74 AGRICULTURAL BACTERIOLOGY. 



contribute to the process, and they are undoubtedly assisted 

 in the action by the aid of insects. 



The first phenomenon that occurs in such a decaying tree 

 trunk is the growth of common fungi. Various forms of 

 mushrooms start their growth on its surface and send delicate 

 mycelium threads into the substance of the wood. These 

 threads grow first underneath the bark and in the superficial 

 layers of wood ; but slowly, though surely, they penetrate the 

 hard wood, and, by the chemical excretions they produce, 

 tend to soften this hard, tough substance. Without the growth 

 of such fungi in the wood there seems to be no means by 

 which the tree trunk can be sufficiently softened for decay. 

 After the wood has become somewhat softened by the fungi, 

 wood-eating insects begin their work upon it, using the wood 

 as food. Simultaneously there begin to work upon the tree, 

 if a sufficient amount of moisture is present, the decomposing 

 bacteria, which, feeding upon the organic substances present, 

 are able to grow in this soft wood, eventually completing its 

 destruction into simple gaseous compounds. The result of all 

 these processes is that the carbonaceous material in the wood 

 is liberated, by being combined with oxygen, and passes off 

 into the air to join the atmospheric store of carbon. The hy- 

 drogen and oxygen are converted chiefly into water, and in 

 their turn enter the atmosphere as water vapor. In this way, 

 by a slow process of decomposition, produced by microorgan- 

 isms, the woody substance is in the course of time converted 

 into simple chemical compounds which join nature's food sup- 

 ply in the air. 



In these ways it is evident that large portions of the carbon, 

 which plants have extracted from the atmosphere by their orig- 

 inal growth, are returned again to the air in the form in which 

 they first existed. The plant seized the carbon in the form ot 

 carbon dioxide and, combining it with water and other sub- 

 stances, built it up into sugars, starches, cellulose, wood and 



