BASIDIOMYCETES 433 



times called summer-buds — are of the colour of rust of iron. They 

 are found in such quantity as to attract public attention, and this 

 provided the name. 



The most familiar example, as it is also economically the most 

 important, is the Rust of Wheat, Puccinia graminis. In June and 

 July the green leaves of the Wheat are often seen to lose their colour 

 (Fig. 329). Yellow patches appear between the veins, and run 

 together into lines that follow the softer mesophyll. The epidermis 

 bursts, and innumerable orange bodies are set free, which are easily 

 carried as dust by the wind. These are the Uredospores of the Rust. 

 A part of a field thus diseased is a centre of infection, and the fungus 

 may often be seen to spread from it down the prevailing wind. This 

 production of spores, which represents so much material robbed from 

 the developing crop, may continue till autumn ; then gradually the 

 patches of disease change in colour to a dark purple-brown. This is 

 due to the formation in them of a new kind of propagative body, the 

 Winter-bud or Teleutospore, which is firmly attached to the straw, 

 and dies down with it, or is removed with the crop. These spores 

 retain their vitality till the next spring, when they germinate (Fig. 



335, P- 436). 



The result of their germination has been shown experimentally 

 to be the infection of the leaves of the Barberry plant, and the pro- 

 duction of a second stage, which appears as red or yellow blotches, 

 thicker than the healthy parts of the leaves that bear them : small 

 dark spots open on the upper surface (spermogonia), and numerous 

 widely gaping cups (aecidia) are clustered together on the lower 

 surface. This stage was first described as 4 ' Cluster-Cups," and 

 regarded as a distinct fungal disease under the name of Aecidium 

 berberidis (Fig. 330). But it is now known that the spores produced 

 by the cups are able on germination to cause a new infection of the 

 leaves of the W T heat plant, which results again in the growth of a 

 mycelium bearing the uredospores. There are thus two stages 

 of the disease, the one on the Wheat or other Grasses, the other on 

 the Barberry. Long before it was proved that these two different - 

 looking diseases were only stages in one life-history, a connection 

 between the two had been suspected. It was thought that the 

 Barberry was in some way injurious to Wheat. But it was not till 

 late in the Nineteenth Century that the cycle was completely demon- 

 strated. A similar hetcroecismal life is now known for numerous 

 species of Rusts. One of the commonest is Puccinia car ids, of which 

 the uredospores and teleutospores are on leaves of species of Carex, 



