Tuesday, 16 February 2016

Plant life is a colourful cabaret (Plant Portrait XIII*)

Roses are red
Violets are blue
Birds like the first
But no insects do

This is bad poetry and not quite correct, but it references some interesting chromatic biology. Reading Richard Mabey's The Cabaret of Plants over Christmas (on the recommendation of Fred Pearce, on the first of the summer series of Talking Plants on RN...) I was reminded that insects and birds see colours differently, and that plants have evolved to take advantage of this fact. This recollection inspired me to write these couplets.

In his chapter 'On being pollinated', Mabey tells us insects such as bees are attracted to blues, as well as shades of purple and ultraviolet. The next most attractive colour for day-time insects is yellow, the partner of blue in greens he notes. No insect has been found with receptors for red, so all colours at that end of the spectrum appear black or dark grey.

A strongly perfumed red flower can still draw in the bugs though. Insects combine scent and colour in locating a food source (for example, a flower) so might not mind the occasional smelly red flower. Terrestrial orchids mimicking female insects and emitting evocative 'pheromones' attract tiny wasps even when fully red such as this Crimson Spider-orchid.

Birds can see red, and they seem to like it. They associate, rightly or wrongly depending on the plant-animal relationship, a red flower with a sweet reward (nectar). That said, they are also attracted to orange, yellow and white flowers. It's been suggested in fact that it may be as much the absence of bees (that can't see red) that attracts birds to red flowers than a particular desire for this colour.

Moths definitely favour white or light-coloured flowers, anything to make the bloom more easily seen at twilight or in moonlight. Bats don't use sight as much so the flowers that attract them can be blood-red and brown, just as long as they are big and hang the right way (as they say). As for black flowers, you can read about these in an earlier post.

While musing on all this, I was also reminded of a conversation in the tea-room at Royal Botanic Garden Sydney a few years ago about the preponderance of particular flower colours in alpine areas. My memory is hazy on the detail but there was talk of strong winds blowing flying insects from the mountain tops and how this might favour plants with particular pollination strategies (e.g. discouraging flowers that depend on insects with big flappy wings).

My botanical colleague and alpine expert Neville Walsh reminded me that in Victoria at least there are bugger all red-flowering plants in the Victorian alps (apart from Grevillea victoriae just below the treeline and couple of others that might blush a little pinkishly). Nearly all the birds above the treeline are insect eaters, and there are plenty of insects for them to eat (Neville recalled with some frustration the large number of seeds he has tried to collect for the Victorian Conservation Seed Bank that were munched by insect larvae).

Elsewhere in the world you get similar patterns, with the number of red-flowering plants decreasing as you rise in altitude. It has been suggested that higher altitudes favour white or yellow, fly-pollinated flowers over blue flowered bee-pollinated plants because there are more flies than bees on our mountain tops (there are certainly plenty of flies in alpine areas such as the Bogong High Plains, but that is more to do with grazing cattle than flower colour). However a study in Norway found that apart from the lack of red flowers, the colour mix as you climb a mountain does not change significantly, and there is no transition from blues to whites (there are some subtleties in these results, relating to the colours that insects detect and we don't, but the basic hypothesis was disproved, at least in this part of the world).

More broadly, evolution from insect to bird pollination is well documented in the plant kingdom, through a variety of mechanisms - including colours, shape and size - and at a variety of times and places over the history of evolution. The simplest transition is where a small change in a chemical pathway - for example the synthesis of the anthocyanin, a water soluble pigment that can be red, purple or blue depending on the pH/acidity - converts a flower from purple to red. This happened in the Morning Glory genus Ipomoea.

Individual flowers can change colour within a single blooming, again to attract or discourage a pollinator. The one I've mentioned in passing a few times in this blog is the Victoria Lily, Victoria amazonica or Victoria cruziana (the amusing and telling story of its naming is part of Mabey's cabaret). The flowers emerge white from below the water each night to attract evening insects, trapping them inside when the flower submerges in the morning. The second night the flower is flushed pink, an unappealing colour to insects and the bedraggled bugs fly away to pollinate another (white) flower.

I also recalled a more recent conversation I'd been told about, this time in a tea-room at Royal Botanic Gardens Melbourne. It was about blue and yellow flowers, which as you now know, are most likely insect pollinated. What captured the curiosity of these botanists was that many plant families or genera include species that have either yellow or blue flowers, but not many other colours. Good examples are in the Rhamnaceae (Ceonotis with blue flowers, Pomaderris with yellow) or Diuris and Iris (with yellow and blue flowered species).

Botanical geneticist Liz James said blue and yellow are 'genetically very close' so a switch is not surprising. Presumably the switch to the other colour means a different cohort of insects become potential pollinators, opening up a new niche for the plant to colonise. That said, a better answer might be found at the tea-table of a zoological research group.

Meanwhile I read on. Mabey's book has been described by a subeditor of the Guardian, as a 'hymn to flower power'. That it is. The stories won't be unfamiliar to anyone who surfs the botanical web or perhaps even reads this blog every now and then, but Mabey creates beautiful sentences and uses some charming words (which is pretty much enough for me to read a book); his writing is way better than my poetry!

Mabey's book does have a bit of the I'm-going-to-tell-you-everything-I-know-whether-or-not-its-off-piste, but I'm exceedingly guilty of that in my own blog and recent book Sprinter and Sprummer. In any case, when the stories are strong enough - and most of them are - they can bear a few diversions along the way.

What makes this book shimmer, and it does, are the personal stories: photographer Tony Evans camped beside a flower for days waiting to get the perfect picture or artist and activist Margaret Mee in Amazonia, painting a moth-pollinated moonflower in the moonlight. And like all good stories, you get fooled into learning stuff along the way.

Images:  red - Callistemon glaucus from Albany in Western Australia; yellow - Acacia drummondii from Stirling Range.; Blue - Gentiana angustifolia from Kew Gardens (and perversely, perhaps, a blue-flowered alpine plant). The Spider Orchid is Caladenia concolour. The Victoria cruziana is in the Water Lily House at Kew Gardens, with the under surface of its leaf being displayed by horticultural student Anne Rostek (I'm not sure of the photographer but it's a commonly used and freely available image).

*Occasional posts are called Plant Portraits (in brackets after the blog title and marked with an asterisk). These are usually about things other than, but including at least passing reference to, plants. Often they will be inspired by a book or something else in my cultural life. The idea is borrowed (very loosely and with due deference) from Milan Kundera's 'Novels, Existential Soundings', in his Encounters. These essays were as much, or more, about things other than the book being reviewed.


Bort said...

My hypothesis for Diuris colouration (and I don't think it's unique to me) is that it is actually secondarily related to insect vision. The hypothesis being that Diuris is a pea-flower mimic with the majority of sympatric pea-flowers being purple (eg hardenbergia) or yellow (eg pultenaea). The selective pressure on the orchid (which produces no nectar reward) is predominantly to remain morphologically similar to the pea-flowers (which do produce a reward), which secondarily constrains them to be purple or yellow, rather than direct selection of visibility to pollinators. It would be tricky to prove this syndrome, but be fun to tackle one day.

Ps. is RBG going to consider embracing the synonimisation of callistemon into Melaleuca at all? ;)

Tim Entwisle said...

Quite true re Diuris and there is a little research out there on this topic (James Insto is a name I recall). And yes re Callistemon and Melaleuca... we'll get there! Tim

Blogger said...

SwagBucks is a very popular work from home site.