64 TEXTBOOK OF BOTANY 



92. Infection of the Wheat. For some reason the spring 

 spores produced on the barberry cannot infect a barberry 

 plant, but they can infect wheat. If one of these spores, 

 carried by the wind or other agency (sometimes, perhaps, by 

 an insect), falls upon a young wheat plant, the conditions of 

 temperature and moisture being favorable, it swells and 

 pushes out a projection (a germ tube). The germ tube grows 

 into a thread which makes its way along the surface of the 

 wheat leaf or stem until it reaches an air-pore. The thread 

 swells at a point just above the pore ; from the swelling a 

 new branch grows down through the pore, and in the spaces 

 between the cells of the host it develops into a branching 

 plant body which may infeet a large area of the wheat plant. 

 Short branches of the fungus grow into the cells of the wheat, 

 whose food supply is used by the parasite in the same way 

 as was that of the barberry, cells. The portion of the rust on 

 the outer surface of the host (the germ tube and swelling) 

 soon withers and disappears. 



93. Summer Spores. As soon as the fungus is well 

 established in the wheat tissues, it proceeds to form spores 



of a second kind. 

 Here also the first 

 step toward spore 

 formation is the 

 appearance of a 

 cushion of fun- 

 gous threads be- 

 neath the epider- 

 FIG. 28. Section through a sorus on the wheat, . ,. , , 

 in which both one-celled summer spores and two- 1 

 celled winter spores are being formed. then the threads 



grow outward to 



form a layer of rather broad parallel basal cells, which push 

 directly against the epidermis of the wheat and finally 

 break it. The break in the epidermis is irregular in shape 

 and often large ; it is called a sorus. From the outer end 



