CHAPTER I 

 INTRODUCTION 



From the time of Aristotle until the eighteenth century it 

 was generally supposed that plants derived their nourish- 

 ment from the soil. This view was questioned by Stephen 

 Hales in 1727, who suggested that plants took some part of 

 their nourishment from the air and he added the remark 

 that light also probably played som^e part in the process. 

 Further progress was possible only after chemists had 

 learnt to distinguish the different gases which constitute air 

 and it was Priestley, the discoverer of oxygen, who made 

 one of the first important contributions to the study of 

 photosynthesis. In 1771 he found that green plants had the 

 ability to 'restore' air which had been made bad, as the result 

 of combustion or by the respiration of animals, to a con- 

 dition in which it could again support combustion and life. 

 Ingen Housz in 1772 showed that light, as well as the green 

 plant, was concerned in the purification of air and that in 

 the dark, plants, just like animals, caused air to become 

 'foul'. This discovery led Ingen Housz to exaggerate the 

 danger of poisoning which might arise when plants are kept 

 in the dark in houses and to the tradition that plants or por- 

 tions of plants with flowers should be removed from the 

 bedroom at night. Senebier in 1782 demonstrated an accel- 

 erating effect of the 'noxious' gas produced by animals and 

 by plants in the dark ('fixed air', or as we now call it carbon 

 dioxide) upon the production by plants of 'dephlogisticated 

 air' (i.e. oxygen) in the light. The introduction of quantita- 

 tive experiments allowed de Saussure to show in 1804 that 

 the gain in weight of a plant during a period of photo- 

 synthesis plus the weight of the oxygen evolved exceeded 

 the weight of the carbon dioxide consumed. He suggested 

 that the difference must be due to an uptake of water. Thus 

 by the early nineteenth century the green plant had been 



