100 



GROWTH AND WORK OF PLANTS 



166. To show that C0 2 is absorbed while is given off by 

 plants. Boil some spring water. This drives off the air and 

 CO 2 which is mixed in it. When the water is cooled a few sprigs 



of Elodea are placed 

 in it as described in 

 paragraph 164. It is 

 now placed in the 

 sunlight. No gas is 

 given off. If oxygen 

 is added by thoroughly agita- 

 ting the water so as to mix air 

 with it, and it is placed in the 

 sunlight, no gas is given off. 

 If CO 2 is now added (by 

 breathing into the water 

 through a glass tube, or with 

 a gas generator), and it is 

 placed in the sunlight, oxygen 

 is again given off. 



167. A chemical change 

 takes place in the gas in 

 the cells of the plant. 

 These experiments indicate that a chemical change takes place 

 in the cells of the plant, for if a simple plant like the pond scums 

 (Spirogyra) is used the result will be the same. Since sugar and 

 starch are formed in the leaves during this process, they are a 

 part of the result or product of this change. 



168. How this change takes place in land plants. The 

 chemical change which takes place with aquatic plants, as indi- 

 cated by our experiments, occurs also in the case of the green 

 land plants. The air is a mixture of a number of gases. The 

 principal and constant gases in the air mixture are as follows. 

 In every 100 parts there are about 21 parts of oxygen (symbol 

 for oxygen = O), about 79 parts of nitrogen (symbol for nitro- 

 gen = N), and a very small fraction of carbon dioxide (symbol 

 for carbon dioxide = CO 2 ). The latter gas is a very poisonous 



Fig. 86. 

 Ready to see what the gas is. 



