DISEASES DUE TO SOIL-INFLUENCE 279 



of the earth-heap that adventitious roots were being abundantly 

 produced from the uninjured cortex of smooth-barked trees, such 

 as the beech and hornbeam, even when the stems were eight 

 inches in diameter. 



Where it has been deemed desirable to preserve valuable trees, 

 excellent results are said to have been got by ringing the stems 

 a short distance beneath the surface of the ground, or at least 

 by removing the bark in patches as far in as the wood. From 

 the callus that formed at these places numerous roots were 

 produced, which by ramifying close beneath the new surface of 

 the heaped-up soil preserved the life of the tree. 



It is scarcely necessary to mention that failure in the natural 

 regeneration of beech woods is often to be traced to the 

 insufficient aeration of soil that is covered with a thick layer 

 of humus. Small seeds especially, that are buried too deeply, 

 frequently fail to produce plants on account of the supply of air 

 being insufficient to replace the carbonic acid gas that is pro- 

 duced during germination. The familiar fact that unsatisfactory 

 results are almost always got when the germination of alder- 

 and birch-seeds is tested in a room, although these seeds ger- 

 minate splendidly when they are sown outside, is probably due 

 to the circumstance that it is only outside that the air in the 

 neighbourhood of the seeds is constantly being changed, owing 

 to the daily variations in the temperature of the soil. In the 

 room the temperature is uniform and the air is comparatively 

 still, so that the carbonic acid gas which is given off during 

 germination cannot be removed quickly enough from the 

 neighbourhood of the seed. Death occurs in heaps of germinat- 

 ing seeds for similar reasons. 



Analogous to the root-rot that has already been described 

 is also the decay of the roots of plants that are culti- 

 vated in glazed pots, which render the free circulation of air 

 impossible. 



POISONOUS SUBSTANCES 



In the narrower sense of the term poisons are taken to mean 

 only such substances as are directly injurious to plant-cells and 

 effect their destruction. Such substances may be naturally pre- 

 sent in the soil, but are more often imported into it. As a rule, 



