198 



f (3) The germination of the teleutospores 

 | giving rise to a promycelium and sporidia. 

 Aecidial J This is a non-parasitic stage, 

 stage. (4) The germination of the sporidia to 



J form a parasitic mycelium which ultimately 

 (^produces secidiospores. 



In our typical example the puccinial and aecidial stages 

 are passed on different host-plants ; such a fungus is said to be 

 heteroecious, and the host which nourishes the aecidial stage is 

 in this case called an intermediate host. Other allied fungi 

 pass both the puccinial and the aecidial stage on the same 

 host-plant andare consequently called autoecious. 



Now in the central areas of India Puccinia graminis is one 

 of the commonest wheat rusts, and it has been found widely spread 

 in places over 600 miles from any species of Berberis which 

 in India are confined to the high hill ranges. In Europe also 

 it has been found that wind-blown secidiospores can ordinarily 

 only infect plants within a radius of 25 yards from the bush 

 on which they arise, and hence it is improbable that wind- 

 blown a3cidiospores could in such cases be responsible for the 

 disease. This rust also is common in parts of Australia, 

 e.g. in Victoria, where no Berberis grows wild and very few are 

 cultivated. Again, if the secidial stage on the Berberis is 

 always necsssary for the continued existence of the fungus, 

 the complete destruction of all species of Berberis should suffice 

 to eradicate the disease in the areas treated. This, however, has 

 not been found to be the case. Finally Aecidium Berberidis is, 

 in India, restricted to a portion of the Himalayan Eange, and 

 it is precisely in this area that Puccinia graminis is extremely 

 rare on cereals. We are therefore driven to the conclusion 

 that the fungus can, and often does, dispense with the a^cidial 

 stage and continues to exist on the wheat, year after year, in 

 the puccinial stage. How this is managed is not yet clearly 

 understood. It has been suggested that the uredospores may 

 pass from the wheat to some wild perennial grass and that the 

 fungus may thus continue to maintain itself on the latter in the 

 puccinial stage until the season for the next wheat crop comes 

 round again. In Europe this fungus has been found on 150 

 different species of grasses, including oats, wheat, barley and 

 rye, these being known as collateral hosts. It has, however, been 

 found that the form on wheat is exclusively limited to the wheat 

 and cannot impart the disease to any other collateral host, 

 with the sole exception of rye and barley, which in rare cases 



