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The Role of Lignin for Fire, Explained

Michael Denton
Photo credit: Patrick Selin, via Unsplash.

Editor’s note: We are pleased to present a series adapted from biologist Michael Denton’s book, Fire-Maker: How Humans Were Designed to Harness Fire and Transform Our Planet, from Discovery Institute Press. Find the whole series here. Dr. Denton’s forthcoming book, The Miracle of the Cell, will be published in September.

In addition to nature’s fitness for photosynthesis, the existence of large woody plants such as trees and the wood they provide for fire-making is only possible because of many other elements of fitness in nature, including the unique properties of an unfamiliar but crucial component of plant cell walls: lignin. 

Lignin is an essential component of all plant cell walls and provides the necessary element of strength for the construction of tall woody trees. Because it is highly resistant to enzymatic catalysis, its breakdown in the soil is slow, allowing the formation of humus, which retains water and minerals in the soil.1 This in turn promoted the growth of large trees and allowed the build-up of vast volumes of undigested vegetation in the Carboniferous swamps, ultimately providing the coal for the steam engines of the early industrial age. Without lignin, there would be no woody plants, no wood, no coal, no charcoal, no fire, no pottery, and certainly no iron and probably no other metals or metallurgy. 

Another “Goldilocks” Combination

Many other conditions must be met for large woody trees to flourish. First, their leaves must be able to lose heat in direct sunshine. This is achieved by a “Goldilocks” combination of fundamental physical phenomena, including evaporative cooling, convection, conduction, and radiation.2 Additionally, their trunks and branches must be made of a strong, durable material providing tensile strength (resisting stretching) and compressive strength (resisting volume reduction) to sustain bending and compressive pressures. The combination of cellulose and lignin in plant cell walls provides the necessary strength and is in all probability uniquely fit for this role. 

In addition to having strong trunks and being able to keep cool in the sun, trees must also have a method of absorbing water and drawing it to their leaves, which in the case of large trees may be many meters above the ground. Water is the matrix of life on Earth and essential for cellular physiology and, in the case of plant cells, for photosynthesis. Water is the source of both the hydrogen atoms for the synthesis of organic compounds, including wood, and of the oxygen atoms, which are released into the atmosphere. Water is also utilized by plants for evaporative cooling, one of the factors that attenuates the temperature of leaves in hot sun. 

Tomorrow, “The Amazing Circulation System of Trees.”

Notes

  1. Matti Leisola, Ossi Pastinen, Douglas D. Axe, “Lignin—Designed Randomness,” BIO-Complexity 2012 (2012). 
  1. James C. Forbes, Plants in Agriculture (Cambridge; New York: Cambridge University Press, 1992), see figure 4.18, page 100, and section 4.9.1, “Thermal injury and its avoidance”; Hans Lambers, Plant Physiological Ecology, 2nd ed. (New York: Springer, 2008), 225–235.