WOODS AND FORESTS. 315 



to speak, along a mountain burn, there are often 

 thickets of Rowan, Sloe, and small Birch trees. 



Thus there is an appropriate fringe to every neigh- 

 bouring association, and both the undergrowth and 

 wood-floor plants show, in these cases, appropriate 

 changes. 



Woods often become diseased, and they may be 

 destroyed by bad management, or by physical changes 

 in the soil. There is always, for instance, the danger 

 of the soil reverting to a peat-moss. If it is badly 

 drained and becomes saturated with water, worms and 

 other animals are unable to live. The air is not able 

 to pass down the worm burrows to the lower levels, 

 where the fungi are working on the humus. Oxygen 

 is therefore not able to reach a particle of say "cellulose," 

 or cellwall substance, which is on the point of breaking 

 up. It follows that it cannot be destroyed, and must 

 remain only partly decomposed ; for the substances into 

 which it would be broken up contain more oxygen 

 than exists in the cellulose. In such a wood, the soil 

 will become a sort of peat, and the special ground 

 fungi will not develop. After a time even the trees 

 will begin to grow badl}^ and eventually die away. 



The special climatic conditions within a wood are 

 very remarkable. The light is greatly diminished ; 

 the intensity of the light in an oak wood is to that of 

 daylight as i to 22 ; in a beech wood it is as i to 85; 

 and even in an ash or birch plantation it is as i to 5 

 or I to 9 respectively. The soil is colder in woods 

 then it is in the open field. The difference is as much 

 as 3.21 per cent, in summer but only 0.02 per cent, in 

 winter. The air temperature is also slightly below that 

 of the open country. The daily changes in tempera- 

 ture are also much less pronounced. The evaporation 



