2 



tissues. As a result of this process oxygen is freed and 

 passes off into the air and carbon is locked up in the tree. 

 About 49 per cent of wood, dry weight, is carbon, most of 

 which is drawn from the carbon dioxide in the air. Its ores- 

 ence in the air is not apparent; and it seems a strange transfor- 

 mation from the invisible form to a bulky substance like wood. 



5. There is another process going on in the foliage, and 

 that is respiration. Trees, in common with animals, breathe; 

 and as a result oxygen is taken from the air and carbon 

 dioxide is returned to it. This process is going on night and 

 day. The other process— the preparation of food — goes on 

 only in the presence of light; so trees give off an excess of 

 oxygen during the day and an excess of carbon dioxide at 

 night. 



6. A large amount of water is passed off through the 

 foliage, especially when the air is warm and dry. The 

 quantity of water passed in this way from the soil into the 

 air is so great that it is doubtful whether forests in all con- 

 ditions conserve the moisture in the soil as well as some other 

 form of covering — a layer of pulverized soil, for example. 

 There is no reasonable doubt, however, that on mountain 

 slopes and hillsides where much of the falling water would 

 be lost but for the obstructions which a forest cover puts in 

 its way, that the balance is in favor of the forest, 



7. The leaves are formed and placed for the work they 

 do. They are formed so that a vast amount of foliage is 

 carried even by a small tree; and they are arranged so as to 

 be exposed to the air and light. Oak leaves are in spirals, 

 so that a leaf does not cut off the light from the one that 

 stands next below it on the twig. Ash leaves are in pairs, 

 each on the opposite side of the twig; and the pair next be- 



