THE MUTUA^ RELATIONS OF ORGANISMS 235 



continue indefinitely, all kinds of material that have been 

 built up into organic compounds by the green plant must be 

 pulled to pieces again so as to be brought back into the simple 

 condition in which the future generations of plants can utilize 

 them. While animals use and break down much of the pro- 

 teids, starches, and fats, there are some substances that ani- 

 mals cannot utilize, and the Fungi are necessary to reduce 

 these substances to a simpler condition. Bacteria everywhere 

 in nature are constantly feeding upon many kinds of organic 

 substances, but primarily upon those that contain proteids or 

 other nitrogenous compounds. The yeasts have a special re- 

 lation to sugar; most of the sugars made by plants, and not 

 otherwise used, are consumed by yeasts in fermentation and are 

 thus brought back to the original condition of carbon dioxid and 

 water. Bacteria and yeasts as well as animals thus feed upon the 

 same substances. But there is other material of harder nature, 

 like wood and leaves, which does not serve as food for animals 

 nor to any great extent for bacteria or yeasts. The molds, 

 mushrooms, and tree fungi seem to be especially designed by 

 nature to attack these hard materials and reduce them to a 

 condition in which they can be destroyed. These larger fungi 

 consist of a mycelium of delicate, branching threads. If one 

 of these plants starts to grow on the trunk of a tree, the my- 

 celium pushes its way through the bark and in among the wood 

 fibers, and eventually grows through the whole substance of 

 the tree, the part visible on the outside of the trunk being 

 only the spore-producing portion that has come to the surface 

 to distribute the spores to other trees. The mycelium, while 

 growing within the wood, produces certain substances which 

 soften the wood and in time disintegrate it, i. e., cause it to 

 rot. A tree attacked by one of these Fungi in time becomes 

 soft and so changed in its chemical nature that it can be utilized 

 as the food of some insect. Eventually the trunk of the tree 

 is converted largely into a soft, pulpy mass, until finally it is 

 wholly decomposed. Its carbon and hydrogen unite with 



