A perspective by Jonathan Silvertown, University of Edinburgh
(c) Jonathan Silvertown
“By leaves we live. Some people have strange ideas that they live by money. They think energy is generated by the circulation of coins. But the world is mainly a vast leaf-colony, growing on and forming a leafy soil, not a mere mineral mass: and we live not by the jingling of our coins, but by the fullness of our harvests.”
Patrick Geddes (1919)
Truly, we live by leaves, though largely in blissful ignorance of this fact. A leaf is a solar panel, a rain maker, a lunch box, a chemistry set, a soil-maker and the inspiration of artists. The reality of our dependence on leaves is easily demonstrated, but the ways and means by which leaves themselves evolved and live are amazing, varied and the story is mostly untold. Every leaf tells a story: not the fanciful tales of future and fortune that charlatans pretend they can read in tea leaves, but natural histories shaped by the necessities of survival and reproduction that can be tested for veracity by science. Not only are leaf stories a closed book, but so too are the adventures of the scientists who discovered them. By leaves they lived.
Imagine that your next meal is sitting upon a plate in front of you. There may be actual leaves there, perhaps lettuce, green onions, cabbage or spinach but even without those, there will be other foods that owe their existence to leaves. No tomatoes, carrots, potatoes, corn, rice or beans without the leaves that feed those plants. No bread without wheat leaves. No meat without leaves, either. This is what Patrick Geddes meant when he said “By leaves we live”. These words were delivered in a valedictory address at the University of Dundee, in which the self-described “comprehensive, synthesizing generalist”, decades ahead of his time, expounded the ecological viewpoint that informed his pioneering work in economics, sociology, politics and town-planning. There are no boundaries to where leaves can lead.
Ironically, if you are a meat-eater, your diet is much more in debt to leaves than that of someone who eats only plants. It takes leaves to feed cattle, sheep and chickens, whether they are fed directly on grass or upon corn. Compared to eating plants directly, meat is a very inefficient way of turning leaves into human food, so more leaves are needed. Only 4% of the energy a cow obtains from leaves during its lifetime ends up in the meat in your hamburger. For comparison, the wheat grains that went into the bun represent 40% of the energy harvested from the sun by the wheat plant. A chicken is a more efficient converter of leaves into meat than a cow, but it only manages about 9%. My point here is not that we should live on empty burger buns, but just that whether you like lettuce in your burger or not, the burger embodies a lot of leaves.
If leaves are the real currency of life, shouldn’t we know how many there are on the planet? This is an obvious question, but no one has ever counted all of them, nor ever shall. I asked Wolfram Alpha, the artificially intelligent website that likes to answer questions about quantities, how many leaves there are. It came up with three numbers: 1558, 2633 and 2555. These are respectively, the year in which the word ‘leaf’ was first written in English, the number of written words that are used more often than ‘leaf’ and the number of spoken words people would rather use before uttering it. Proof indeed that leaves are a neglected subject.
When scientists can’t fix a number on something very big, they estimate it. One way to do this is to count the number in a small sample and then to multiply up. I asked Wolfram Alpha how many stars there are in our galaxy and, as if we live by stars, it confidently answered: 200 – 400 billion. This is calculated from the volume of the Milky Way (8 thousand billion cubic light years) times the average star density in a cubic light year. Easy. Now, if we can estimate the number of stars in the Milky Way, how hard can it be to estimate the number of leaves on one lonely planet?
But hold on, not all leaves are the same size, nor are there as many on a daisy as on an oak tree. Most plants have innumerable numbers of leaves. Has anyone ever been nuts enough to count the leaves in even a small sample of a field of grass? I confess that I have. I wanted to know how to increase the biodiversity of grassland on a nature reserve near Oxford where the habitat was dominated by just a couple of grass species, to the exclusion of meadow flowers of all kinds. So, two colleagues, James Bullock and Bronwen Clear-Hill, and I spent a couple of years repeatedly counting grass leaves in many small plots on the reserve, to see whether grazing with sheep could control the dominant grasses. Well, to tell the truth, James and Bronwen did most of the counting and James also had to put up with feeble jokes about bullocks in fields. Long story short: the grasses just kept growing back, filling every gap where seedlings of other species might be able to establish themselves.
One of the things we discovered was that leaf densities varied between seasons and between different grass species. By the end of the growing season in October, a very common grass called “creeping bent”, which, as it happens, also describes what you look like when you’ve been counting grass for two years, could reach densities of more than 10,000 leaf shoots per square metre. That is more than 100 million in a hectare. In Britain, there are 4.5 million hectares of grassland like the one we studied, so at the seasonal peak of the year, we can estimate that there are 450 trillion leaf shoots in just this one kind of grassland. Or in other words, a thousand times more than the highest estimate of the number of stars in the galaxy. In grasslands, there is an astronomical number of leaves under our very feet. In fact the number is so big that I propose that we should stop using astronomy as our benchmark for ‘huge’ and use botany instead. How many stars are there in the galaxy? Oh, a botanical number!
This is a sample from the introductory chapter to a new book that Bot Soc member Jonathan Silvertown is currently writing. More information at Jonathan’s website www.JonathanSilvertown.com or you can contact him via Twitter @JWSilvertown
 Boardman, P. (1978) The worlds of Patrick Geddes: Biologist, town planner, re-educator, peace-warrior. Routledge and Kegan Paul, London.
 Smil, V. (2013) Chapter 4. What It Takes to Produce Meat. Pages 112-175. Should We Eat Meat? John Wiley & Sons.
 Dai, J. et al. 2016. Harvest index and straw yield of five classes of wheat. Biomass and Bioenergy 85:223-227.
 This assumes chicken is fed on grain (60% conversion: Dai et al. 2016) and that 15% (Smil, 2013) of that is turned into meat, so 60% X 15% = 9%.
 Bullock, J. M., B. Clear Hill, and J. Silvertown. 1994. Tiller dynamics of two grasses – Responses to grazing, density and weather. Journal of Ecology 82:331-340.
 NERC. 2007. Countryside Survey. UK Results. Chapter 3. Enclosed Farmland: Arable and Horticulture and Improved Grassland Broad Habitats NERC.