LEAF AND SEEDLING DISEASES 173 
the Continent. The rusts are, for the most part, charac- 
terized by a very complex life-cycle, and many of them 
grow at different periods of the year on two different hosts 
and bear four different kinds of spores in the course of their 
life-cycle. Species which migrate from one host to another 
and back again are called heteroecious, to distinguish them 
from autoecious species, which live always on one host or on 
the ground. 
The discovery of heteroecism is one of the landmarks in 
the history of plant pathology. The first rust in which it 
was demonstrated is the black rust of wheat (Puccinia 
graminis), a fungus which has caused a great deal of damage 
to wheat crops all over the world. It had long been suspected 
that wheat rust was encouraged by the presence of plants 
of barberry, and a decree was promulgated in Rouen as early 
as 1660,1 ordering the destruction of barberry in the vicinity 
of wheat fields. But it was not till 1865 that de Bary showed 
that this rust actually infects barberry plants in spring, and 
that wheat leaves are again infected in early summer by 
spores produced on the barberry plant. 
The life-cycle of a typical heteroecious rust will be better 
understood by following the accompanying diagram (fig. 71). 
Starting from the bottom left-hand corner we have the 
mycelium in the tissues of the host a (e. g. the wheat plant). 
This mycelium bears red or rust-coloured pustules on the 
surface, called uredosori, which contain numerous spores 
called uredospores. These spores again infect the host 
A, sending their germ-tubes through the stomata, and 
giving rise to fresh mycelium which bears more uredospores, 
and so on through the summer. Towards the end of the 
season mycelium on the same host produces another kind 
of spore (dark brown or blackish in the case of the black 
rust of wheat), called the teleutospore. The teleutospores 
may appear in special pustules (feleutosori) or side by side 
with, and in the same sori as, the uredospores. These 
teleutospores, which are thick walled, are the perennating 
organs of the fungus. They fall on the ground and remain 
there during the winter. They contain enough food reserve 
1 Wheitzel, 1918. 
