268 GRASSES OF IOWA. 



layer of cells of the cups also break, thus exposing a large 

 number of one celled spores borne in chains. These spores 

 arise from short stalks contained at the base of these cups; the 

 cluster cup spores are known as aecidiospores. They are trans- 

 ported by the wind and other agencies, and have the power to 

 germinate soon after maturity. When the proper host — a 

 grass like bent grass, oats or wheat — occurs, the germ tube 

 enters by way of the stomata, or the so-called breathing pores. 

 The germ tubes produced by the spore of Aecidiu)it berberidis 

 are simple or branched, and in fourteen days usually give rise 

 to ihe uredo spores, which occur in definite spots called sori. 

 These spots occur in great number along the veins of the leaves. 

 Before breaking open, the tissues of the leaf are somewhat 

 paler at those places. The nourishment afforded by the host 

 causes a vigorous mycelium to form, which soon collects in 

 places, pushes the epidermis out, and an oraage-colored pustule 

 is formed; this is known as the uredosorus. 



A section through a diseased sorus shows that an abundance 

 of the vegetative mycelium grows between the cells of the 

 plant, and in some cases haustoria penetrates them. This pus- 

 tule contains a large number of one-celled, round or elliptical, 

 spiny, orange-colored spores, the uredo spores. This spore 

 has two membranes, the outer exospore being provided with 

 wart-like projections. The inner endospore is provided with 

 several pores through which the germ tube appears. These 

 spores germinate in from three to foar hours; they can thus 

 start a general infection. These spores, carried by the wind, 

 rain or insects to aaother part of the same or another plant, 

 germinate. The germ tubes branch and spread over the sur- 

 face, but the tube cannot enter the host, a grass of some kind, 

 such as wheat, oats or barley, unless it reaches the opening of 

 the stoma, since it cannot bore through the epidermal cells. A 

 single sorus contains hundreds of spores, and as a single plant 

 may contain hundreds of pustules, it can readily be seen that 

 rust must become quite general. 



The red rust stage is followed by the black rust stage, known 

 as the teleuto stage. The sori are brownish-black in color, and 

 frequently occupy the same place that the uredo stage did. 

 The spores are dark brown in color, two-celled and smooth, 

 having attached to them a persistent stalk known as the pedi- 

 cel. The teleuto spores do not germinate till the following 

 spring, when each call pro luces a germ tube, the promyceliunL 



