Minnesota Plant Life. 



51 



Fig. 11. — Patches of wheat-rust, natural 

 size and enlarged. The red rust stage. 

 After Atkinson. 



of wheat-rust. They cannot convert themselves into each other 

 but are independent plants as distinctly as are oats and rye. It is 

 well-known that there are two stages of wheat-rust, one of which 

 develops in the early summer and autumn and is known as the 

 red rust, the other developing in late summer and autumn and 

 known as the black rust. The char- 

 acteristic colors of the two stages 

 are given by masses of spores grow- 

 ing in layers upon the plant-body of 

 the rust which in turn consists of a 

 network of parasitic threads living 

 in the tissues of the wheat plant, the 

 skin of which is burst by the fungus. 

 It is the same plant body which 

 produces the red spores forming 

 red streaks on the wheat leaf that 

 afterwards produces black spores 

 in equally enormous numbers occasioning the black rust stage. 

 The red or "summer" spores are ovoid, spiny bodies, properly 

 described as single cells. Their walls are thinner than those 

 of the black rust. The black, or "autumn" spores have 



smooth walls and are divided into two 

 cells by a cross partition situated near 

 the centre of the somewhat elongated 

 and pointed or rounded body. Red- 

 rust spores, when separated from their 

 stalks by the wind, may be carried 

 throughout the summer to other 

 wheat plants, and thus the infection 

 spreads possibly over a whole field. 

 The black-rust spores remain dor- 

 mant during the winter upon the stub- 

 ble and debris of the field. In the 

 spring each of the two cells of the 

 black-rust spore develops a tiny jointed body upon which four 

 very small thin-walled, colorless spore-cells are produced. Now 

 is the time selected by the wheat-rust for the periodic change of 

 habitation. It is known that the small spores thus produced 

 do not so readily germinate if blown upon wheat plants, but 



m 



I 



Fig. 12. — Patches of wheat-rust, nat- 

 ural size and enlarged. The black 

 rust stage. After Atkin.son. 



