THE TRANSFORMATION OF CARBON 51 
into a brownish powder and is incorporated into the soil, contrib- 
uling to the formation of the humus. This destruction of woody 
tissue is also brought about by microérganisms, but in this case it 
is not bacteria that are at first concerned. 
The first phenomenon that occurs in such a decaying tree trunk 
is the growth of larger fungi. Various forms of mushrooms and 
tree fungi start their growth on its surface and send delicate myce- 
wood 
\ 
\ 
\ 
\ 
10 
ceufooo % ix BAGTE RIA 
\ 
VARIOUS CARBON 
COMPOUNDS BUT 
EVENTMWALLY ALL 
REDUCED TO CQ, 
IN ATMOSPHERE 
Fic. 13.—The carbon cycle. 
lium threads into the substance of the wood. These threads grow 
first underneath the bark and in the superficial layers of wood; but 
gradually they penetrate the hard wood and, by the chemical 
excretions they produce, soften this hard, tough substance. With- 
out the growth of such fungi in the wood there would seem to 
be no way of softening the wood sufficiently for decay. After the 
wood has been somewhat softened by the fungi, wood-eating in- 
sects begin their work upon it, using the fungi largely as food. 
It is probable that bacteria also may assist in this matter, but 
the larger fungi are chiefly responsible for the destruction of the 
