THE WORK OF LEAVES 195 



water) as it will conveniently hold, and then inserting 

 a small vial partly filled with clear lime-water. Set 

 the bottle away for a day or two in darkness. Prepare 

 a control bottle in which no leaves are placed. Both 

 bottles should be sealed air-tight. In which is more 

 carbon dioxide produced, as shown by the lime-water 

 test? As the result of this experiment we may say 

 that in the dark a green plant behaves in regard to 

 respiration like a colorless plant or an animal. 



There is a popular belief that plants are unhealth- 

 ful in sick-rooms at night because 'they vitiate the air. 

 As a matter of fact, it would take a very large number 

 of plants to do as much harm in this respect as a 

 single candle. 



Since the leaves have work to perform during the 

 day, as well as during the night, we should expect to 

 find the process of combustion going on then also, since 

 it is this which furnishes energy to do work; yet we 

 have found that carbon dioxide is used up and oxygen 

 produced by the leaves in daylight. Careful determina- 

 tion of the relative amounts of the gases has shown 

 that both processes take place simultaneously; the 

 sunlight furnishes energy for the work of starch- 

 making which results in the production of oxygen ; the 

 combustion which results in the production of carbon 

 dioxide furnishes energy for work of other kinds. 



Since the energy which is absorbed in the making 

 of starch is all given out again when the starch is 



