DISEASES OF CEREALS AND GRASSES 51 



• 

 These contain spores, but their use is not definitely 



known. On the under side of the leaf, at the 

 same time, cup-like openings appear, called cecidia. 

 These eecidia are seen as reddish-yellow raised 

 patches, and they are filled with ai^cidiospores. 

 The secidiospores are roundish cells with smooth 

 outer coats ; they retain their power of growth for 

 a short time only. They may germinate on the 

 barberry leaf, but the germ tube does not enter ; 

 if, however, they are carried by the wind to the 

 stem or the leaf sheath of the wheat plant, the 

 germ tube readily enters the inner tissues, develops, 

 and in the course of 8 to 10 days aerial hyphae 

 break through the surface and bear at the tip the 

 uredospores ; which in mass appear as red pustules. 

 These uredospores are readily carried by the wind 

 to healthy wheat plants, thus causing infection. 



Experiments in America have demonstrated that, 

 in that country at least, the rust is able to survive 

 the winter on the over-wintering wheat plants, pro- 

 ducing an abundance of uredospores in the spring, 

 so that the presence of the barberry is not an es- 

 sential for the maintenance of the disease. Indtted 

 it has been found that sometimes the mycelium 

 remains alive in the seed, and so infects the new 

 crop. 



Note. — According to Erikson there are six biological varieties 

 of P. gramijiis^ each having its ascidial stage on the barberry. 



For preventive measures see p. 54. 



4* 



