PLANT PHYSIOLOGY 131 



and placing it in the sunlight. Minute bubbles of gas will soon 

 make their appearance on the plant, which will rise through the 

 water and pass off into the air. If these bubbles are collected in 

 an inverted funnel (Fig. 67) and tested chemically, the gas 

 proves to be oxygen. All green plants liberate oxygen when 

 growing in sunlight, a process that is exactly the reverse of the 

 respiration of animals, which absorb oxygen gas and liberate 

 carbon dioxid gas. 



Photosynthesis is the foundation of all life, since the life of 

 all animals as well as plants depends upon starch. Its relations 

 to various external conditions are as follows : 



Chlorophyll. Photosynthesis is dependent upon chlorophyll 

 and hence occurs in green plants only. Moreover, in these 

 plants, photosynthesis occurs only in those cells that contain 

 chlorophyll, and thus chiefly in the palisade and mesophyll cells 

 of the leaf, although it may take place in other cells if they 

 contain chlorophyll. 



Sunlight. Photosynthesis is dependent upon sunlight and 

 therefore never occurs in plants unless they are in the light. 

 The vigor of the process is dependent also upon the intensity 

 of the sunlight. It is most active in direct sunlight, less so in 

 diffused daylight, and stops entirely when light is withdrawn. 



Carbon Dioxid. Photosynthesis is dependent upon the 

 presence of carbon dioxid. Those plants which live in the air 

 will always have plenty of carbon dioxid, since the air contains 

 this gas. Water plants depend upon the gas dissolved in water. 

 The dependence of photosynthesis upon carbon dioxid can be 

 shown if a green water plant is placed in sunlight in ordinary 

 water, when bubbles of gas (oxygen) arise from it, showing the 

 presence of photosynthesis. If, however, this plant be placed 

 in a dish of boiled water which has been cooled, the bubbles do 

 not arise from its leaves, showing that photosynthesis does not 

 occur. Boiling the water drives off the carbon dioxid dissolved 

 in it, and the plant, having no carbon dioxid at its command, 

 cannot carry on photosynthesis. 



