108 GROWTH AND WORK OF PLANTS 



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 green parts of 

 plants during this process, they are a part of the result or pro- 

 duct of this change. 



172. 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 



Fig. 90. 

 The splinter lights again in the presence of oxygen gas. 



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

 gen =N, see paragraph 26), and a very small fraction of carbon 

 dioxide (symbol for carbon dioxide=CO 2 ). Carbon dioxide is 

 a suffocating gas. We have already noticed that it is easily 

 absorbed by water. Water is a compound of two gases, hydrogen 

 two parts and oxygen one part, and the symbol for water is there- 

 fore H 2 O. The land plants absorbf some of the carbon dioxide 

 from the air, and it is also formed within the plant cells of all 

 plants during respiration. But whether formed in -the plant 

 cell or absorbed from the air, as soon as it comes in contact with 

 the water in the cell it is absorbed. When carbon dioxide is 



* For reference. 



j" The process of transpiration keeps the cell wall saturated with water. 

 This provides the water for the solution of the CO 2 which in the dry con- 

 dition (anhydride condition) could not diffuse through a dry cell wall fast 

 enough. This is believed to be one of the important functions of trans- 

 piration. In the case of aquatic plants the CO 2 in the water is dissolved 

 and is therefore CH 2 O 3 = carbonic acid. 



