252 STRUCTUKAL BOTANY 



The rust occurs commonly in summer on the leaves 

 and stems of Wheat, Eye, and Oats, as well as on various 

 wild grasses. In this condition the parasite is easily 

 recognised, for it forms conspicuous long, rusty red or 

 orange streaks between the veins of the leaf or along 

 the surface of the stem. When the rust is mature, 

 we see that these streaks are made up of a fine powder, 

 bursting out through the epidermis of the host-plant, 

 the powder consisting of the conidia or uredospores of 

 the Fungus. Thus it is only the fructification of the 

 parasite which is visible externally. 



The vegetative part or mycelium is hidden in the 

 tissues, and requires very careful microscopic examina- 

 tion for its detection. It does not spread throughout 

 the whole plant, but is limited to isolated patches of 

 the particular organs attacked. The mycelium consists 

 of a dense- web of excessively fine hyphse, growing 

 luxuriantly between the cells of the parts affected, 

 and also sending out haustoria, which penetrate the 

 cavities of the cells themselves. The mycelium is 

 multicellular, the transverse septa, however, only occur- 

 ring at long intervals. The elongated form of the rust- 

 streaks is due to the fact that the Fungus attacks the 

 soft tissues, lying between the longitudinal bands of 

 fibres, which accompany the vascular bundles of the 

 leaf. 



The fructification characteristic of the rust-stage of 

 the Fungus is produced in great quantities throughout the 

 summer months. Preparatory to its formation, certain 

 of the more superficial hyphae pack themselves closely 

 together, forming a dense layer just below the epidermis 

 of the host-plant. From this layer the conidia are formed. 

 Each conidium is a single cell, borne at the end of a 



