CHAPTER VIII 



THE BREATHING OF PLANTS, OR RESPIRATION 



We cannot live without breathing night and day. Neither 

 can plants. All parts of a plant breathe. If plants are 

 supplied with too much moisture, so that the roots do not get 

 enough air, the plant looks yellow and sickly and the leaves 

 may fall. Plants living in moist places, like the arum or 

 water-lily, have large air chambers running through the stem 

 for supplying air to the roots. The hollow stems of pumpkins 

 admit air to the parts underneath the large leaves. 



To show that Air Passages are continuous through- 

 out the Plant. — Provide a flask with a stopper containing tw^o 

 holes. Into one insert the stem of a leafy shoot, into the other 

 a glass tube. They should fit so tightly that no air can enter 

 the flask. By placing your mouth over the end of the tube 

 withdraw the air. Bubbles of air will rush from the cut end of 

 the plant as long as the air is withdraw^n. The air enters the 

 plant through the stomata. 



A little over three-fourths of the air (seventy- seven parts) 

 consists of nitrogen ; about one-fifth is oxygen, and a very small 

 part of carbon-dioxide. Carbon dioxide is a gas compounded 

 of one part of carbon and two of oxygen, which hold on to 

 each other very closely. CO.2 is the symbol or short form of 

 writing this gas. 



The proportion of nitrogen and oxygen in the air can be 

 nicely shown by the following experiment : invert a glass jar 

 with straight sides over a pan of water. Notice how high the 

 water stands in the jar. Now lift the jar and place under it a 

 piece of phosphorus fastened to a cork to keep it afloat. 

 (Phosphorus should always be kept and cut under water.) 



