CRYPTOGAMS 



317 



C. Rusts 



Material. — A leaf of wheat affected with red rust ; a leaf or a stalk 

 with black rust. Some barberry leaves with yellowish pustules on the 

 under side, which under the lens look like clusters of minute white corollas. 

 These are popularly known as " cluster cups." As the spots on barberry 

 occur in spring, the red rust in summer, and the black rust in autumn, 

 gather the specimens as they can be found, and preserve for use. 



The orange leaf, or brown, rust {Puccinia rubigo-vera) is more common 

 in some parts of the country than the ordinary wheat rust {Puccinia 

 graminis), but the two are so much alike that the directions given will 

 do for either. If the orange leaf-rust (so named from its color, and not 

 from any connection with orange leaves, the logical relation of the words 

 being orange leaf-rust, and not orange-leaf rust) is used, the cups and 

 pustules should be looked for on plants of the borage family — comfrey, 

 hound's-tongue, etc. The orange leaf-rust of apple is caused by a fungus 

 which will serve to illustrate the same class of parasites. 

 The "teleuto" stage of tliis will be found on cedar 

 trees, in the excrescences commonly known as "cedar 

 apples"; the "cluster cups" on the leaves of apple 

 and haw trees affected with the disease. 



359. Red rust. — Uredo stage. Examine 

 a leaf of " red rusted " wheat under the lens, 

 and notice the little oblong brown dots that 

 cover it. These are clusters of spore cases, 

 and are the only part that appears above the 

 surface. Viewed under the microscope, the 

 red rust will be seen to consist of a mycelium 

 (see Fig. 452), which ramifies through the 

 tissues of the leaf and bears clusters of single- 

 celled reddish spores that break through the 

 epidermis and form the reddish brown spots 

 and streaks from which the disease takes its 

 name. These spores, falling upon other 

 leaves, germinate in a few hours and form 

 new mycelia, from which, in six to ten days, 

 fresh spores arise. Formerly this was thought to complete the 

 life history of the fungus, to which the name of Urcdo was 

 given. It is now known, however, that the red rust is merely a 



450 



Figs. 450, 451.— 

 Leaf of wheat af- 

 fected with orange 

 leaf-rust (Puccinia 

 rubigo-vera), uredo 

 stage : 450, upper 

 side of leaf ; 451, 

 under side. 



