22 PLANT PHYSIOLOGY 



all nature. Not only the food which plants themselves use, but 

 the food of all animals as well, including man, comes from plants. 

 Carnivorous and omnivorous animals get all their food directly 

 or indirectly from plants. 



This process of building food starts with the manufacture of 

 the carbohydrates, — the sugars and starches; and this construc- 

 tion of simple carbohydrates from inorganic materials such as 

 water and carbon dioxide is called carbon assimilation or photo- 

 synthesis. The term " photosyntax " was proposed by Barnes 

 in 1893 and later changed to photosynthesis, which is the term 

 now commonly used in America, although in England and other 

 European countries, the phrase " carbon assimilation" is still in 

 common use. 



Photosynthesis. — Photosynthesis is the manufacture of simple 

 carbohydrates from carbon dioxide and water by the chlorophyll of 

 the green plant in the presence of light. Photosynthesis is hence the 

 most important organic process in nature because upon it all the 

 other life processes depend. For this reason it has been called 

 "the most fundamental fact in science." 



Historical. — Bonnet (1769) noticed bubbles coming from grape 

 leaves immersed in water. From boiled water no bubbles were ob- 

 tained, but the phenomenon remained without explanation. 



Priestley (1771) noticed that mice living in a confined space 

 gradually vitiated the air and made it unfit for continued life. 

 He sought some method to restore the air to its original purity 

 and placed some plants under a bell jar under which the mice had 

 previously been. He found that after some time the air was im- 

 proved and again capable of supporting animal life but, unfor- 

 tunately, he was not always able to repeat the experiment, and 

 he could not understand why sometimes the air was improved by 

 the plants and sometimes not. 



Ingen-Housz, a Dutch physician (1779), showed that this purify- 

 ing process was effected only by the green parts of plants and only 

 in the light. He did not know why plants should have this effect 

 nor did he have any idea concerning the importance of the matter 

 in the nutritional economy of the plant, but arrived at the con- 

 clusion that plants had two kinds of "respiration," — nocturnal 

 and diurnal. Influenced by the philosophical ideas of his time, 

 he thought that this was a purposeful reaction for improving the 

 air for the benefit of man Neither did Ingen-Housz know precisely 



