205 



can be crumbled in the fingers will illustrate the difference. 

 For the decay and destruction of structural timber fungi 

 are mainly responsible, their hyphae excreting enzymes which 

 destroy the wood tissue, as has been made clear in the examples 

 of Fomes and Trametes, and it is therefore important to 

 remember that, as a general rule, the conditions nrrost favour- 

 able for the development of such fungi are: 



(1) A supply of organic food materials such as carbohy- 



drates and proteid substances which are com- 

 monly found in sapwood. 



(2) Warmth. 



(3) A liberal supply of moisture and absence of free 



ventilation. 



(4) A supply of oxygen. 



Thus sapwood, especially if kept damp, quickly decays, 

 and in the case of posts driven into the ground we frequently 

 see that decay spreads most rapidly in that portion of 

 the post situated in the upper, well-a3rated, layers of soil. 



Seeing also how fungal hyphse may spread from a decayed 

 piece of wood to a sound piece in contact with it, and how 

 easily the minute spores are distributed, it is clear that the 

 greatest care must be taken if we wish to preserve sound 

 wood from infection. 



SECTION III. INFLUENCES OF THE SOIL ON PLANT DEVE- 

 LOPMENT. 



183. For the healthy develop- Presence of 



ment of plants the soil must contain a sufficient supply of all ^eoewaiy 

 the essential mineral salts. These may, for instance, be su it a ble Con- 

 so diminished by the competing roots of neighbouring dition and in 

 plants as to cause disease and death, the absence of ^ lltab | e . 

 any one of them making growth impossible. Further these 



salts must exist in such a state of combination as to enable 

 the plant to make use of them as food materials, and they 

 must not be present in excess. The best nutrient solution 

 for most phanerogams, for example, should not contain more 

 than 0*5 per cent, of salts. 



184. Any substance which may Presence of 



cause disease or death in plants may be broadly classed as a Substances. 

 Poison. If the solution of salts in the soil becomes concentrated 

 beyond a certain limit, rarely exceeding five per cent, owing to 



the osmotic processes being interfered with, the roots of plants 



