PHOTOSYNTHESIS (Neo-Lat., from Gk.
cpt.7.)s, plus, light synthesis, tion). The process by which the green parts of plants construct carbohydrates under the influ ence of light. Carbohydrates are substances like sugars, starch, etc., which serve as the most im portant foods, both for plants and animals. The materials out of which the plant builds these foods are carbon dioxid and water. The idea cur rent in the middle of the last century. and per sisting even yet, that plants obtain their carbon compounds chiefly from the soil is erroneous. Few of the materials absorbed from the soil are fairly to be considered as foods. Carbon dioxid is present in normal air in minute quantities, usually less than three parts in 10.0011. Yet it is absorbed by the aerial parts of plants, particu larly the leaves. in sufficient quantities. because it finds its way by diffusion at high speed through the minute openings in the epidermis (stomata, q.v.) into the intercellular spaces which are bounded by moist cell walls. (See AEltAriox.) In the water of these it readily dissolves and dif fuses to the interior of the cells. where muler proper illumination it is being; used as rapidly as it be absorbed. (See It is not possible for the carbon dioxid to he dissolved in any considerable quantity by the superficial cells themselves. since the walls of these arc more or less completely waterproofed to prevent evaporation. This effectually prevents solution and consequent diffusion of the carbon dioxide. The water necessary for food-making is obtained by absorption (q.v.) through the roots. The amount requisite for this purpose, however. is very small in comparison with the great quanti ties needed to replace that lost by evaporation. (See TRANSPIRATION.) Carbon dioxide dues not merely dissolve in the water, but enters into loose chemical union with it. producing carbonic acid, which is highly unstable, as compared with the very stable carbon dioxide. Then-fore it may he readily decomposed and its components rear ranged to form a new substance. This is done by the living protoplasm.
The necessary energy is only obtainable when the plant is supplied with light of sufficient in tensity. To obtain the requisite energy plants have developed in the cells lying near the sur face mimite protoplasmic bodies called chloro plasts, holding various green and yellowish pig ments called colhbetively chlorophyll (q.v.). These pigments absorb only certain wave lengths of light, as shown in the accompanying absorp lion spectrum (Fig. 1). They derive their sole significance from their association with the living protoplasm, or enzymes (q.v.) produced by it. Like chemical action may he produced by proto plasm when energy is supplied to it in other ways, or when radiant energy of different wave lengths than that absorbed by chlorophyll is ob tained. Thus certain bacteria containing purple pigments absorb of the invisible ultra-red rays, and these organisms are said to produce carbohydrates.
It is possible to determine in sonic measure the influence of light Wa ye,: of different leimths. It
appears that the most efficient lengths are those which appear as red, orange, and yellow' to our eyes. It has been clearly shown that absorbed light supplies the energy and that the chloro 'Mists do not act merely as a screen to cut off in jurious or useless radiations. a theory which was mime maintained with some vigor. The intensity of light adequate for photosynthesis approaches that of the brightest diffuse daylight. No plants are able in nature to secure sUllielent carbon dioxide to utilize all the energy obtained from direct sunlight, though ninny of them are adapted to such conditions. Photosynthesis is nearly pro portional to the intensity of the light. and though it is observable by proper methods in twilight. it has not. yet been observed in so weak illumination as full Moonlight. Artificial lights of various kinds are efficient in proportion to their intensity. Horticultural experiments in the United States and EnJand have shown that electric light may be used for foreino• certain plants in winter to a more vigorous development of foliage and vegeta tive organs. See Et.eCra0-r1 LTVRF of PtAxrs Scarcely any two cells of the plant receive the same amount of light. Those. therefore, which lie on the upper side and are first reacted by the light are in the most favorable position for photo synthesis. As many as possible are developed by leaves in this position. Ideed, the palisade re gion (see pcnds for its mode of ment on the action of light. The internal yells. receiving only that which has passed one or more external layers, work at a slower rate, although they are usually better situated for receiving an adequate supply of carbon dioxide. %%inflow plants. receiving the light only a very lim ited portion of the sky, at most 30 to 50 per cent., are more or less starved, because the energy received is inadequate in amount for the neces...ary food-making. A comparison of the total energy received by the leaf and the amount stored in the carbohydrates formed show, that a relatively small amount of the kinetic energy has become Potential in these eompounds. Inability to determine the exact quantity of carbohydrate formed and to separate the energy absorbed by the ehloroolasts from that :absorbed by the other portions of the plant make such computations un certain. From them, however, it appears that 1 per cent. m r less of the total energy received by the leaf reappears in the starch of starch-pro thwing leaves. The products of photosynthesis are probably unlike in different plants. Stan' is often spoken of as the chief product of this process. It is probably not only a secondary' product of photosynthesis, but it is certain that many plants do not form it at all. :Most of the larger seaweed., for example, make other carbo hydrates and mach of the carbohydrate food in the ordinary land plants is in the form of sugars, cellulose. gums. etc.