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So, Why are plants smelly then?

As part of the University of Reading British Science Week outreach activities Dr M and colleagues Oli Wilson, Alex Dean and Katie Cooper hosted an event for local families addressing this aromatic question!  As Dr M pointed out to the attendees, this event also forms part of Reading Botany 2019, a year-long series of events celebrating 50 years botany masters teaching at Reading and over 100 years of botany research more widely.

Over 60 adults and children were welcomed to the Harborne Building biology laboratories at the School of Biological Sciences on Sunday 10th March 2019.  The larger group was divided into three smaller groups who alternated between three especially designed smelly plant activities:

Oli Wilson introducing lemon scent

1 Lemon Scent!  Oli Wilson started off by thinking about what smells actually are, and why plants might want to deploy them in the first place. Focusing on an array of lemon-scented species, we used microscopes to look at their architecture, noticing how several, such as stinging nettles (handle with gloves!), had surfaces covered in tiny spines (trichomes). We then discussed potential reasons why a number of different and quite unrelated plants smelled of lemon, e.g. Lemon Grass (Grass family), Lemon Thyme (Dead-nettle family) and Monterey Cypress (Cypress family – a conifer!).

The images below show some of the young participants sharpening their microscope skills observing Stinging Nettle trichomes – with protective gloves on! – and the wonderfully aromatic Mugwort (Artemisia).

The concept of convergent evolution was discussed and the advantages of using a common chemical language in a plant’s interconnected world.  Participants discussed how all of these lemon scented plants were producing limonene, and for all of them to use the same chemical, despite being so distantly related, suggested that there must be advantages to that. One advantage might be that limonene is just very effective at its job, so it has evolved several times; another might be the ability to signal alarm to neighbours (and to hear/smell warning signals from them in turn) about attacks. Related to that, participants also briefly looked at the ‘wood wide web’ concept of inter-connectedness among plants, which underlies the inter-communication between plants. This last theme is explored in the YouTube video here.

Alex Dean and his smelly (plant) families!

2 Smelly Families! Alex Dean explained that plants emit a wide variety of smells from their flowers, leaves, stems and roots. These smells have a variety of roles both for attracting pollinators, as well as preventing damage from insect herbivores or infection by fungi and microbes. In this session participants were treated to an exploration of how plant scents are found across a variety of plant families, while also showing diversity within a family.

In order to smell as many plants as possible participants played Alex’s “smelly plant game” in which 30 squished, mushed, or otherwise broken and crushed plants were placed in little pots. Participants sniffed the contents without any clues as to what the original plant might be. The aim of the game was to identify the smell, think about why it might smell like that, and also what it might smell similar to. The lucky winners were awarded their prizes (a lime, an onion, and some ginger!), and Alex explained a little about how these chemicals are useful for both the plant, and to us for food, medicines, therapy etc.

Dr M ‘s smelly, sticky conifers!

3 Dr M’s Smelly Conifers! Dr M started off by spelling out the love a joy of plants and the many things they do for us and how they are too often overlooked in favour of the superficially more charismatic animals.  Then he produced some flowering stems of a plant called Daphne with its heavily scented flowers and asked why it might smell like this?  Younger members the group were quick to suggest (correctly) that it could be to attract pollinators like bees. Dr M then showed pictures of a pretty pink flower called Red Valerian and asked why it might have pretty pink flowers and what they might smell like? Again the suggestion was sweet smelling to attract pollinators.  Dr M then encouraged his group to sniff from a herbarium folder of dried Valerian material, the smell is actually not sweet at all, rather it is rather gross and stinky like old socks or a sweaty sports changing room after match! The chemical responsible is Valeric acid, it has a very unpleasant odour.  Dr M used this example to make the point that some (to humans at least) unpleasant smells may be attractive to different pollinators, e.g. flies are rather partial to ammoniacal aromas which to our noses smell like public toilets!

Dr M then moved to a different group of smelly plants, the conifers. He showed the group piles of branches from different conifers collected from the Harris garden.   Dr M asked them to examine several species noting the different leaf forms – scale-like, awl-like, liner and needle-like.  Dr M encouraged the group to use hand lenses provided to examine the detail of these lovely plants, admiring the brighter sheen of the leaves, the scaly twigs and even making out the rows or patches of stomata on the underside of the leaves. Dr M guided the group in the correct use of the hand lens, holding it to the eye and moving the object (twig or leaf) to the eye into focus.

Dr M then got the group snapping the stems and crushing the leaves to release the characteristic terpene-rich sticky resin.  The group then tried to characterise the smells – pine, resin, parsley, citrus – were all detected by different participants but there was often no general consensus for any one plant, this reflects the individualistic feature of the human nose!  The session concluded by pondering what the function of the terpene-based resins might be.  Anti-herbivore defence, healing damaged trunks and branches, as well as helping maintain seed dormancy in the closed cones were all suggested as well as anti-microbial properties protecting the conifers from bacterial and fungal infections.


Dr M says: Participants and staff alike had an illuminating and fun afternoon exploring some of the science surrounding the nature of smelly plants.

We have had some great feedback, one participant saying: “Thank you for this brilliant event.  All 3 sessions were fascinating and have changed the way I think about plants and trees.  My children loved the interactive nature of sessions and will now be making good use of their hand lenses.  We will look out keenly for any future botany events!”  

We plan to develop these activities plus some more for the forthcoming “why are plants smelly 2” schools outreach project for the International Fascination of Plants Day on Wednesday 15th May (a few days before the actual FoPD but Weds is better for the schools we want to bring to Reading), watch this aromatic botanical space!




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