2Q4 THE PARASITIC DISEASES OF PLANTS. 
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 the breathing pore of the plant, the 
stomata. 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 directions and may in time pene- 
trate to all parts of the plant. Living thus as a parasite and drawing 
its sustenence 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 repro- 
ducing 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 gejrminate before it can do any injury. In order 
to germinate it must absorb moisture, a fact that explains the great 
influence of the weather upon the fungoid diseases. In moist 
weather the spores find plenty of moisture upon the surface of the 
leaves, while in dr^ weather the necessary moisture is lacking. Once 
it has germinated and its mycelium has entered the plant, it finds 
