Fungus Diseases 37 
sapping its life, — true parasites; others again are in some 
stages of development parasitic, and in other stages only 
saprophytic. 
There are fungi living on leaves, others living on the roots, 
others on the bark, and again others living on the wood of 
the tree, producing its decomposition, or what is commonly 
known as ot. 
The young plants or seedlings are exposed to a large num- 
ber of fungus enemies, which do not trouble them in later life, 
but we shall here only discuss those occurring in older trees. 
Of the many interesting phenomena in the life history of 
the fungi one of the most important is that many not only 
pass, like the insects, through different phases or forms of 
development, but in these different phases live on different 
“hosts,” as the plants are called on which they feed. 
The well-known rust of wheat lives in one phase of its 
development in the ground, as many others do, and in an- 
other on the barberry bush; some fungi, active on the juniper 
tree, are found in different stages of development on apple 
and other trees. A rust which attacks the needles of pines 
is dependent for its development on ragweed or groundsel 
(Senecio). Lately a new pest attacking our white pine has 
been introduced from Germany, and this passes one stage 
of its development on currant bushes. 
The presence of a certain plant may, therefore, be a cause 
of the spread of the disease on another, and the removal of 
the host plant as well as of the infected trees or parts of 
them is often the only means of getting rid of the trouble. 
In so far as specific enemies exist on given species, if an 
epidemic has set in, it may become practically necessary to 
replace the species by another in order to avoid the spread 
or renewal of the trouble. 
The fungi which attack the foliage usually destroy only 
