plants
I
have found that the various trapping mechanisms seen in modern carnivorous
plants can be broadly classified into four different types; Flypaper Traps,
Pitfall Traps, Motile Traps and Lobster-pot Traps (see Figure 4.).
The
most common form of trapping mechanism (the form which is present in the most
modern genera) is the Pitfall Trap; these traps have evolved independently on 5
occasions (twice within Bromeliaceae, and once in the Nepenthales, Rosids and
Ericales), and are generally composed of a single leaf which has developed into
a funnel or pitcher-like shape which holds water (hence the common name pitcher
plant). An exception to this are the carnivorous bromeliads, whose traps are
composed of a rosette of tightly packed leaves, which holds water in the
middle. As the bromeliad carnivores are very different in morphology from other
pitfall 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 the
traps from flooding with too much rainwater, however many individual species
have adapted the lid to perform a variety of other functions. The peristome
(rim of the opening) varies greatly between different species, being not much
more than a rolled edge in Sarracenia
and often highly developed in Cephalotus
and Nepenthes; it is often smooth
with a slippery surface. The lid, neck and occasionally peristome of pitfall
traps produce nectar, which is used to attract insect prey with the sugary
liquid. Ultraviolet patterns which resemble those flowers use to direct
pollinators are also present in some species (Ellison and Adamec, 2018). When
the prey lands, they will often slip on the smooth, waxy surface of the
peristome, and fall into the pitcher chamber of the trap, which is often water
filled (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, and
instead the prey drowns in the water. This water may contain digestive enzymes
produced 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 pitcher
chamber. Carnivorous bromeliads similarly utilise ultraviolet patterns and a
slippery surface, but instead have adventitious roots growing into the water
tank, which absorb minerals (Christenhusz, Fay and Chase 2017).
The second most common form of trap is the Flypaper Trap. These traps are often composed of leaves with glandular hairs which secrete a sticky substance at their tip. In addition to these secretory glands, many (but not all) flypaper traps also possess glands which produce digestive enzymes and absorb minerals from the digested prey. Flypaper traps are often designed to catch small flying insects, which are attracted to the leaves by the sparkling dew (sticky substance), which in some instances produce ultraviolet patterns similar to the markings of some flowers (Ellison and Adamec, 2018). Once the insect lands, the sticky ‘glue’ prevents them from escaping and, in most cases, suffocates the insects as the ‘glue’ clogs up their spiracles and trachea (tiny holes and tubes, along the sides of their body, which they use to breathe). At this point, some flypaper traps (such as those in Drosera) also act as motile traps, as their hairs and leaves fold inwards to wrap around the insect (Ellison and Adamec, 2018). The prey is then digested and the minerals adsorbed. Despite its seeming complexity, this is the simplest form of trap (with the possible exception of the pitfall traps of certain Poales).
![]() |
Figure 6. Drosera leaf life drawing and cross section. Ab, Abaxial leaf surface; DG, Digestive Glands; GH, Glandular Hair; Gu, ‘Glue’ (mucilage); TS, Trapping Surface. By Julian Kiely. |
Motile
traps are perhaps the most enigmatic of the carnivorous plants, and include the
famous Venus Fly-Trap (Dionaea muscipula).
The morphology of motile traps varies greatly between the handful of genera
which have developed them, but the significant element in all of them it the
use 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. As
such these traps are sometimes referred to as ‘snap-traps’ (Ellison and Adamec,
2018). Another form of highly advanced motile trap is the bladder, or suction
traps of Utricularia species, which
grow on subterranean or aquatic stems where they are surrounded by water.
Bladder traps are highly derived leaves which take the form of a hollow bladder
with a trap door at one end. They work by pumping water out of the trap, creating
a partial vacuum within them; when small prey stimulate hairs around the
trapdoor, 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 Pinguicula
species also move in response to prey; in Pinguicula
this movement is slow and minimal, but in Drosera
it is often more rapid and noticeable (in D.
glanduligera, the edges of their leaves possess snap-tentacles, which hinge
inwards when stimulated, to further entrap prey; Ellison and Adamec, 2018). The
lid Nepenthes gracilis also acts as a
passive motile trap, utilising raindrops which hit the lid to throw prey into
the pitcher (Ellison and Adamec, 2018).
![]() |
Figure 7. 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. |
The
final form of trap is the lobster-pot trap (sometimes called a pigeon trap in
Genlisea and Sarracenia
psittacina). The basic mechanism of these traps is to have a one way
entrance, so that the prey, once inside are unable to back out. In Darlingtonia (which is generally
considered a pitfall trap) and S.
psittacina, these traps are modified pitfall traps, where the lid and
peristrome have fused and curved inwards (Figure 8.), to produce a small
opening which funnels prey inwards, but prevents them from escaping back out of
that opening. The pigeon trap, while similar in its trapping mechanism, is
different 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 at
intervals along the corkscrew like trap), and many stiff hairs within the tube
pointing away from the opening. These hairs prevent any prey which enters the
trap from moving backwards, so the prey moves slowly deeper and deeper into the
trap before dying. Pigeon traps are generally submerged within water where,
particularly in the case of S. psittacina,
a standard pitfall trap would not work.
![]() |
Figure 8. 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 can
see the diversity of trapping methods utilised by modern carnivorous plants is
truly astounding, and reflects the many different prey items which they catch. There
are many different deviations from the basic design of each trap and thus the
above descriptions provide only the briefest of overviews. Carnivorous plants
are so incredible and diverse that, even at the end of this series of blogs, I
will have only scratched the surface of their biology and diversity. Next time
I will introduce all the different genera of carnivorous (and possibly
carnivorous) plants, talking more about the specific adaptations of each genus,
and the species within each genus. I will also delve a little more into their
basic phylogeny (although, this will be covered in more detail within Part 3).
But
for now, I hope you have all enjoyed this blog and found it interesting and
informative. As it is my first proper article I’m still getting used to the
writing style, so I apologise if I have rambled a little bit. Please let me know
what 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 palaeobotany
blog there hasn’t been a single sign of a fossil, but trust me, I will be
getting to that soon enough; so, if it helps, you could always think of this as
just 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,
Julian Kiely
References
Beentji, H. (2016). The Kew Plant Glossary: an illustrated
dictionary of plant terms. Second Edition. Kew Publishing, Royal Botanic
Gardens, Kew.
Christenhusz, M. J. M., Fay, M. F.,
Chase, M. W. (2017).
Plants of the World, An Illustrated Encyclopaedia of Vascular Plants. Kew
Publishing, Royal Botanic Gardens, Kew.
D’Amato, P. (2013). The Savage Garden Revised. Second
Edition. Ten Speed Press, Berkeley.
Ellison, A. M., Adamec, L. (2018). Carnivorous Plants: Physiology, Ecology,
and Evolution. Oxford University Press, Great Clarendon Street, Oxford, OX2
6DP, United Kingdom.
Ellison, A. M., Gotelli, N. J. (2009). Energetics and the evolution of
carnivorous plants—Darwin’s ‘most wonderful plants in the world’. Journal of
Experimental Botany, Vol. 60, No. 1, pp. 19–42.
International Carnivorous Plant Society. Carnivorous Plant Trapping Mechanisms. https://www.carnivorousplants.org/cp/carnivory/trapping
Rose, F., O’Reillyn, C. (2006). The Wild Flower Key: How to identify
wild flowers, trees and shrubs in Britain and Ireland. Revised Edition. Penguin
Books, 80 Strand, London, WC2R 0RL, England.
Slack, A. (1979). Carnivorous plants. Ebury Press,
National Magazine House, 72 Broadwick Street, London, W1V 2BP.
Thorogood, C. J., Bauer, U., Hiscock, S.
J. (2017). Convergent
and divergent evolution in carnivorous pitcher plant traps. New Phytologist,
Vol. 217, pp. 1035 – 1041.
USD Plant Database. Sarracenia purpurea L. purple pitcherplant. https://plants.usda.gov/core/profile?symbol=sapu4
Don’t forget Philcoxia! Also, the evolution of the pitfall trap in Bromeliaceae could not be considered independent on two occasions as this evolutionary adaptation is commonly adopted throughout the family. The carnivorous syndrome may have been adopted by the two genera independently however.
ReplyDeleteOh Crikey! Thanks for reminding me! I only came across Philcoxia recently, in the research for the next post of this blog series, so I'm not very well acquainted with it. I didn't realise that it has been shown to be a true carnivore, although, so far the only research into it I've done has been very cursory. Also, thank-you for letting me know about the Bromeliaceae traps. From the reading I've done it's thought that Catopsis and Brocchinia developed carnivory independently, but of course the trap structure is shared by many Bromeliads. Do you know of any other carnivorous/ pseudocarnivorous Bromeliads?
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