, Uredospores. 



The primary generation appears in the early spring, and originates 

 either by infection from the promycelial spores, aecidiospores, or, it may 



Spennatia 



Sporidiola, 



Uredospores 

 Teleutospores 



Uredospores 

 FIG. 6. 



he, from a perennial mycelium. The spore-layer is usually distinguished 

 by its larger size, and the corresponding injury it causes ^ to the host- 

 plant; while the secondary generation, produced by infection from the 

 preceding uredospores, has' a smaller spore-layer. 



A well-investigated and very striking case is seen in Triphragimum 

 ulmariae, in which the primary" and secondary uredospores generally re- 

 semble each other; but the sori of the former are much larger and pro- 

 duced in great abundance, occurring; on the stalks and midribs of the 

 leaves, while those of the latter are small and scattered on the under sur- 

 face of the leaf. So striking is the difference that a special name has been 

 proposed for the sorus, epiteosporiferotis and epiteospore for the primary 

 spore; but it is quite superfluous, since the larger and more prominent 

 sori of the primary generation may be accounted for from the strong 

 vigour of growth in "the plant at the time when the first infection occurs. 



The wintering of the rust-fungi, in the form of the uredo, depends 

 on the nature of the rust itself, and also on that of the host-plant. If 

 any portion of the host-plant remains green and succulent during the 

 winter, then the fungus has an opportunity of surviving, and it is thus 

 seen that climatic conditions have a deal to do with the persistence of 

 the fungus. When the winter is mild and green vegetation flourishes, the 

 mycelium of the rust fungus may continue to grow, and may even produce 

 spores; whereas, if the winter is severe, and the mycelium does not 

 remain in the perennial parts of the plant, then the continuance of the 

 fungus is likely to be by teleutospores, which can last through the winter 

 on dead stems or other decaying vegetable matter. This so-called winter- 

 ing of the uredo depends so much on the climate that in a mild climate 

 the fungus may perpetuate itself exclusively by uredospores ; whereas, 

 under severe conditions, it has to resort to teleutospores. 



A very striking case is recorded by Lagerheim 1 , in Uromyces fabae 

 (Pers.), De Bary, which in Europe passes through the three stages of the 

 aecidio-, uredo-, and teleuto-spore, while in Ecuador it only produces the 

 uredo-form. Heteroecism, or alternation of generations, "is an arrange- 

 ment suited to conditions where the seasons are variable, but in an equable 

 climate such as Ecuador there is no occasion to produce such a variety of 

 spore-forms, and so the fungus adheres to the one which serves its purpose 

 best. Australia is also a case in point as far as Puccinia graminis is con- 

 cerned, and it is necessary to remember that our seasons are the reverse of 

 those of the Old World. Our cereal crops are generally sown in the 

 aufuimn months of April and May, or even earlier, and the harvest is 



