Ecological Explainers: Physical Defences

Plants are constantly fighting a battle against critters that want to eat them. We see the result of this battle in the physical defences that plants have evolved over time. Plants make use of different types of defence mechanisms; from tough waxy armour to chemical-laden barbs. These defences may just discourage an enemy from chowing down on the plant or even cause their enemy to have a long and painful death. Whatever it takes to avoid being someone’s dinner. Here I am focusing on the spiky plant parts that we have all hurt ourselves on at some point.

In botany, we class spiky plant parts into 3 major types:

  • Thorns
  • Spines
  • Prickles


Thorns are sharp and pointed branches. They have vascular tissue, can be branched and can have leaves.

Discaria toumatou, Mt Iron (C.Simpson-Young)

Discaria toumatou, a plant endemic to New Zealand, has branches that have developed into thorns. The giveaway is that they are green which means they have vascular tissue and are photosynthesising. Whilst it is native to New Zealand it can often become weedy because it’s the only plant that survives the munching of sheep thanks to their epic thorns. Mt Iron, Wanaka (C.Simpson-Young)


Spines are modified leaves or parts a leaf. They don’t have vascular tissue and don’t photosynthesise, and tend to be hard and dry. Spines can develop from the little stem that holds the leaf (petiole), main cluster of veins in the leaves (midrib), veins in general or the stipule.

Spines of Citrus australasica or Finger Lime (C.Simpson-Young)

The Finger Lime (Citrus australasica) have what we call Stipular Spines. They have developed from the stipule, which is a little bit of tissue that occurs at the base of the leaf stalk (C.Simpson-Young)

Spines of a Solanaceae (C.Simpson-Young)

The spines of this Solanaceae species follows the veins in the leaves, indicating they develop from the tissue of the veins (C.Simpson-Young)

Pereskia grandifolia Kew 2015 (C. Simpson-Young)

Pereskia grandifolia, a species of Cactus, has spines that grow out of a pale lump of tissue called areole (a highly modified stem). Kew 2015 (C. Simpson-Young).


Prickles are spiky outgrowths from the bark of a plant. They are usually small, and grow from the cortex and epidermis layers of the bark. These layers don’t have vascular tissue, so prickles don’t either.

Rose Prickles (C.Simpson-Young)

The terms spine, thorn and prickle are often used interchangeably outside of botany. For example, rose thorns are actually prickles because they are outgrowths of the epidermal layer of the stem (C.Simpson-Young)

In summary, the three main forms of spiky things found on plants are thorns (spiky stems), spines (spiky leaves and leaf bits) and prickles (spiky outgrowths of the bark). There are some other pricks and piercers like modified roots and trichomes, but I’ll save them for another post.

Armstrong, W.P. (2007) Botany 115 Vegetative Terminology: Modified Roots, Stems and Leaves. Wayne’s Word: Online Textbook of Natural History, accessed Feb 2018  <;

Biology Online Directory (2017), Areole, accessed Feb 2018 <;

Raven, P., Evert, E., Eichhorn, S. (1986) Biology of Plants, Fourth Edition. Worth Publishers: New York.

Missouri Botanical Gardens (2017), Pereskia grandifolia var. grandifolia, accessed Feb 2018 <;


Buds and the Bees: Sexual Deception in Orchids

Sexual deception is probably the weirdest and most wonderful methods used by plants to attract pollinators. Many plants use deceit of some kind to get little critters to spread their pollen to another flower, whether that is one plant species mimicking the petal arrangement of another, or a flower smelling like rotten flesh to attract pollinating flies. However, not many species use sexual deceit to attract pollinators. True sexual deception has only been described in the family Orchidaceae (and to a lesser extent, in a species of daisy, Gorteria diffusia).

Paracaleana minor or the Small Duck Orchid can be found on rocky slopes and gravelly soils in sclerophyll shrublands, woodlands, grasslands and forests across the Australian South East.

Paracaleana minor (Photo: Casey Gibson)

Paracaleana minor (Photo: Casey Gibson)

Sexual deception is what it sounds like. Some orchids have evolved to look and smell like a female insect and send out a silent “come hither” to the male insect who thinks he is about to get lucky. He tries to get it on with the flower, while the flower plops a sac of pollen (called pollinia) on the head of the foolish insect. He flies away and finds himself falling for the orchid’s trick again, and leaves the pollen sack behind on the second flower.

Calochilus campestris or Copper Bearded Orchid can be found in moist sclerophyll forest on the East Coast of Australia and in Tasmania. Their flowers are pollinated by the male scoliid wasp (genus Campsomeris). You can see that the labellum has evolved to resemble a wasp, with colourful hairs and even some glands that look like eyes.

Calochilus campestris (Photo: Justin Chan)

Calochilus campestris (Photo: Justin Chan)

These orchids are exploiting the mate-seeking behaviour of male insects, usually Hymenoptera, a large insect order that includes bees, ants and wasps. The orchids exude a chemical that smells like the mating pheromones of the female of the pollinating species. This cross-species communication is made possible because both insects and plants use the same chemical compounds in their cuticle wax layers to reduce water evaporation. Insects use these chemicals for communication, and some orchids have evolved the ability to co-opt the compounds in their own protective waxy coating for producing insect pheromone-like smells. In addition to this sneaky smelly trick, the bottom petal structures (labellum) will often mimic the shape, colour and texture of the female. The result is flowers that look super weird and often very beautiful.

Sexual deceit has evolved independently across the world in unrelated orchid genera. About 400 sexually deceptive species can be found in Australia, Europe (one genera, Ophrys), the Mediterranean basin, South Africa (2 species from the genus Disa), and in South and Central America. However, it is likely that there are species that are yet to be described.

Caleana major or the Flying Duck Orchid is pollinated by male sawflies (and will resprout from tubers as a backup). The sawfly lands on the labellum which spring shut and forces the sawfly against the column of the flower transferring pollen. They can be found across the East and South of Australia on sandy or gravelly soil in sclerophyll forest, shrubland and heathland.

Caleana major (Photo: Casey Gibson)

Caleana major (Photo: Casey Gibson)

Giant thanks to Casey Gibson and Justin Chan for letting me use their photos. Check out their Instagram: Casey and Justin

There are loads of wonderful resources out there on this topic, with examples from across the world. Below are a few.

Sexual deception in orchids by Alun Salt on the AoB Blog

Deceptive orchids: luring wasps for pollination by Emma Young for Australian Geographic

The deceptive sex lives of orchids by Danielle Clode and Sue Double for Ockham’s Razor, Radio National





Benitez-Vieyra, S., Medina, A.M. and Cocucci, A.A. (2009) Variable selection patterns on the labellum shape of Geoblasta pennicillata, a sexually deceptive orchid. Journal of Evolutionary Biology 22 (11): 2354-2362

Cozzolino, S. and Widmer, A. (2005) Orchid Diversity: and evolutionary consequence of deception? Trends in Ecology and Evolution. 20 (9): 487- 492.

Davis, L. (2016) The Ducks. Native Orchid Society of South Australia <;

Ellis, A. and Johnson, D. (2010) Floral mimicry enhances pollen export: the evolution of pollination by sexual deceit outside of the Orchidaceae. American  Naturalist. 176(5): 43-51.

Jersáková, J. and Johnson S. and Kindlmann, P. (2006). Mechanisms and evolution of deceptive pollination in orchids. Biological Reviews. 81: 219-235.

Johnson, S. and Nilsson, L. (1999) Pollen carryover, geitonogamy, and the evolution of deceptive pollination systems in orchids. Ecology. 80:2607–2619.

Schiestl, F. and Cozzolino, S. (2008) Evolution of sexual mimicry in the orchid subtribe orchidinae: the role of preadaptations in the attraction of male bees as pollinators. BMC Evolutionary Biology, 8(27):1-10

Schiestl, F. (2005) On the success of a swindle: pollination by deception in orchids, Naturwissenschaften. 92 (6): 255-264.

Smithson, A, and Gigord, L. (2001) Are there fitness advantages in being a rewardless orchid? Reward supplementation experiments with Barlia robertiana. Biological Sciences 268 (1475): 1435-1441.

Ecological Explainer: Head Inflorescences

Flowers come in many shapes and sizes. Sometimes a bunch of flowers grow so close together that they trick you into thinking they are one flower.
 The Birds and Bees
 So I probably don’t need to tell you that a flower is the part on the plant that holds the reproductive organs. In their simplest form you have the female organ (which includes an ovary and a landing pad for pollen) and the male organ (a little capsule that flaps around in the wind letting pollen out into the world). There are pretty petals fringing out around all this hardware. Their job is to lure the animals that are going to spread the pollen between flowers. Plant sex happens when the pollen from a male organ lands on a female landing pad, male sex cells go down into the female ovary and…BAM….fertilisation. The flower turns into a fruit and the merged sex cells turn into seeds.

 A Bouquet
 The flower I have just described is a super simplistic one. Think of a lily. But there are an infinite number of variation on this theme! We have all seen the amazing variety of colours that flowers come in. Think about the different shapes. From orchids to tulips. Some flowers are both male and female, some are either male or female. We don’t think much about it but grass has flowers. The wheat we are the fruits that develop from the very plain flowers (their pollen spreads on the wind so they don’t need to impress anyone). The flowers on a plant can be arranged in loads of different ways. When there are a bunch of flowers sitting on a stem close together it is called an inflorescence. These also come in many different forms. Today I want to talk about a type of inflorescence called a head.
 Heads Up
 For my birthday I got a microscope. One of the first things I did was pull apart a daisy ‘flower’ into its parts. So based on all the above, it may not come to you as a surprise when I tell you daisies are not a single flower, but a number of flowers arranged to look like a single larger flower. This is called a head. Each of the individual flowers is very squat and they all sit very close together. The most common group of plants with this arrangement is the daisy family (or the Asteraceae family). In this family you have 2 types of flowers that make up the head inflorescence: 1) disk flowers and 2) ray flowers. The petals we see are single petals on the ray flowers which ring around the outside of the inflorescence. Making up the middle of the head inflorescence is lots of disk flowers.


On the left a disk flower and on the right a ray flower.


And this is what it looks like when you put it all together! The curly structures sticking up out of the flowers are the female reproductive structures (called a pistil)