THE FUNGI : ASCOIMYCETES AND BASIDIOMYCETES 295 



Urediniales 



The Urediniales are Basidiomycetes in which the basidia are formed 

 as outgrowths from a special type of spore. The life-cycle is complicated 

 by the production of a number of different spore-types w^hich succeed 

 one another. They are all obligate parasites, mostly on the tissues of 

 higher plants. In many types two dissimilar host plants are involved in the 

 life-cycle, and there is a definite alternation of spore-types from one host 

 plant to the other. 



As an example of the group we shall consider the life-history of Puccinia 

 grominis, the common Rust of Wheat. 



Puccinia graminis (The Black or Stem Rust of Wheat) 



This Fungus illustrates the condition known as heteroecism, in which 

 the life-cycle of the species involves two distinct host plants. The alternate 

 hosts in this case are the Wheat plant {Triticum vulgare) and the Barberry 

 {Berberis vulgaris). A hundred years ago this Rust was a serious plague of 

 wheat fields in Britain, but the removal of Barberry bushes, following upon 

 the discovery of their part in the life-cycle of the parasite, has reduced its 

 attacks almost to insignificance. 



Life-history of the Fungus 



It will be convenient to start the life-history with the basidiospores, 

 which are carried by wind to the leaves of the Barberry in early spring. 

 There the spore germinates and the primary hypha penetrates into the 

 epidermis directly (Fig. 287). From there it branches freely, forming a 

 uninucleate, monoploid mycelium between the cells of the mesophyll. 









m.' ^. 



Fig. 287. — Puccinia graminis. Gennination of basidiospore 

 infecting leaf of Barbern-. {After Allen.) 



