THE FUNGOUS DISEASES OF PLANTS 317 
discussion of bacteriology, but their very close relation to germ 
life makes it necessary to consider them briefly. 
As stated on an earlier page, the higher fungi are characterized 
by developing a mycelium. This delicate branching, usually color- 
less thread, grows in profusion in or upon the substance that 
furnishes the fungus with its nourishment. It is this mycelium 
that makes these plants especially adapted to live as parasites 
upon plants. A spore of some fungus falls upon the surface of a 
leaf and germinates, sending out its tiny thread. This finds 
some opening into which it can thrust its way. Sometimes the 
opening is a wound in the cuticle, but in other cases it is a breath- 
ing pore of the plant, a stoma. Once it has entered through this 
cuticle it finds the tissues soft and moist and there is nothing to 
prevent its growing through the plant. The mycelium can 
readily grow among the plant cells, winding its way in all direc- 
tions and may in time penetrate to all parts of the plant. Living 
thus as a parasite and drawing its sustenance from its host it 
naturally produces more or less effect upon the plant life, resulting 
in what are called plant diseases. 
The mycelium is the growing part of the plant, but not its 
reproducing part. These plants are reproduced by spores. 
Although differing in their method of origin, the spores are always 
minute, microscopic bodies, produced in immense numbers by 
the fungus. Generally, though the fungus grows below the surface 
of its host, the spores are produced on its surface. The mycelium 
is usually out of sight, while at certain spots the parasite breaks 
through the cuticle of its host in order to produce spores and 
discharge them into the air. The mycelium is white or colorless, 
but the spores show a variety of colors. It is evidently by these 
superficial spores chiefly that the fungus is spread from plant to 
plant. 
When one of these spores gets carried to the surface of another 
plant it must first germinate before it can do any injury. In 
order to germinate it must absorb moisture, a fact that explains 
