MILDEWS, RUSTS AND SMUTS 
65 
of dead leaves, etc., are called saprophytes ; those that derive 
their food from living plants, or less frequently from living 
animals, are termed parasites. As would be expected, there 
is no hard and sharp line between these two groups ; 
some fungi, so far as is known, can only exist as sapro¬ 
phytes ; others again, can only obtain their food from 
some living organism, and are sometimes spoken of as 
obligate parasites. Others again, that are generally sap¬ 
rophytes, possess the power of deriving their food from 
living plants, and thus become parasites for the time being. 
Such have been called facultative parasites. 
The plant on which a fungus is parasitic is called the 
host-plant, the host being a compulsory one, and parasitic 
fungi gain an entrance to the host by different means. 
When a spore or conidium alights on a suitable living 
leaf, germination takes place if the necessary amount ci 
moisture is present, hence plants are generally infected 
during the night, or during damp, dull days, a fact of some 
value in connexion with the prevention of diseases, it having 
been proved that plants covered with a screen, to prevent 
the deposition of dew during the night, remained practically 
free from infection, whereas plants of the same kind, not 
so protected during the night, become badly diseased. 
The first product of a conidium, which usually consists 
of a delicate thread-like body, is called a germ-tube, this is 
produced on the surface of the leaf, and soon enters into the 
tissue of the leaf, either through a stoma, or directly through 
the epidermis, the tip of the germ-tube secreting an enzyme, 
or substance which enables it to penetrate the wall. 
Once inside the leaf, the germ-tube soon produces a 
copious, branching mycelium which eventually produces 
the spores. Parasitism is an acquired habit on the part of 
fungi. It must be obvious to everyone that when the 
spores of fungi are dispersed by wind, insects, and other 
agents, such spores must necessarily alight on practicall3/ 
every kind of plant growing in the neighbourhood, but it 
is well known that a given parasitic fungus does not grow 
indiscriminately on every kind of plant. On the other 
hand, most fungi are confined to a few allied host-plants. 
Why ? Miyoshi, a Japanese botanist, has shown that if 
a very thin sheet of mica, through which a number of very 
minute holes have been pierced, is placed on the surface 
of gelatine, containing a substance suitable as food for a 
fungus, and fungus spores are then sown on the surface 
of the mica one of two things happens. Either the germ- 
