54 A PRIMER OF BIOLOGY 



associated with the haemoglobin or red-colouring 

 matter of the blood and forms a feeble compound 

 with it, known as oxyhaemoglobin. This loosely 

 combined oxygen is carried by the blood to such a 

 seat of activity as, let us say, a muscle. There oxida- 

 tion of muscle substance, or of organic compounds 

 present in the muscle, takes place, thus liberating 

 energy which enables the muscle to do work, i.e., to 

 contract. Amongst the substances formed as a 

 result of this oxidation is the gas, carbon dioxide, 

 which is transferred from the contractile cells to the 

 blood, and thence by its means back to the lungs 

 from which it is expired. The difference between 

 the air inhaled and that exhaled shows approximately 

 how much tissue destruction has taken place. Thus 

 inhaled air consists approximately of 79 per cent, of 

 nitrogen, 21 per cent, of oxygen, and '04 per cent, 

 of carbon dioxide, but exhaled air consists of only 

 16 per cent, of oxygen, and about 4 per cent, of carbon 

 dioxide, the percentage of nitrogen a neutral gas 

 remaining approximately constant. 



It may readily be shown that in plants the process 

 of respiration is, in principle, fundamentally the 

 same the method of entry and exit of the gases, 

 however, differs, for in them there is no special respi- 

 ratory apparatus beyond the intercellular spaces in 

 the tissues themselves. Air enters by the stomata 

 in green parts or by special pores, left in older parts 

 which have become covered with cork, known as 

 lenticels, and diffuses to all parts which may require it, 

 there to break down organic substances and so release 

 energy required for carrying on vital processes. So, 

 too, the carbon dioxide formed diffuses outwards and 

 finds its way to the exterior by the same channels. 

 During the day, while the green cells are exposed to 



