THE WHEAT RUST 6i 



89. Spring Stage of the Wheat Rust. — The first plant 

 attacked by this rust is not the wheat, but the barberr3^ 

 The branching threads of the fungus Uve in the leaves, young 

 stems, and fruits of the barberr}-. The threads, made up of 

 rather short cells attached end to end, grow for the most 

 part in the spaces between the cells of the host ; but they 

 gi^'e off, here and there, short branches which make their T\-ay 

 by means of a cell-wall- dissolving enzym into the cells of 

 the barberry. These short branches digest and absorb the 

 food materials of the host. Sometimes, though not usually, 

 they Idll and absorb the H^ing matter of the host cells them- 

 selves. Alore often the growth and di\'ision of the host 

 cells are abnormally stimulated, so that an infected leaf is 

 swollen, especiallj- on the under side (Fig. 27, .4), in the 

 regions in which the rust threads are most abundant. How- 

 ever, although such parts become unusually large, the bar- 

 berry plant as a whole is deprived of much of its food, and 

 so is more or less weakened. After the rust has become well 

 established in the barberry tissues, it proceeds to form spores. 



90. Spring Spores. — These spores are formed in small 

 cup-shaped structures. The cups appear most commonly 

 on the under sides of the leaves of the barberry (Fig. 26, B), 

 projecting from the surface and just visible to the naked eye ; 

 but sometimes they grow on the upper sides of the leaves, on 

 the fruits, or on the j'oung twigs. The cups are formed in 

 groups, and so are called cluster cups. The part of the leaf 

 on which a cluster of cups is borne is usually yellowish for a 

 short distance about the cluster ; and the leaf tissues in 

 this yellow spot die early. 



The formation of the cups and of the spores can best be studied 

 in a cross section of a leaf. At the place where a cup is to appear, a 

 cushion, composed of fimgous threads packed closely together, is 

 formed beneath the epidermis of the leaf. Then the ends of the threads 

 turn directly toward the epidermis and grow so as to make a basal 

 layer of rather large vertical cells (Fig. 27, B). The pressure caused 

 bj- the growth of this layer and of the cushion of threads beneath it 



