412 



irXGOt'S DISEASES OK PLANTS 



the upper epidermis. At maturity the flask-shaped body consists 

 of an indefinite wall, later giving rise to numerous filamentous 

 branches within, most of which project inward toward the center, 

 the majority bearing on their tips small rod-shaped or oval conidia. 

 Other filamentous hyphae emerge from the mouth of the pycnidium 

 as hair-like processes. The hyphae making up the pycnidium are 

 all tinted yellow or orange in color, the coloring matter being first 

 present in the protoplasm and later deposited in the cell walls. 

 The spots in which first the pycnidia and later the aecidia are pro- 

 duced are pale yellowish to orange in color, and the leaf is some- 

 times considerably thickened. 



The aecidia are organized in the mesophyll tissue near the lower 

 epidermis. In general, each aecidium is differentiated and developed 

 following the formation of a weft of filamentous hyphal elements. 

 According to Richards there is first formed at the base of the 

 hyphal mass a well-differentiated short, thickened hypha. By the 

 division of this hypha there arise numerous fertile branches, or 

 young conidiophores, each of which originates a chain of spores. 

 Every alternate cell in the chain becomes a perfect spore ; the 

 others are small and temporary, remaining for a time as wedge- 

 like structures between the spores. The outer border, or inclosing 

 layer, consisting of differentiated hyphae, forms a definite peridium. 

 Prior to the rupture of the epidermis, the fruit body has a more 

 or less spherical form, and it consists merely of the sheath, or 

 peridium, inclosing the numerous chains of spores. The increase 

 in size of the spores breaks the peridium as w r ell as the epidermis, 

 and the aecidia appear superficially in the cluster-cup form (Fig. 

 202). The spores there exposed separate readily and are dis- 

 tributed. The aecidiospores are more or less spherical and vary 

 from 14 to 26 fji in diameter. This spore germinates, and upon the 

 different grass hosts it penetrates the stomata, producing the my- 

 celium of the uredo stage. 



The uredosori generally occupy linear areas, yet upon some 

 hosts they may be in the form of small circular dots. They show 

 a considerable amount of coloring matter when young, and when 

 mature appear yellowish brown. They are ovate, 10-15 X 20-35/4, 

 with rather thick walls, the outer of which bears numerous echinu- 

 lations or small spine-like appendages (Fig. 203, a). There are four 



