104 



mar)' place in the economy of life. It is 

 the process by which the energy of the 

 Sun is captured and converted to the 

 uses of the living cell. It is, in addition, 

 tlie beginning process in the transfer 

 of atoms from the inorganic world to 

 the organic. It serves as the very bridge 

 of life— the bridge by which star stuff 

 passes over and becomes life stuff. 



In this alchemy through which the 

 nonliving is converted into the living, 

 the green plant stands almost alone. 

 Neither the human body nor that of 

 any animal is able to synthesize sugar or 

 other organic compounds from inor- 

 ganic materials. The animal kingdom 

 thus is parasitic on the vegetable. There 

 are certain micro-organisms, such as 

 purple and green bacteria, which trap 

 sunlight and use its energy to make 

 carbohydrates, but their production is 

 small in comparison with the vast out- 

 put of green plants. To live, an animal 

 must either eat vegetation or else eat 

 other animals which have fed on vege- 

 tation. There is no other bridge to the 

 free energy and rich material stores of 

 the physical world. 



A TRAIN OF DISCOVERIES 



The discovery of photosynthesis 

 and the exploration of its hidden proc- 

 esses has been the cumulative work of 

 investigators in many countries, span- 

 ning a period of nearly two centuries. 

 The first contributor was Joseph 

 Priestley, the eighteenth-century Brit- 

 ish clerg}'man whose hobby was chem- 

 istry. Here is how he reported his pio- 

 neering experiment of 1771: 



I have been so happy as by accident 

 to hit upon a method of restoring air 

 which has been injured by the burning of 

 candles and to have discovered at least 

 one of the restoratives which nature em- 

 ploys for this purpose. It is vegetation. 

 One might have imagined that since com- 

 mon air is necessary to vegetable as well 



PHYSIOLOGY 



as to animal life, both plants and animals 

 affected it in the same manner; and I own 

 that I liad tliat expectation when I first 

 put a sprig of mint into a glass jar stand- 

 ing inverted in a vessel of water. But 

 when it had continued growing there 

 some months, I found that the air would 

 neither extinguish a candle nor was it at 

 all inconvenient to a mouse which I put 

 into it. 



In this way Priestley stumbled upon 

 the green plant's faculty of producing 

 free oxygen. His observation is one of 

 the landmarks in the histor\' of chem- 

 istry. It started a train of experimenta- 

 tion, and other discoveries followed. 



In 1779 a Dutch physician, Jan 

 Ingen-Housz, working in England, re- 

 ported that plants indeed "have the 

 ability to correct bad air" but perform 

 this office only in sunlight. He also no- 

 ticed that "this service is not performed 

 by the whole plant, but only by the 

 leaves and green stalks." 



Three years later came the next 

 step. This was the observation by an- 

 other clergyman. Pastor Jean Senebier 

 of Geneva, that the plant performs its 

 office of purification only if some 

 "fixed air" is present in the atmosphere. 

 Fixed air was the name for carbon di- 

 oxide. 



It was not long before oxygen and 

 carbon were isolated and recognized as 

 elements, and the process which Priest- 

 ley had observed in his sprig of mint 

 became interpreted as one in which 

 green plants, on exposure to light, ab- 

 sorbed carbon dioxide and released ox)'- 

 gen. Chemists guessed that the ox)'gen 

 was produced by the breakdown of the 

 carbon dioxide; but if so, what became 

 of the carbon? Ingen-Housz came up 

 with the answer in 1796. The carbon, 

 he said, was utilized in the plant's nu- 

 trition. Photosynthesis was not just a 

 benevolent scheme to purify the air 

 for the benefit of man and the animals, 

 but was also a process for obtaining car- 

 bon and building it into organic ma- 



