164 Chapters in Modern Botany CHAP. 



plants, which are lean and blanched, actually weigh less 

 (when dried) than the seeds did. Instead of a gain in 

 organic matter, there has been a loss, due to the living 

 and breathing of the plant. In other words, in the absence 

 of light the plant is not green, and is unable to utilise the 

 carbonic acid gas of the air. So we may say of the leaves 

 that they are those green parts by means of which the plant 

 is in sunlight able to utilise the carbonic acid gas of the air 

 and to build up organic substances. 



Another experiment of profound importance is readily 

 made. Take a piece of some vigorous water-plant, such as 

 the Canadian pondweed Elodea (Anacharis) canadensis, so 

 common in streams and canals ; place it in a vessel of 

 water into which some carbonic acid gas (easily produced 

 in a separate vessel by decomposing carbonate of lime with 

 hydrochloric acid) may with advantage have been intro- 

 duced. Place the vessel with the pondweed in the sunlight, 

 and watch the little bubbles of gas which appear on the 

 leaves and ascend to the surface. Place a funnel over the 

 plant, and catch the bubbles of gas in a test-tube filled with 

 water and inverted over the tube of the funnel. Gradually 

 as the bubbles ascend into the test-tube they displace the 

 water, and if you have patience you will at last have the 

 test-tube full of the gas. If you carefully remove the test- 

 tube and push into it a smouldering match, it will glow 

 brightly and perhaps catch fire again, or in other less rough- 

 and-ready ways you can show that the gas in the tube is 

 very rich in oxygen. The leaves, therefore are organs 

 which in sunlight give off oxygen, this being another aspect 

 of their power of utilising the carbonic acid gas. The 

 oxygen which they liberate results from the breaking up of 

 the molecules of carbonic acid gas within the plant. 



That the Canadian pondweed absorbs, like other water- 

 plants, its requisite supply of carbonic acid gas from what 



