WAYS OF THE WILD.

Auckland Star, Volume LXVI, Issue 22, 26 January 1935, Page 1 (Supplement)

WAYS OF THE WILD.

CHLOROPHYLL ESSENTIAL TO PLANT LIFE. (By A. T. PYCROFT.) I have been asked why grass is green. My questioner has been told that it is due to chlorophyll and wishes to know what chlorophyll is and why it is green, and also what makes the different colours which we sec. A great authority, Sir William Bragg, referring to colours, says the chemists tell us that the green of trees and grass, and plants is due to the universal presence of a certain substance which is known as chlorophyll from two Greek words meaning light green and leaf, literally leaf green. Atoms and Molecules. To mako quite clear what he is about to describe Sir William Bragg informs us that the whole world is made up of atoms, of which there arc 92 different kinds. Nature's first step in all her constructions is to assemble small companies of atoms into molecules. The atoms arc extraordinarily permanent, but

molecules can easily be broken up, and afterwards reassembled, or, it may be, grouped in new combinations. Nature in this respect is like a compositor in a

printing office who lias before him type representing the various letters of the alphabet. He puts them together to form words, cacli having its own meaning; he can make different words in numbers far exceeding that of his different letters. Ho can break up his type, distribute it and reassemble it in new ways. Likewise the number of different kinds of molecule is practically infinite. Nature has selected that which we call chlorophyll to play a part which is essential to plant life. It is certainly one of the wonders of tlio world that so narrow a selection should be made from so wide a choice, and that the chosen molecule should be used so universally for the one purpose. When the sun's rays strike leaves and grasses the chlorophyll absorbs a certain band of wave lengths lying mainly in the red end of the spectrum. It also absorbs other portions of the spcctrum to a greater or less extent, particularly the blue. What is left is reflected and scattered, and gives us the green which we know so well. It has been said that chlorophyll absorbs a certain part of the sun's light. It is, in fact, a pigment, the most important pigment in the world. There is a connection between the action of a pigment and a wave theory of light. It depends on an effect which we call "resonance," and is easily exemplified in the analagous case of sound. Sit down before the piano and gently depress one of the keys, so that the damper is taken off the strings and yet the note is not struck. Sing that note strongly, and the string will pick it up and respond. The only necessary condition is that your note must be in unison with that of the string. In this case the string is deriving energy from the sound waves sent out by vour voice. Now the molecule, like the the string, has certain natural modes of vibration or notes. Their frequencies are, of course, exceedingly great, far greater than that of the string or any other note that we can hear. But the frequency of the light waves is correspondingly great and can set the chlorophyll molecules into vibration. Just as you have roused the string to activity, so the sun sings to the leaves, gives them their energy and starts tliem on the way of living. Colours Which We See. Colours such as those of the flowers are made up of molecular types. Thee are certain forms which, slightly modified in various ways, give to flowers their different colours. The colours of hair and wool and hides, and again of minerals, earths and ochres of various kinds usc> as painting materials, are due to pigments. If we consider the colour of the sea and sky we find causcs of colour of a character entirely different. Colour is formed by a sorting process. The air is an assemblage of atoms and molecules of various gases,

oxygen and nitrogen principally, with small proportions of carbonic acid, argon and others. All tlie atoms and molecules are very small in comparison with the lengths of waves of light which we can see, even of the waves at the blue end of the spectrum. When the rays of the sun cross the atmosphere the various waves are all turned to one side. The amount is extremely small, but the air is of great extent. The blue is turned aside more than the red. So when the rays sweep past overhead, and we look up, we receive into our eyes the scattered light and we receive more blue than red, for which reason the sky is blue. Dust in the air scatters, and sometimes the particles are large enough to scatter all the colours of the

spectrum. Then the sky become? white or grey. In hot dry countries this is often the case, and when the rain come? and the dust is brought down the sky is literally washed clean and becomes blue again. The smoke of a cigarette looks blue when it rises from the burning tip, but grey when it has boon through the mouth, and become coated with moisture, so that its particlcs are enlarged. The Colour of the Ocean. The deep blue of the ocean also derives its colour from selective scattering by the water molecules. Nearer the shore the colour becomes lighter and greener on account of the scattering by particles in suspension. Of course, the colour of the sea is often influenced by the colour of the sky. A smooth sea or lake reflects the hills and c'.'uds, as in a mirror, and we sec nothing of any colour due to scattering by the water molecules. When a ripple spreads over the surface the little slopes reflect the sky above and repeat its blue, or its grey if the sky is leaden with cloud. it is in the deep, clear ocean that the true scattering is to be seen. A ray of light striking the surfaii of srn->oth, clear water is not scattered in all directions from the place where it strikes. The surface is not therefore visible, there are no details upon it to be distinguished. A tree ovcrhangii.g clear water casts no sliad">w on the surface. There is, however, a shadow if the water is muddy, because light is scattcied where the sunshine falls upon the wat-'r and the particles which it contains, and the contrast is obvious.

When a wave breaks and llie water is filled with air bubbles, the innun;riable surfaces in the foam reflect the white light entirely and sr.ronirly. »' c see the same effect where water of fh« stream falls over the stones and imprisons the bubbles of air. toa.i. * tends to white, 110 matter what colour of the liquid may be as for ample tho foam on a o" ,s , forira * also a piece of colourcJ ™„i,w ■■«•» • white streak.