Part I. Colour and appearance of photosynthetic organisms

The driving forces leading to the continuing optimisation of biological colour on Earth are, first, colouration by default, arising from the reflection of unwanted radiant energy in energy‐absorbing systems; second, colour which has evolved from the combination of the light reflected from one organism and colour vision of that organism or another; and third, colour which arises from a coloured biochemical which has been optimised for a purpose other than colouring. Biological organisms must live in harmony with their environment. Specialist organic chemicals have evolved to maximise absorption of energy at those wavelengths controlling the life‐support systems of primary producers, for example, photosynthetic bacteria and plants. The energy absorbed is governed by the organism's pigments, which have evolved through adaptation to its environment. Red seaweed is an example of the successful application of this principle. It is proposed that leaves of plants have evolved to be green in a similar manner. The action‐spectrum peaks for two‐stage photosynthesis, phototropism, photoperiodism, and stomata opening all occur at those wavelengths at which skylight and light transmission through leaves are at their maxima.