246 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



between the loose cells of the softer leaf-tissue, and if 

 we supposed a very, minute creeping organism to 

 enter one of the stomata, it would find itself in a 

 labyrinth of inter-cellular passages : supposing it able 

 to traverse these, it could pass from any part of the 

 leaf to any other between the cells ; or it could 

 emerge again from the leaf at thousands of places 

 other stomata. In traversing the whole of the 

 labyrinth, however, it would pass over many millions 

 of times its own length. Moreover it would find these 

 intercellular passages filled with a varying atmosphere 

 of diffusing gases oxygen, nitrogen, the vapour of 

 water, and carbon-dioxide being the chief. It would 

 also find the cell-walls which bound the passages 

 damp, with water continuous with the water in the 

 cells. If we suppose our hypothetical traveller 

 threading the mazes of these passages at night, 

 and able to perceive or test the changes which go on, 

 it would find relatively little oxygen and relatively 

 much carbon-dioxide in the damp atmosphere in the 

 passages ; whereas in the daylight, if the sun was 

 shining brightly on the leaves, it would find the atmo- 

 sphere rarer, and relatively little carbon-dioxide 

 present, but an abundance of oxygen. These gases 

 and vapour would be slowly moving in and out at the 

 stomata by diffusion, the evaporation of the watery 



