Proc. Nat. Acad. Sci. USA Vol. 68, No. 11, pp. 2875, November 1971

Photosynthesis Bicentennial Symposium: Introduction by the Chairman

KENNETH V. THIMANN Crown College, University of California, Santa CruG, Calif. 95060

In early August 1771 Joseph Priestley, chemist of Birmingham, mainly from the underside of the leaves. But the biological England, and codiscoverer of oxygen, performed his famous "climate" was not ready for the correct interpretation of the experiment with the mouse and the mint plant. This experi- bubbles and Bonnet's discussion was wholly in terms of heat- ment provided the beginnings of our understanding of that ing and the consequent expansion of gases at the leaf surface. remarkable process whereby the organic matter of our bio- In the following symposium Eugene Rabinowitch presents sphere is produced and our atmosphere continuously purified. It is perhaps not widely known that the discovery was al- the sequential development of the leading ideas in the action most made some 20 years earlier by Joseph Bonnet, another of light on leaves, and then the current developments in the very active researcher of the time. For Bonnet observed that physiological, biochemical and photochemical aspects of the when leaves were immersed in water and exposed to sunlight, field at the present day are summarized by some of the leaders bubbles of gas were produced. He even noted that they came in the study of these aspects of photosynthesis in this country.

Proc. Nat. Acad. Sci. USA Vol. 68, No. 11, pp. 2875-2876, November 1971

An Unfolding Discovery

EUGENE RABINOWITCH State University of New York, Albany, New York, 12203

About 1648, a Dutch alchemist, van Helmont, grew a willow "Experiments and Observations on Different Kinds of Air". tree in a bucket of sand, and found that the sand weighed as This book included certain experiments on animals and much afterwards as it did before the tree had grown to sizable plants, among them the discovery that plants can restore the proportions. Although the law of conservation of matter was air that had been made unfit to support the life of animals established only a hundred years later (it was proposed by by burning candles in it. When a candle had burned in an zen- Lomonosov in Russia in 1748, and by Lavoisier in France in closed space until it went out, a mouse placed in the remaining 1770), van Helmont wondered where the material of which the air soon died. On August 17, 1771, Priestley put a sprig of mint tree was formed had come from, and decided that it must have into a quantity of air in which a wax candle had burned out been from the water used to wet the sand. and found that on the 27th of the same month, another candle In 1727, Stephen Hales, an English minister-naturalist, sur- burned perfectly well in it. A mouse was found to survive in mised that plants drew -a part of their nutrition from the air, this "restored?" (or, as it was then called, "dephlogisticated") and that sunlight may play a role in "ennobling the principles air. of vegetables". In 1971, we celebrate the 200th anniversary of this momen- Between 1770 and 1785, several chemists in England, tous experiment. France, Germany, and Scandinavia devised methods to catch, However, Priestley's observation was only the first step in transfer from vessel to vessel, and study the properties of dif- the unveiling of that basic process of life on earth that we now ferent gases. Oxygen, nitrogen, chlorine, carbon monoxide, and call photosynthesis. What he had shown is that plants con- carbon dioxide were discovered; the composition of air and vert "phlogistic" into "dephlogisticated" air-we would de- that of water were established by their experiments. scribe it now as replenishment with oxygen of air that has One of the most significant discoveries of this period, often become deficient in it; he had not shown what were the condi- called the age of pneumochemistry, was the discovery of oxy- tions needed for plants to do this. gen by Joseph Priestley, a nonconformist English minister, In the spring of 1779, Jan Ingenhousz, a Dutch court physi- who ended his stormy life as a self-exiled refugee in Pennsyl- cian to the Austrian Empress Maria Theresa, rented a villa vania. In 1776, he published an epoch-making book called near London to pursue Priestley's discoveries. In three summer 2875 Downloaded by guest on September 30, 2021 2876 Rabinowitch Proc. Nat. Acad. Sci. USA 68 (1971)

over 500 before the end of months, he performed experiments; In 1796, Ingenhousz, who at first disbelieved Senebier's dis- the year, a book entitled "Experiments Upon Vegetables, Dis- photosynthesis, the Common Air in coveries concerning the role of "fixed air" in covering Their Great Power of Purifying reversed himself and proclaimed that carbon dioxide, taken Sunshine, and Injuring It In the Shade and at Night", was discovered that plants im- up from the air, is the only source of all organic matter pres- written and published. Ingenhousz ent in plants (and thus, indirectly, also in animals). This prove the air not slowly, by "vegetating" in it (as Priestley had Aristotelian belief that thought), but rapidly, by a "photo-chemical" reaction, taking meant breaking with the traditional He plants draw their nutrition from the "humus" of the soil. place in green leaves and green stalks exposed to sunlight. doctor, Julius Robert Mayer- thus identified the green plant pigment (which we now call Finally, in 1845, a German factors without one of the codiscoverers of the law of conservation of energy- "chlorophyll") and sunlight as two catalytic in addition to acting as a gigan- which plants cannot "improve the air". realized that the plant world, in Ge- tic chemical factory, which produces organic matter from in- In 1782, a Swiss pastor, Jean Senebier, published, a giant power station, neva, a three-volume treatise under the title "Physico-Chemi- organic raw materials, represented also in converting sunlight into chemical energy. The stored energy is cal Memoirs on the Influence of Solar Light Changing the or Nature of the Three Kingdoms, particularly that of the Vege- released suddenly in the combustion of wood or fossil fuel, table Kingdom". What Senebier added in this work to Ingen- slowly in respiration of plants and animals, and is by far the was the discovery that plants im- most important source of life energy on earth. housz' earlier observations became prove the air only if they are provided with a gas then called With this discovery, -the equation of photosynthesis "fixed air", which we now call carbon dioxide. Considering the complete: minute amounts (0.03%) in which carbon dioxide is present in the atmosphere, this was a remarkable discovery. -CO2 + H20 + Light 02, In 1804, another Swiss scholar, Nicolas Theodore de Saus- (Senebier) (de Saussure) (Ingenhousz) (Priestley) sure, who was one of the earliest chemists to make careful ana- + Organic Matter + Chemical Energy lytical measurements (and was armed with the knowledge of (Ingenhousz) (Mayer) the law of conservation of matter), published a treatise called "Chemical Studies on Vegetation". He determined that the The discovery of photosynthesis took (not counting the pre- sum of the weights of the oxygen produced by plants and the scient suggestions of van Helmont and Stephen Hales) from organic matter formed in them is considerably larger than the 1771 to 1845, and involved one Englishman, one Dutchman, weight of carbon dioxide consumed, and surmised that the dif- two Swiss, and one German. Two were churchmen, two medical ference must have been supplied by water-thus reviving the doctors and one a professional chemist-a characteristic ex- insight of van Helmont (as Ingenhousz had revived the in- ample of the international and interdisciplinary nature of ma- sights of Stephen Hales). jor scientific progress. Downloaded by guest on September 30, 2021