Something special happens when the human eye and green leaves meet. The investigative eye sees one thing and the imaginative eye another.

All humans share an evolutionary legacy beneath the thin veneer of educational, cultural and gender differences. We can assume that, abnormal pathologies such as colour blindness excepted, each human eye is much like every other in the way it collects light, turns light excitation into nerve impulses and sends impulses to the visual centres of the brain.

The cognitive aspect of seeing is another matter altogether and can sometimes persuade us that the eye itself must be somehow built differently in different people. Both Goethe and WE Gladstone, no less, thought that the eyes of the ancient Greeks might have been physically distinct from our own.

They were puzzled by the very few references to the colour blue in their literature (the word occurs only twice in the whole of Homer, and one of these alludes to the colour of Hector's hair). Perhaps the most famous passage from the Odyssey runs ...and flashing-eyed Athena sent them a favourable wind, a strong-blowing West wind that sang over the wine-dark sea...

The colour-world of the Homeric Greeks is (to us) a strange one, where the sea is red, hair is blue and the colour of tears, honey and even blood is chloros (green).

It isn't necessary to travel far in time and space to find many such alien world-pictures. Here in Wales a common name for houses and farms like the one pictured is Maes Glas - maes meaning field and glas blue. So are the fields really blue in Wales?

The answer lies in the way the Welsh language divides the continuum of colours from green through blue to grey and on to brown. Just three words cover the range - gwyrdd, green (but not the green of fields); glas, for all the tones from the vivid hue of the rain-soaked turf through to the edge of grey (glas is also the Irish for blue and in Irish, rabbits are glas); and llwyd, grey or brown. Blue, green, grey, brown - the colour world of the rural Celt.

The colours of foliage run the full range from the dark, glaucous turquoise of some pine needles to the luminous yellow of ginkgo in autumn. The English language historically provides us with a marvellously rich vocabulary to describe this span - colour names like orpiment, bice, smaragdine and crash.

Can it really be that human visual acuity has generated the need for the four hundred or so (according to Roget) adjectives and pigment names that describe greenness and yellowness? A plant breeder, and particularly a breeder of grasses, will certainly encounter a vocabulary-testing range of colour variation in the populations and treatments from which the best lines will be selected.

Green is also significant for colour theory and practice in the visual arts. In the Ostwald system, influential in twentieth century European painting, more than a quarter of the colour circle is taken up by shades of green.

In his book Concerning the Spiritual in Art (1910) Wassily Kandinsky wrote: Green is like a fat, very healthy cow lying still and unmoving, only capable of chewing the cud, regarding the world with stupid dull eyes. Perhaps it was his exasperation with the (perceived) placidity of green which caused him largely to eliminate it from his later works.

Kandinsky notwithstanding, there is good reason to think of green not only as a particularly active and vital colour (the colour of plant growth, which is necessary to support all life on earth), but indeed as a dangerous one.

When chlorophyll is present in leaves and absorbing sunlight, but for some reason photosynthesis (the conversion of carbon dioxide and water to oxygen and sugars) is prevented, the result is that so-called active oxygen species are produced. These damage membranes and other components of the leaf, and the result can be death of the cells. So under the wrong circumstances chlorophyll can turn into a destroyer.

The same principle is used when some compounds related to chlorophyll are used as anticancer drugs, designed to kill malignant cells by means of the active oxygen species they can generate.

Linguistic and cultural clues tell us that something special happens when the human eye beholds green foliage. Physiology and genetics help us to understand why this should be.

The Old World apes (including Homo sapiens ) have true three-colour vision. In the retina of the human eye there are cells receptive to visible light of short, mid and long wavelengths (the S, M and L receptors respectively). The wavelength sensitivity of each receptor type is determined by whether it possesses the S, M or L variant of the light-absorbing protein opsin. If we graph the wavelengths of light reflected by a typical green leaf, we find that the resulting curve sits squarely on top of the sensitivity spectrum of the L receptor.

 It is pretty certain that this convergence is not an accident. Molecular genetics shows convincingly that the S, M and L variants of opsin evolved from a common ancestral form by stepwise changes in the DNA sequence of the corresponding genes.

It is believed that a major factor responsible for evolution and spectral tuning of the L form of the receptor was green leaves, the dominant feature of the colour environment of our primate ancestors. Significantly, the L receptor of trichromatic animals such as goldfish, over which foliage would not have had a strong tuning influence during evolution, is not aligned at all with the leaf reflectance spectrum.

The gene for S opsin is on human chromosome 7. Genes for L and M opsins are arranged in tandem on the X chromosome, which is why red-green colour blindness is sex-linked and much more common in men than in women.

It may also (though this may be straying into dangerous gender politics territory!) explain the richness of the colour environment, perceptiveness and vocabulary of women compared with men.

It is certainly true that all male squirrel monkeys, and all homozygous females, are typical New World primates in being dichromatic. But some heterozygous females with variant genes that give them one M and one L allele have full trichromatic vision. It is said that the rest of the social group follow these individuals around because they are better at spotting the choicest fruit against the background of tree foliage.

The red channel of human vision seems to represent a direct line of communication between foliage and the brain. Is it too speculative to suppose that experiencing the greenness of plants is essential for mental wellbeing?

Humans evidently have a need to surround themselves with green plants (David Lee calls this urge chlorophilia), even in an era when an increasingly urbanised population has largely lost touch with the agricultural origins of the food it eats and the clothes it wears. Evolution may have left humans with an intrinsic mental itch that only visual contact with green plants is able to scratch.

We seem to be genetically, physiologically and psychologically designed to respond to leaves in a special way. A consequence of this is that the eye is a particularly sensitive instrument for quantitative botanical work.

There are commercial electronic instruments available that can measure non-destructively the greenness of a leaf, such as the Minolta SPAD, a device widely used in agriculture. However, an experienced plant scientist can estimate greenness by eye with a statistical accuracy that matches the SPAD and in some cases can even exceed it.

Superb instrument as the eye may be, we would like something even better. A spin-off from our participation in the Grass Art project with Heather Ackroyd and Dan Harvey has been an exploration of digital imaging and computer processing to increase colour discrimination to many times the sensitivity of the human eye.

In conclusion, we may like to convince ourselves that when we choose plants for their colour, we are wielding the power of selection over mere passive vegetation. But before humans were humans, plants were choosing the individuals who could most sensitively respond to their colours - by driving evolution of the visual system.

From the scientist's perspective, human eyes are still pretty good tools for plant selection and improvement although new methods of genetic analysis can use more and better data than eyeballing can provide.

We think the answer lies in the kinds of imaging technologies inspired by (literally) new views of leaves that have come from working alongside artists in the medium of grass. Our scientific viewpoint may in turn catalyse new directions for their artworks.

In this sense, the contemporary dialogue between artists and scientists reflects the age-old interplay of human evolution and plant domestication.