g2 
in the autumn are allowed to remain exposed to the 
weather through the winter, they will, under suitable con- 
ditions, germinate in a few hours in March or April. In 
this case the process of germination of the spore differs 
from any of those dealt with in previous chapters. Each 
of the two cells of the spore sends out a delicate tube, 
which, after growing to two or three times the length of 
the spore, becomes divided into four cells or segments. 
Then a delicate peg-like branch is put out from each of 
these segments, and a minute, oval spore is formed at the 
tip of each peg. The question now arises as to what 
becomes of these teleutospores and the small oval spores 
or sporidia they produce. For many years this was not 
understood, although trials again and again proved that 
the teleutospores or their sporidia were unable to cause 
new infection on the wheat plant. 
The clue to the problem was discovered by De Bary 
in the old belief of farmers that outbreaks of rust were 
in some way connected with the presence of barberry. He 
first made a careful study of the yellow fungus which 
occurs on barberry, and then proved the connection of this 
with the rust on wheat. If one of the diseased areas on 
the barberry is closely examined it is found to arise on 
a swollen part of the leaf, and on the under side a number 
of small, yellow, cup-like bodies are produced. From 
these cluster-cups yellow spores, known as aecidiospores, 
are liberated as a fine dust. A section through the diseased 
area shows that the cluster-cups arise as round masses of 
fungal filaments beneath the skin of the leaf. Within 
the ball of fungus, aecidiospores are produced in chains, 
and when this growing ball bursts the skin of the leaf, it 
opens at the apex and the edges turning back give the 
structure the appearance of a minute cup or bowl. The 
aecidiospores arise from threads at the base of this, and 
as they ripen and are set free new spores are produced. 
Now De Bary discovered that when sporidia from the 
teleutospores on wheat are sown on barberry leaves in the 
spring, infection takes place and the cluster-cups just 
described are produced in two or three weeks. Not only 
did he prove this, but he also showed that when the 
aecidiospores from the cluster-cups are sown on the leaves 
of wheat, they germinate, and like the uredospores send 
germ tubes through the stomata and thus infect the leaf. 
Pustules containing uredospores and later teleutospores 
