150 A TEXT-BOOK OF BOTANY 
means of these summer spores the rust may spread through 
a field of wheat and into adjoining fields with great 
rapidity. 
Later in the season, on the stubble 
and on plants not removed in the har- 
vesting, black lines and dots appear, 
which are masses of a very different kind 
of spore sent to the 
surface by the myce- 
lium (Fig. 141). This 
spore, which is two- 
celled and has a very 
| heavy wall, is the 
Fic. 141.—The winter yyy fer spore; for it is 
spores of wheat-rust. 2 a 
in this form that the 
rust usually endures the winter. 
In the spring the winter spores, lying 
where the plants on which they were 
produced have decayed, begin to ger- 
minate, each one of the two cells send- 
ing out a short filament. This filament 
is not a parasite, but a saprophyte, and 
usually consists of four cells, each one 
of which sends out a little branch, at 
the tip of which a small spore is pro- 
duced (Fig. 142). These may be called 
early spring spores. 
These early spring spores are scat- 
tered by the wind; and those falling 
upon barberry leaves germinate, the 
new mycelia entering and spreading 
through the leaves. In this phase the 
rust is parasitic upon an entirely dif- 
Tia. 142.—A_ winter 
spore of wheat -rust 
germinating, each fila- 
ment producing four 
cells, each of which 
sends out a branch 
that produces at its tip 
a spore (early spring 
spore),— After Tu- 
LASNE. 
ferent host, and one that holds no relation to wheat. The 
mycelium in the barberry leaves sends to the leaf surface, 
