THE GROWTH OF THE SHOOT FROM THE BUD 49 



In order that the chlorophyll may do its work, the plant must 

 be supplied with carbon dioxide. This gas is always being breathed 

 out by plants and animals, so that there is always some in the air. 

 If it is prevented gaining access to the plant, starch cannot be 

 made. To demonstrate this, a pot containing a green plant 

 clover does very well should be placed under a bell-jar. By the 

 side of the plant a small bottle containing a strong solution of 

 caustic potash should be placed, to absorb the carbon dioxide 

 given off by the plant. 

 Through the india- 

 rubber cork of the 

 bell-jar a bent glass 

 tube is passed. The 

 U -portion of the tube 

 is filled with soda of 

 lime, which absorbs 

 the carbon dioxide 

 from the air passing 

 through it into the 

 bell- jar. Thus no car- 

 bon dioxide reaches 

 the plant. After about 

 twenty-four hours the 

 leaves of the plant 

 may be tested, and 

 will be found not to 

 turn blue when soaked 

 in iodine. In trying 

 experiments connected 

 with carbon assimila- 



FlG. 29. Apparatus for excluding carbon dioxide. 

 S, soda of lime. C, caustic potash. 



tion it is always best to leave the plant in the dark a day or so 

 before beginning the experiment, in order that any starch then 

 the present in the leaf may be withdrawn from the leaves through 

 stem to the root or the other part of the plant. 



The work of the chlorophyll is to separate the carbon from 

 the carbon dioxide. With the carbon thus obtained and with 

 water from the roots starch is made. The oxygen of the carbon 

 dioxide is returned to the air ; for it is the carbon alone that the 



VOL. III. 4 



