We take plants entirely for granted, farming them for food and timber, cultivating selected garden plants for pleasure and chemically killing others (“weeds”) that don’t please our eye, and so on. We have no concept of plant welfare in the way we have of animal welfare – no concern for the comfort and state of mind of the individual plant.
"That's simple, genius," you may retort, "plants don't have feelings or minds." But, things are not that straightforward. Plants are incredibly successful and sophisticated life forms and may have inner lives. Michael Pollan wrote a fascinating review titled The Intelligent Plant in The New Yorker (December 23rd, 2013).
Plants are easily the dominant life form on Earth, constituting 80 per cent of total biomass – bacteria constitute 13 per cent and fungi 2 per cent. Humanity is only a trace life form on Earth, constituting about one-10,000th of the total biomass.
Biological life is broadly divided into autotrophic and heterotrophic organisms. Plants are autotrophs and animals are heterotrophs. Plants are much more biochemically versatile than animals. Autotrophs can synthesise their own food from simple chemicals available in the environment (CO2, N2, H2O). Heterotrophs cannot do this and must eat other organisms, both plants and animals, for nutrition.
Green plants inhale CO2 from the air and, using the energy of sunlight, combine it with water to synthesise glucose in a process called photosynthesis. Glucose is then “burned” (oxidised) to generate energy for the plant and also to make cellulose and other biochemicals needed by the plant.
Plants can survive removal of 90 per cent of their mass
Autotrophs are the primary food producers, occupying first place in the global food chain. Heterotrophs come next – herbivores (plant eaters) feed on autotrophs and are placed second on the food chain; carnivores (meat eaters) and omnivores (eat all types of organisms), including humans, are placed on the third level of the food chain.
Oxygen is released to the atmosphere when glucose is photosynthesised. Animals (and plants) inhale oxygen and use it to “burn” glucose and other fuels. This burning process releases carbon dioxide to the atmosphere, to be inhaled by plants. All life on Earth lives in one big interdependent cycle.
Animals can either fight or flee from predators. Plants cannot flee but have evolved very sophisticated means of fighting predators. For example, a plant can secrete chemicals that poison the predator or render the plant taste extremely unpalatable. When attacked by caterpillars, some plants, eg corn, secrete volatile chemicals that attract nearby wasps to feed on the caterpillars.
Because plants are so susceptible to predators, their body-plans are designed very differently to animals. Various vital functions are localised to specific sites in animal bodies. If an animal loses a significant part of its body it dies. Plants, on the other hand, have a delocalised organisation and the plant can survive removal of 90 per cent of its mass.
Senses
Animals have five senses – sight, hearing, touch, taste, smell. Plants have their own versions of these senses. Sight: plants respond differently to different types (wavelengths) of light. Hearing: playing sound recordings of a caterpillar chomping on a leaf stimulates the plant to secrete anti-caterpillar toxins. Touch: some leaves eg Mimosa pudica, curl up when touched. Smell: plants secrete and detect volatile odour molecules, essential for survival strategies such as deterring pests and attracting pollinator bees.
Are plants intelligent? If we define intelligence as the ability to solve problems, eg defence strategies against predators, plants may show intelligence. Also, plant root tips are extremely versatile. They can sense gravity, moisture, light, pressure, hardness, volume, nitrogen, phosphorous, salt, toxins, microbes and signals from neighbouring plants. Roots approaching an impenetrable object or a toxic substance change direction before making contact. Roots can tell whether nearby roots are self, kin or stranger and compete with strangers for root space but share resources with kin. Forest trees recognise offspring and funnel nutrients to them until they are big enough to fend for themselves.
A major difference between plants and animals is speed. Plants are stationary and interact with the environment slower than animals do. Otherwise plants live using a range of capacities and senses analogous to animal capacities. If aliens, living in a vastly speeded-up dimension, arrived on Earth, we would look to them like plants look to us. I’m sure we would like them to ponder us a little more deeply than we ponder plants.
William Reville is an emeritus professor of biochemistry at UCC