olds, in order that
we may better understand our apple and potato rots, as well as other
plant diseases.

If you cut a lemon and let it stand for a day or two, there will
probably appear a blue mold like that you have seen on the surface of
canned fruit. Bread also sometimes has this blue mold; at other times
bread has a black mold, and yet again a pink or a yellow mold.

These and all other molds are tiny living plants. Instead of seeds they
produce many very small bodies that serve the purpose of seeds and
reproduce the mold. These are called _spores_. Fig. 112 shows how they
are borne on the parent plant.

[Illustration: FIG. 112. TANGLED THREADS OF BLUE MOLD
The single stalk on the left shows how spores are borne]

It is also of great importance to decide whether by keeping the spores
away we may prevent mold. Possibly this experiment will help us. Moisten
a piece of bread, then dip a match or a pin into the blue mold on a
lemon, and draw the match across the moist bread. You will thus plant
the spores in a row, though they are so small that perhaps you may not
see any of them. Place the bread in a damp place for a few days and
watch it. Does the mold grow where you planted it? Does it grow
elsewhere? This experiment should prove to you that molds are living
things and can be planted. If you find spots elsewhere, you must bear in
mind that these spores are very small and light and that some of them
were probably blown about when you made your sowing. When you touch the
moldy portion of a dry lemon, you see a cloud of dust rise. This dust is
made of millions of spores.

[Illustration: FIG. 113. MAGNIFIED ROSE MILDEW]

If you plant many other kinds of mold you will find that the molds come
true to the kind that is planted; that like produces like even among
molds.

[Illustration: FIG. 114. A MILDEWED ROSE]

You can prove, also, that the mold is caused only by other mold. To do
this, put some wet bread in a wide-mouthed bottle and plug the mouth of
the bottle with cotton. Kill all the spores that may be in this bottle
by steaming it an hour in a cooking-steamer. This bread will not mold
until you allow live mold from the outside to enter. If, however, at any
time you open the bottle and allow spores to enter, or if you plant
spores therein, and if there be moisture enough, mold will immediately
set in.

[Illustration: FIG. 115. A HIGHLY MAGNIFIED SECTION OF DISEASED PEAR LEAF
Showing how spores are borne]