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 use many different types of defence mechanisms; from tough waxy armour to chemical-laden barbs. These defences may do as little as discouraging an enemy from chowing down on the plant, or as much as causing an enemies’ long and painful death. Whatever it takes to avoid being someone’s dinner. In this Ecological Explainer, I’m focusing on those spiky plant parts that we see quite often..that all of us have probably hurt ourselves on at some point.

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

  • Thorns
  • Spines
  • Prickles

Thorns

Thorns are sharp and pointed branches. 

They have vascular tissue, and so some will be green telling us they are able to photosynthesise. They 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

Spines are modified leaves or parts of leaves.

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); the main cluster of veins in the leaves (midrib); other smaller veins; 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

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)
  • 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  <http://waynesword.palomar.edu/ecoph30a.htm&gt;

Biology Online Directory (2017), Areole, accessed Feb 2018 <https://www.biology-online.org/dictionary/Areole&gt;

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 <http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=273816&isprofile=0&=&gt;

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