GROUP I. THALLOPHYTA : FUNGI. 



299 



Order I. Uredineae. This order comprises those parasites which are 

 generally known as Busts, on account of the rusty appearance which 

 they give to their host-plants at a certain stage of their life-history, when 

 they bear at the surface a great number of orange-coloured spores. 



Puccinia G-raminis affords an example of the most complex life-history 

 with heteroecism : that is, that the different forms inhabit different hosts. 

 It infests Wheat, Rye, and other Grasses, and developes its mycelium in 

 the tissues of the young plants. During the summer it produces groups of 

 simple sporophores, at the apex of each of which a single oval spore, 

 termed a uredospore, of en orange colour, is formed by abstriction (Fig. 

 178, III) in consequence of the grea,t development of cells at these points 

 the epidermis of the host is 

 ruptured, and the groups of 

 uredospores are visible on the 

 surface as rusty spots. These 

 uredospores are scattered by the 

 wind, and infect other Grass- 

 plants ; on reaching a leaf, the 

 uredospore germinates at once, 

 forming a hypha which enters 

 through a stoma into the in- 

 terior of the leaf, where it de- 

 velopes into a mycelium bearing 

 uredospores. This stage in the 

 life-history is termed the Uredo- 

 form. 



Later in the season, when the 

 tissues of the hosts are becoming 

 hard and dry, the Uredo-form 

 no longer produces uredospores, 

 but dark-coloured often com- 

 pound spores, known as teleuto- 

 spores (Fig. 178, 77), developed in 

 the same way as the uredospores. 

 The teleutospores remain qui- 

 escent during the winter. When 

 they germinate in the following 

 spring, one or both of the cells 

 gives rise to a small, free, non- 

 parasitic mycelium (promycel- 

 ium), from each of the cells of which a delicate conidiophore is produced, 

 which developes a small conidium (termed a sporidium) by abstriction at 

 its apex (Fig. 179). 



The sporidia are scattered by the wind, and if they fall on the leaves of 

 the Barberry they germinate, giving rise to a hypha which pierces the 

 epidermis of the leaf, and then forms a dense mycelium in the inter- 

 cellular spaces of the mesophyll. At certain points the tissue of the leaf 

 is hypertrophied, forming cushions which project on the under surface. 



c. 



A. 



FIG. 179. Germination of teleutospores of 

 various Uredineae : A of Puccinia Graminis ( x 

 40C) ; B of Melampsora ( x 300) ; C of Coleo- 

 sporium ( x 230) ; t teleutospore ; pm promy- 

 celium ; sp sporidia. 



