How a Tree Lives 71 



(e.g. a jam, pot) which ivill go comfortably into the 

 basin. Small wad of cotton ivool. Methylated spirit. 

 Lime water. Small flask with tightly fitting rubber 

 cork. Vaseline. Glass tube (\J-shaped as in Fig. 37) 

 to Jit tightly into cork. Test-tube which will go into the 

 ft ask, containing ^ doz. 1 doz. peas or beans which 

 have been soaked in water for 12 hours. 



(b) For each pair of pupils. Small flask half 

 filled with lime water. Cork fitted with two glass tubes, 

 longer of which passes into the lime water at one end 

 and is fitted at the other with 9 in. of rubber tubing. 

 Pair of bellows. Two fresh leaves, part of whose 

 blades have been covered with tinfoil for 24 hours 

 before picking. Bunsen burner. Tripod stand and 

 wire gauze. Glass rod. Beaker of water. Two small 

 glass dishes, one containing alcohol and the other 

 alcoholic solution of iodine diluted with water to a 

 pale brown colour. Dish of plain water. 



To understand how and why a plant breathes you 

 must first know something about the air with which we 

 are surrounded. 



This air is of three sorts, all mixed together : the 

 first gas, oxygen, is necessary to us for life and is very 

 strengthening. It would however be too strong for us 

 if we always breathed it pure. Nitrogen, the second, 

 is not able to support life and serves only to weaken 

 the effect of the oxygen. Besides these two gases even 

 the purest air contains small quantities of a poisonous 

 substance called carbon dioxide or carbonic acid gas. 

 When people, animals, and, to a lesser extent, plants 

 breathe they take in oxygen and nitrogen from the air, 



