PHOTOSYNTHESIS 101 



light by a suitable hood, and it is even desirable to add a third, in the light 

 without a plant. There are, however, three errors current in connection 

 with this experiment. First it is said in many books that a flame burns 

 the entire 21% of oxygen in the space to carbon dioxide, whereas it never 

 burns more than about 2j% before the flame is extinguished. The rise of 

 water in the jar is not due to the removal of the oxygen (for that is always 

 replaced by an equal volume of carbon dioxide), but to the expulsion from 

 the bottle of some of the air when heated and expanded by the flame; and 

 the rise is the smaller the less the heat is. And it is especially an error to 

 consider that the burning of the candle at the close of the experiment proves 

 not only that the carbon dioxide has been removed, but the oxygen has been 

 restored. In fact, although the latter is true, the experiment proves only 

 the former. 



The student should make sure of his knowledge of the amount 

 of carbon dioxide in the air, upon which he will find matter in 

 Part III of this book. At this point also he should inform him- 

 self, through the literature, upon the absorption of carbon by 

 plants in forms other than that of carbon dioxide, and upon the 

 ecological correlations thereof. The mode of absorption of 

 the carbon dioxide and release of the oxygen will be considered 

 later under Absorption. 



The use of carbon dioxide in photosynthesis, shown by the 

 preceding experiment, raises the question as to its proportional 

 combination with the hydrogen and oxygen of water to form 

 the photosynthate. Taking into account the known composition 

 of the substances concerned, viz., CO 2 , H 2 O, C 6 Hi 2 O 6 , it will be 

 evident that the most probable, if not the only possible propor- 

 tions of these substances involved would be expressed by the 

 following equation : 



6CO 2 + 6H 2 O = C 6 H 12 O 6 + 6O 2 . 



This implies, however, that oxygen is released in the process, 

 a probability so striking as to demand experimental testing : 

 Is oxygen released in photosynthesis? 



This may be determined directly by the analysis of the gas of a 

 closed chamber in which photosynthesis has taken place, but it can 

 be ascertained more simply by testing the gas shown by observation 

 to be given off during photosynthesis by water-plants. 



EXPERIMENT. Fill a graduated test-tube or cylindrical graduate 

 with water, and support it inverted over a glass jar of water of con- 



