On Wheat Mildew. 
499 
motes dancing in the sunbeam. But though so minute, they 
are mighty agents for good or for evil, because of their extraor- 
dinary quantity. 
The spores of fungi are limited, to some extent, in their 
operations, by the fact that each spore can germinate only on 
the species of plant that is proper to it. If the seed of a flower- 
ing plant be supplied with suitable heat, moisture, and air, it 
will germinate in any soil, and maintain a vigorous life or other- 
wise, in accordance with the character of the soil. But happily 
the spores of these fungi nmst not only have the necessary- 
physical conditions required by the seed, but they will fail in 
establishing themselves unless they further find these conditions 
associated with that particular species of plant with which their 
life-history is associated. Were it not so, the spores produced 
in a single season would be more than sufficient to clothe every 
inch of the surface of the earth with a dense mould. 
The red spores of the barberry fungus will produce a myce- 
lium only when they germinate on the leaf or stem of wheat, or 
of some other grass. And they can germinate there only when 
they can obtain a sufficient supply of moisture. 
It is a very general notion that mildew and other blights are 
" in the air," or are produced by fogs or mists. To some extent 
these notions are true. The farmer has observed the atmo- 
spheric conditions favourable to the growth of the spores, and 
without being aware how they quickened into life the every- 
where present spores, they give 
Fig. 3.— The Spores of the Bust in 
Wheat. 
the physical conditions the 
credit of being the efficient 
producers of the blight. But 
just as dry grain remains for 
any length of time in the barn 
without germinating, so the 
spores of the potato-fungus 
rest on the potato, or those of 
the barberry-fungus on the 
wheat, without germination, 
if there be no free moisture 
accessible" to them. A slight 
reduction of temperature, when the w^arm air of summer or 
autumn is saturated with moisture after rain, liberates some 
of the aqueous vapour which had formed an invisible ingredient 
of the atmosphere, and a mist is produced. This mist supplies 
the spore with the moisture it needs, and germination begins ; 
a small tube is pushed out, and, finding its way to one of the 
minute openings or stomates of the leaf, it passes through into 
the tissues, where, finding suitable food, it rapidly grows. In 
2 K 2 
