The title of this post is a question I hear fairly frequently, often yelled in frustration and usually followed by a myriad of questions. There are certain organisms that people are astounded to find out are from a different kingdom altogether.
In this post, we’re going to look at three things that are not plants, discuss what makes them different and I’m going to apologise in advance for the stress and confusion it may cause.
Our first “not a plant” may not come as a total shock, so I felt it would be a gentle immersion into the world of things that are not plants. It may shock you, however, to find out that corals are in fact animals. Corals belong to the Cnidaria phylum along with jellyfish.
Cnidarians are aquatic, invertebrate animals that have specialised stinging cells called ‘Cnidocytes’. Cnidocytes are used in prey capture and as a defence mechanism. If you’ve ever been unfortunate enough to accidentally step on a jellyfish, you will be familiar with the uncomfortable sensation these cells cause. Many species of coral have these stinging cells in their tentacles.
Corals may look like beautiful underwater gardens, but they share little in common. Each coral structure is made up of a group of identical polyps from a coral species. When grouped together, they secrete calcium carbonate skeletons which produce these iconic structures. Reefs are made up of many corals, and they are home to a huge array of animal species.
Reefs are now considered one of the most vulnerable ecosystems on Earth. They’re affected by many human activities, including pollution, over-fishing, urban runoff, and bleaching and rising sea-levels due to greenhouse gas emissions. To help restore coral reefs, 3D-printed polyp “houses” were developed in the US. They mimic the skeletons of natural reefs to encourage regrowth and are already proving successful .
Sticking with marine organisms, this next creature may come as a surprise. One of the things I dislike most when I’m at the seaside is the sensation of slimy seaweed tangling around my legs as I swim. Although the name may suggest this sluggish specimen to be a plant, seaweed is actually a macro-algae and only some seaweeds can be classified as plants. I wrote briefly about photosynthesising algae in a previous post which includes green algae, under which some seaweed falls.
Seaweed doesn’t have a formal definition, but roughly groups together a few types of multicellular algae: red, brown and green. Of the three, only green algae falls under the plant classification. Because algae doesn’t have a formal definition, it is hard to pinpoint why some are algae, but they’re importantly not plants because they do not have usual plant tissues. However, they’re very plant like in appearance, and have structures called fronds, consisting of stipes and blades, like stems and leaves.
This begs the question, do these fronds have benefits? The answer is yes, many. For a start, seaweed creates an important ecosystem that acts as a nursery for fish and a food source for many marine species. Seaweed is also farmed in many countries, from Japan to Wales and is a good source of many elemental nutrients. It has also been used medicinally as an antibiotic and to encourage calcium uptake by bones .
Agar jelly used in labs is also derived from seaweed and there’s a good story behind how it was discovered. In 17th century Japan, an innkeeper left some Seaweed outside their mountain inn and it froze overnight. Upon thawing, the innkeeper found that the impurities had gone and it had thickened into the jelly-like substance we’re familiar with . The repeated freezing and thawing process is still used in agar production today.
Of the three I have listed today, this final group comes as the biggest surprise to most people I tell. The humble mushroom, although regularly found growing on and around plants, is most definitely not one. Mushrooms are from the Fungus kingdom, making them as distinct from plants as animals, and there is one thing that absolutely separates them from plants structurally.
Plants have cell walls made of cellulose. Whereas, Fungi have cell walls made of chitin. Chitin contains nitrogen while cellulose does not, and cellulose is comparatively much weaker than chitin. Chitin is so strong in fact, that it is also the substance that makes up the exoskeleton of arthropods (such as crabs), the beaks of cephalopods (such as squids) and the scales of fish.
Today, mushrooms are a popular icon in fashion and art. They’re often associated with magic and fantasy. Gnomes are often depicted sitting on them whilst fishing in ponds. But this cute imagery of fairies and pixies lazing dreamily on toadstools hasn’t always been the case. In the 1800’s, there was a superstition against toadstools in Britain, coined “fungophobia” by William Hay. He wrote of them,
“They are looked upon as vegetable vermin only to be destroyed.”
However, we have long since moved past this notion and now enjoy mushrooms for their many benefits. As well as being popular in many cuisines, mushrooms used to be the main source of many textile dyes prior to the industrial production of synthetic dyes. Psychoactive mushrooms containing psilocybin have recently been researched for their long-term spiritual effects in users . Psilocybin is now being investigated for its use against alcohol dependence  and anxiety in advanced-stage cancer patients .
The takeaway message from this post is, if it looks like a plant and it acts like a plant, it could be an animal, algae or fungus. Organisms are beautifully diverse, but sometimes the lines between them can be visually blurry and it’s not until you look at their micro-structures that you realise they’re not the same at all.
 Crook, L. (2020). Coral skeletons crafted from 3D-printed calcium carbonate could restore damaged reefs. Dezeen.
 Dillehay, T. D. et al. (2008). Monte Verde: Seaweed, food, medicine, and the peopling of South America. Science.
 Guiry, M. D. Agars: Uses and Utilization. The seaweed site: information on marine algae.
 Griffiths, R. R. et al. (2008). Mystical-type experiences occasioned by psilocybin mediate the attribution of personal meaning and spiritual significance 14 months later. J. Psychopharmacol.
 Bogenschutz, A. A. et al. (2015). Psilocybin-assisted treatment for alcohol dependence: a proof-of-concept study. J. Psychopharmacol.
 Grob, C. S. et al. (2011). Pilot study of psilocybin treatment for anxiety in patients with advanced-stage cancer. Archives of general psychiatry.
3D-print reef from Reef Design Lab: Alex Goad