174 LEAF AND SEEDLING DISEASES 
to live till the spring and then to send out a small germ 
tube of limited growth, called the promycelium, and this 
bears a few, generally four, very small spores, which are 
known as sporidia. The sporidia are very light, and being 
lifted by the wind settle on the leaves of neighbouring 
plants. But, though they may germinate, they inevitably 
perish unless they happen to come in contact with a plant 
& 
——. &. On the ground 
§& No ' 
ae 
Myce, ius 
m 
Y ee J | 
%, ny 3 On Plant B 
P| = % ayn 
~y. 
Gm 
On Plant A 
Fig. 71. 
of some definite species, the host B. In the case of the 
black rust of wheat, this plant is the barberry. The germ- 
tube which grows from a sporidium pierces the cuticle and 
enters an epidermal cell of the host B, and develops into 
a mycelium which fills the tissues, and gives rise to another 
special type of pustule, also usually rust-red in colour, 
which is called the aecidium. The aecidium pustules contain 
spores, known as aecidiospores, which are incapable of 
infecting plant B, but infect plant a by sending their germ- 
tubes through the stomata of the epidermis. 
The mycelium on plant B generally produces, in addition 
to aecidia, other pustules, known as spermogonia (or pycnidia), 
which bear minute spore-like bodies, which are generally 
