FUNGI n 



The rust of wheat (Pucdnia graminis) has a more com- 

 plicated life-history, possessing four distinct structures 

 connected with reproduction, and, like many of its allies, 

 lives on different host-plants during different stages of its 

 development. Heteroedsm is the term used to denote 

 fungi that require to grow on two distinct hosts to com- 

 plete the full cycle of their development. 



The spring condition of wheat rust, called the Aecidium 

 or ' cluster cup ' stage, occurs on the leaves, young shoots, 

 sometimes also on the flowers and young fruit, of the bar- 

 berry (Berberis vulgaris\ under the form of clusters of 

 minute cup-shaped structures with white, fringed margins, 

 and filled with golden-yellow spores (Fig. 2, 8 and 10). 



Other bodies, considered to be in some way connected 

 with reproduction, known as Spermogonia^ are produced by 

 the fungus on barberry leaves, accompanying the aecidium 

 condition, but nothing definite is known respecting them. 



The mature aecidium spores are scattered by wind, and 

 those that happen to alight on the leaves of wheat or 

 various other grasses, germinate if the surface is damp ; 

 the germ-tube enters the tissues, where it forms a mycelium, 

 and in a very short time produces dense groups of rust- 

 coloured uredospores, which burst through the tissues of 

 the leaf, forming the rust-coloured streaks on its surface, 

 known as wheat-rust (Fig. 2, i and 2). Uredo or summer- 

 spores are produced in immense numbers throughout the 

 summer months, thus securing the spread of the disease. 



During the autumn teleutospores or winter-spores are 

 produced by the mycelium that gave origin to uredospores 

 earlier in the season. The streaks formed on the leaves 

 by the teleutospores are much darker in colour than those 

 produced by the uredospores (Fig. 2, 5). The teleuto- 



