AN INTRODUCTION TO PLANT BIOLOGY 111 



is absorbed at all times of the day and night by the surface 

 of the plant in contact with the air; and the gas then diffuses 

 or osmoses from cell to cell, for all living cells must have a 

 continual supply of oxygen. If some mushrooms be placed 

 in a closed jar for many hours, it can be demonstrated by 

 chemical tests that the air in the jar has lost oxygen. This 

 will be the same no matter whether the jar is kept in light 

 or in total darkness. 



A bean plant kept in darkness in a closed jar will act like 

 the mushroom at all times ; that is, it will take up oxygen. 

 But the same plant kept in a closed jar for several hours in 

 sunlight will give off oxygen. Apparently this is the reverse 

 of the mushroom or the green plant in darkness ; but let us 

 withhold our conclusion until we have reviewed previous 

 lessons dealing with carbon dioxide in leaves. 



We have noted ( 99) that carbon dioxide is used rapidly in 

 making carbohydrates (in photosynthesis) while the plant 

 is in light; but in darkness the plant makes no carbohy- 

 drates and consequently uses no carbon dioxide. Now, in the 

 combining of the elements of carbon dioxide and of water to 

 make sugar or starch, the carbon of the carbon dioxide is 

 used, but the oxygen is not needed and is set free in the cells 

 of the leaves.* The amount of oxygen thus freed is very 

 much more than the plant needs for its oxygen-supply; 

 that is, it is very much more than the same plant in darkness 

 would absorb from the air; and the result is that there is 

 excess oxygen to be given off to the air. The truth is that 

 the living cells throughout the plant require oxygen the same 

 in light as in darkness ; but while the starch-making goes on 



* Readers who have studied chemistry will be interested in the proportions 

 of CO 2 , H2O and O in starch-making as follows: (6 CO2+5 H 2 O)* = 

 (C 6 H 10 O5+602) Z , which means that to every six molecules of carbon dioxide 

 and five of water there will be one molecule of starch (C 6 H 10 O5) and six 

 molecules of free oxygen. This formula simply gives the proportions ; for 

 starch is some unknown multiple of C 6 HioOs. 



