THE FUNGI 255 



vertical unicellular stalk (see Fig. 105, E, u). They arise 

 in great numbers close together, forming large groups or 

 sori ; in each sorus the development begins near the 

 middle of the mass and spreads centrifugally. As the 

 sorus develops, the epidermis lying above it is burst, 

 and the ripening conidia are exposed to the air. 



The single conidium, as it matures, acquires a rather 

 thick cell-wall, consisting of two layers, the exospore 

 and endospore, the outer of which is of a brown colour 

 and is covered with short spines, while the inner is 

 colourless. There are two germ-pores or thin places 

 in the cell- wall, one on each side of the spore (see 

 Fig. 105, E, u). In the cell-contents a quantity of oily, 

 orange-coloured pigment is present. 



These conidia are called the uredospores, because 

 they were formerly regarded as belonging to a distinct 

 genus Uredo the species of which are now known 

 to represent merely a particular form of fructification 

 of the Puccinia. These uredospores become detached 

 from their stalks, and are scattered by the wind and 

 possibly also by the agency of insects. They are capable 

 of immediate germination, and give rise to the same 

 form of the Fungus as that which produced them, 

 growing on the same kind of host - plant, or at least 

 on an allied species. The hyphae grow out from the 

 germinating spore through the two germ-pores (see 

 Fig. 105, F). If germination takes place upon a Wheat 

 plant or other host of the Grass family, the hyphse 

 grow along the surface of the 'epidermis until a stoma 

 is reached, through which an entrance into the inter- 

 cellular spaces of the host is effected. Thenceforth the 

 hyphse at once proceed to develop a new mycelium, from 

 which new crops of uredospores arise. This form of 



