Field Bindweed – Convolvulus arvensis – busy doing what?
Common sense, ‘facts’, things taken for granted, taken as read: the history of science shows that all these can be swept aside like old toys by some confident new research. The idea of unchanging, fixed species? Darwin saw to that. Solid matter? Smashed by nuclear physics. In botany? Well, Darwin and evolutionary theory are still transforming our view of how plants came to be what they are today. However, considering that much less funding goes to botanical research than other sciences, I wonder if other surprises still await us. Yes, the way a seed germinates and grows is clever, but after that, plants just stand there alone, stupid, blind, waiting to be picked, or eaten, or trodden on, or strimmed – don’t they? Isn’t that how they’re different from animals?
I’d like to hazard a guess at what a new surprise might be: plants have a social life. Since I’ve taken more time to observe plants and read about them in the last year or so, my view of them has changed. Here are some examples.
Plants communicate. Not in the ‘talk to your geranium’ sense, or the ‘scream when they’re cut’ sense, but more commonly with chemicals. In the book I reviewed in my last post, Weeds, Richard Mabey writes that
The air and the soil are busy with constant streams of chemical messages – plant pheromones – designed to deter predatory insects, seduce pollinators, kill off competitors, encourage companion plants and warn other plants of insect attack.
These pheromones can be volatile compounds evaporating from the leaves or soluble chemicals exuding from the roots into the water in the soil. The roots of field bindweed (Convolvulus arvensis – in photo at top) secrete something which inhibits the germination of most grain crops. The seeds of the striking thornapple (Datura stramonium ) can release chemicals which inhibit cabbages and tomatoes.
Thornapple – Datura stramonium – the poisonous weed that looks like a garden flower
The growing tip of the plant dodder, which is parasitic on tomatoes, spirals round till it senses tomato leaf chemicals and then grows straight toward their source.
Plants are interdependent. Gardeners often use the phrase ‘companion plants’ to describe plants which grow well together: my Agenda du jardinier bio (organic gardener’s diary) lists dozens under ‘Voisinage favorable’: plant garlic near tomatoes but away from artichokes, celery near beetroot but away from salads etc. Pheromones may be at work here too.
An even closer association is between plant roots and beneficial fungi – I had heard of truffle oaks of course, but I was surprised to read in a botany textbook that ‘most higher plants have an association with soil fungi’. Yes, ‘most higher plants’: estimates are as high as 95%. A root which cohabits (‘is infected with’ seems too value-laden a term) with a symbiotic fungus is called a mycorrhiza. The fungal threads can cover a huge area and help the plant source scarce minerals such as phosphates and nitrates, as well as water. In return the fungus receives carbohydrates from the green plant. This short clip shows how it works:
A plant that combines many of these features is Cistus monspeliensis, which I wrote about on this blog here. As well as helping absorb nutrients, the fungus on its roots secretes a toxin which stops other seeds germinating – and it’s true that each Cistus usually sits in a bare patch of ground.
To give a few more examples, they’re also particularly important in trees of northern temperate areas, such as oaks, birches and conifers; and in heathers – Erica and Arbutus. Many orchids can’t even germinate without a particular fungus, which may account for their appearance in patches, from seeds germinating within the area of ground which contains fungus. This makes evolutionary sense: plants originated in the seas and first colonised wet areas. Fungal help would have been invaluable in spread to drier habitats, and once the solution was found, why evolve another?
Here’s forestry specialist Professor Suzanne Simard explaining that a forest is really a community, whose members have different roles:
You can make the most of mycorrhizae in organic gardening by inoculating your seeds and plants with fungal spores: see here:
One point I came across often is that industrial-scale grain growing goes against this process: the grain-bearing species are the least likely to have mycorrhizae; they therefore need higher levels of chemical fertiliser than other crops; and application of fungicides and other processes further reduce the biological activity of the soil. All in all, we’re getting some insights in how to live with Nature, which, as Richard Mabey has said, is bigger than us.
In the second part of this theme, I’ll look at the lifestyles of plants – and their relationship with humans. Meanwhile, what more appropriate song title for this post than Stevie Wonder’s 1979 ‘Secret life of plants’?