The unfolding of a green leaf is a miracle of the earth. At its heart is the secret of life itself, and the explanation for how plants grow.
Each leaf is an economic unit powered by the sun.
Supported by vessels (veins) that transport water and nutrients a leaf transforms carbon dioxide (CO2) into carbon (C) and oxygen (O2), makes sugars (glucose) from starches, and dissolves them in water to be transported through vessels (veins in leaves and wood vessels in the tree trunk) to build cellulose; and then, more leaves, stems, wood, flowers, fruits and seeds.
Leaves are central to the survival of life on earth.
From the canopy of mighty rainforest trees to the shrubs, soft herbs and grasses that feed us all, leaves are the fundamental units of production and the ultimate expression of sustainability. As long as you breathe you should care about leaves!
Photosynthesis is the link between the Sun and life on Earth.
The whole process of photosynthesis is driven by energy from sunlight.
Photo = Light (energy) and Synthesis = to blend or create; together mean ‘to create using light’.
The overall process of photosynthesis is described by the equation:
6 H2O + 6 CO2 + sunlight → C6H12O6 + 6 O2
The most important chemical reactions in photosynthesis involve the release of oxygen from water, and the breaking up of carbon dioxide to help create glucose (sugars) (C6H12O6) from starch.
In the wet rainforest, water sitting on the leaf surface can slow gas exchange and inhibit the capture of sunlight.
Plants use different methods to keep their leaves free of water. Some have waxy and shiny leaf coatings, while others have hairy surfaces that repel water. In the rainforest one of the most distinct leaf features are drip tips.
Drip tips occur at the apex of the leaf, where the central vein leads to the leaf point.
Leaves generally narrow to the tip, and elongate to form the drip tip. Water that falls on the leaf (either as rain or mist) gathers on the surface and is moved toward the leaf tip by gravity. Water accumulates at the edges of the leaf and when the leaf can no longer support the weight of the water, droplets separate off. Leaf margins can be entire with a single drip tip, or leaves can be serrated with each tooth having the ability to shed water.
While there is inevitably some competition for water and other resources in the rainforest, the main competition is for sunlight.
Leaves in the closed canopy of the rainforest can be so tightly arranged that less than 5% of available light reaches the rainforest floor. This means that smaller plants in the understory layers need to be shade tolerant and creative about how they capture light.
Leaf size is one of the important strategies plants use to increase exposure to available light. Tropical and subtropical rainforest plants have particularly large leaves. By aligning leaves toward the sun and by having a larger surface area, sunlight capture by plants can be maximised. When day and night time temperatures decrease in response to increasing altitude and/or latitude, leaf size also decreases.
Rainforest plants tend to have dark green leaves, and this relates directly to the amount of light they can absorb. Black (or dark) objects absorb all light while white (or lightly coloured) objects reflect light. Therefore, darker leaves absorb more light. Rainforest plants growing in the shaded understory can have red or purple leaf undersides that allow them to absorb reflected light, or light with longer wavelengths, and leaf forms that align and maximise exposure to light at different times of day.
- Richards, P. 1996. The Tropical Rain Forest: An Ecological Study. (2nd) Cambridge University Press, UK.
- Royer, D.L., Kooyman, R.M., Little, S.A. and Wilf, P. (2009) Ecology of leaf teeth: a multi-site analysis from an Australian sub-tropical rainforest. American Journal of Botany 96: 738-750.
- Wright, I.J., Dong, N., Maire, V., Prentice, I.C., Westoby, M., Díaz, S., Gallagher, R.V., Jacobs, B.F., Kooyman, R.M., Law, E.A., Leishman, M.R, Niinemets, Ü., Reich, P.B., Sack, L., Villar, R., Wang, H., Wilf, P. 2017. Global Climatic Drivers Of Leaf Size. Science 357: 917–921.