AN INTRODUCTION TO PLANT BIOLOGY 



113 



The following experiment shows how air may enter the leaf 

 and pass through the stem. 



(D) Select a wide-mouthed bottle and a cork or rubber stopper 

 with two holes. Take a leaf with a small round petiole and push 

 the petiole into a hole in the cork and almost 

 to the bottom of a bottle. Also fit a glass 

 tube into the other hole of the cork. Fill 

 the bottle half full of water. Use vaseline 

 to make the apparatus air-tight. Apply suc- 

 tion to the glass tube (a small bicycle pump 

 with the leather on the plunger reversed, so 

 as to "draw" air out, and connected by 

 rubber tubing with a reversed valve from a 

 bicycle tire will answer, if an air-pump is 

 not at hand). Air will exude from the cut 

 end of the petiole and appear as bubbles in 

 the water. The experiment may be reversed 

 by attaching the petiole to rubber tubing 

 leading to a bicycle pump, and forcing air 

 from the pump into the tubes of the petiole FlG V 39 Apparatus to 



show that air can enter 



the leaf. 

 burger.) 



(From Stras- 



and out through the leaf, which should be 

 held under water so as to make air-bubbles 

 rise from the leaf surface. 



106. Excretion of Carbon Dioxide from Plants. The 



oxygen absorbed by plants and distributed to all their living 

 cells is used in the cells in a process of slow oxidation or 

 chemical union of oxygen with foods and other substances 

 in the cells. This is the same as in animal cells. Such 

 oxidation is constantly going on among the particles of 

 living plant cells, and one of the substances formed is carbon 

 dioxide (CO 2 ). When we remember that substances con- 

 taining carbon are abundant in cells, we can understand why 

 oxidation of cell-substances should form a compound of carbon 

 and oxygen (C0 2 ) . For example, when sugar is highly heated 

 the effect is first to drive off the water and leave carbon, 

 which then burns and disappears in the air as a gas (CO 2 ). 

 Something similar occurs in living cells when any substance 

 made from carbohydrate foods burns. The result is carbon 



