444 



BOTANY OF THE LIVING PLANT 



surface. This stage was first described as " Cluster-Cup?," and 

 regarded as a distinct fungal disease under the name of Aecidium 

 berberidis (Fig. 376). But it is now known that the spores produced 

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



FIG. 377. 



Longitudinal section of a leaf of Wheat, showing a tuft of Uredo-spores bursting 

 through the epidermis. Highly magnified. (After Marshall Ward.) 



leaves of the Wheat plant, which results again in the growth of a 

 mycelium bearing the uredo-spores. 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 



FIG. 378. 



Germinating Uredo-spores, showing various stages of development of the germ- 

 tubes, a, b, c,. Very highly magnified. (After Marshall Ward.) 



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 heteroecismal life is now known for about fifty 

 species of Rusts. One of the commonest is Puccinia caricis, of which 



