354 [ ASSEMBLY 
which take their nourishment from the living host, without killing 
it, and in some cases without interfering with its functions, like 
many of the mildews, rusts, peach curl, etc. 
Probably those most to be dreaded are the half-saprophytic kinds. 
They may grow for some time in the host before killing the tissues 
and beginning the work of disintegration. In this case there is a 
struggle between the fungus and its host. The fungus requires 
a certain grade or quality of food, it must be, we will say, not 
above a certain degree of vitality; if the vigor of the host can be: 
raised to exceed this point, the fungus is successfully repulsed, and 
the disease overcome. Again, in other instances, the fungus hay- 
ing a different habit of life, finds a foothold at some weakened part, 
as upon old leaves, a sickly branch, or through a wound, and by 
slowly diverting the nourishment of the plant to its own uses, 
finally lowers the vitality until it Invades and destroys the whole 
plant. But some fungi are more aggressive than either of these, and 
at once strike at the most highly vitalized parts of the plant, blast- 
ing the tissues wherever they touch. It would be folly to fight such 
a disease simply by enhancing the vigor of the plant, for this is 
only providing the fungus with a better supply of the nourishment 
which it prefers. No more foundation exists for the beliet that a 
fungus cannot successfully attack a healthy plant, than that a well 
man cannot be made sick by a contagious disease. If we admit, as 
is most likely true that these saprophytes are helped in their work 
of disorganization by bacteria, especially the kinds that most quickly 
kill the host, such as the rots, the facts are not materially changed, 
for in this case the bacteria without the assistance of the fungus 
would have no power to break down the tissues. py 
This brings us to the consideration of true parasitic fungi. If 
we select for an example such a fungus as the lilac mildew (Micros- 
phera Friesii Lev.), which causes the upper surface of lilac leaves 
to turn white in late summer, we shall have to do with a fungus 
which completes its growth upon a living host, from which it draws 
its nourishment, and with so little disturbance to the normal funce- 
tions that one can scarcely call it a disease. In the nearly related 
pea and hop mildews the hosts are usually unable to supply sufficient 
nourishment for both host and parasite, and the host, in conse- 
quence, fails to develop fully. In the cases just mentioned the para- 
site lives wholly upon the outside of the plant, but it is quite as com- 
mon to find it penetrating the tissues, and living inside. A simple 
case in point is the peach curl. The mycelium of the fungus causing 
this disease ramifies through the tissues of the leaves and. young ~ 
shoots of the tree, and makes itself almost a part of them. There is 
no marked disturbance of functions till the fungus begins to fruit, 
causing the leaves to become hypertrophied, to ripen prematurely 
and fall away; but the part of the fungus remaining in the tree 
lives on perennially. It is only when the tree is unable to meet the. 
full demand for nourishment that we see any lowering of vigor. 
