Dr. James C. Jackson wrote: "I think it may be said with perfect truth, that no living organism, of whatever species, whose subject has a brain, a pair of lungs, stomach, bowels and back-bone, can ever be equal in the exhibition of capacities, if it be kept in shaded sunlight, to what it would be if permitted to follow out its own habits in unshaded sunlight. * * * Superior qualities are uniformly found existing in animals of the same species as these live in unshaded sunlight. This is just as true of humans as it is of animals; whoever lives habitually in the sunlight grows strong. This is not only true of the body itself in its various parts, but is true of the intelligent and responsible faculties which reside within the body. If women lived in the open air as much as men do, they would have capacities as much greater than now as men have now greater capacities, than they would have if they lived in homes like women."--How to Treat the Sick Without Medicine.

The manner in which sunlight is used to produce the effects that follow is not well understood, and many theories, some of these very ridiculous, have been offered to explain its use. That it is used in some way similar to the uses of vitamins seems certain to me. I look upon it as a catalyzer and its action that of catalysis. A catalytic agent or substance is one which possesses power to instigate a chemical reaction without itself being transformed or destroyed in the course of the process.

Most of the chemical changes with which the chemist is familiar require something to "touch off" the reaction. Thus explosives require a jar or a shock to cause them to explode. Hydrogen and oxygen gases, if mixed in the dark do not unite. If mixed in the light they unite explosively. Photography is based entirely upon the power of light to instigate chemical change or reaction. That plants and animals make use of this power of sunlight is certain.

Sunlight is vitally important in the nutritive processes of both plant and animal life. Perhaps we cannot call it a food, but we can, at least, call it an accessory nutritive factor. Its office would seem to be somewhat like, if not identical with that of the vitamins. Take away sunlight and all life upon earth would perish. In the tropics, where the sunlight is most abundant, life exists in greatest profusion. In those portions of the earth where nights are longest and days are shortest, and where long winters prevail, life is either absent altogether or it consists of poorly developed forms.

Under the influence of light, plants both excrete and absorb oxygen. The absorption of oxygen goes on continuously; but its excretion takes place only when the plant is exposed to light. The plant leaves absorb carbon dioxide from the air. They employ the carbon in producing starches and sugars and release the oxygen which may again be used by animals.

Light enables plants to assimilate carbon dioxide and convert it into plant substances. The carbon dioxide is transformed into formaldehyde and this in turn is polymerized to sugar by the action of light. A carbohydrate is thus formed by plant metabolism under the influence of light.

Photosynthesis is the manufacture of carbohydrates out of carbon dioxide and water in the chlorophyll-containing parts of plants exposed to sunlight. Both chlorophyll and xanthophyll are associated in the process of photosynthesis; chlorophyll being the most important. The radiant energy acting in the synthesis of carbohydrates has been shown to be located in the visible spectrum. Red, orange and yellow light rays are considered most important light rays in plant assimilation, blue and violet show the least synthetic energy.

Green leaves chiefly absorb the red rays and only absorbed rays are chemically active. It has been said that, "It is the red rays which make a green world; it is red rays which make life possible; and the rosy cheek is in truth on fire with the red light hidden in the green leaf."

The starch of fruits and some plants, like cane and beets, is converted into sugar in the ripening process. This conversion requires the action of light. Heat will accomplish part of the work, but the perfection of these sugars requires the work of ultra-violet rays.

Sunlight is essential to the production of both the green coloring (chlorophyll) of the leaves and the many colors of the flowers, stems, leaves and fruits. The beautiful colors of flowers cannot be produced or perfected without light.

Sunlight helps plants to tear down compounds and to synthesize new ones. Both phases of catalysis are represented in its work. It aids them in transforming one of its products into another. The chemical effects of light are related to the processes of photo-synthesis, photolysis, photopolymerisation, photo-oxydation, and reduction, photoisomerisation.

An examination of a leaf in the early morning reveals little or no starch. After a few hours of exposure to the sun's rays, plenty of starch will be found in it, the quantity increasing with the length of exposure to the sun. If two pieces of cork or cardboard are pinned closely to opposite sides of the leaf, the covered part of the leaf will be much whiter, in a few days, than the rest; and if the rest of the leaf is left exposed to the sun's rays, a test with iodine will show the presence of much starch in the healthy green part, while the paler or covered part contains little or no starch.

A plant kept in darkness grows colorless, flaccid and stunted. Given sunshine, it soon regains its color and unfolds bud, leaf, flower and fruit. Moss, mould and fungi are all that can grow in a cave. Deprived of sunlight, the plant dies outright or puts forth a sickly, colorless growth. If any rays of light chance to filter through the coverings of the plant, the plant will bend towards the light in its effort to receive the little understood, but nonetheless actual benefits of the light. If it fails, it soon withers and dies. The pale colorless plant deprived of sunlight, is said to be etiolated.

The stalk of a potato that sprouts in the cellar will be as white as chalk and as tender as bleached celery and the substance of the potato will be exhausted without a new vegetable being formed. Put the potato out-of-doors, where it will receive sunshine, and it will put forth green leaves, its stalk will become thick and strong, it will grow and produce more potatoes.

Etiolation is the change in appearance and structure of the plant caused by growth in absence of light. Chlorophyll is lacking in etiolated dicotyles and monocotyles, and its absence makes the yellow pigment, carolin (formerly called etiollin), evident. Red light, free from blue or violet rays, produces all the etiolation, except lack of chlorophyll. The more refrangible portion of the spectrum is the important portion in determining growth and structural modification in plants. Etiolation is not limited to monocotyles and dicotyles, but appears in gymnosperms, ferns, mosses, algæ and fungi.