164 BOTANY 



the bark from any tree infected with bracket fungus, and you will find the 

 silvery threads of the mycelium sending their greedy hyphae to all parts of 

 the wood adjacent to the spot first attacked by the fungus. This fungus 

 begins its life by the lodgment of a spore in some part of the tree which has 

 become diseased or broken. Once established on its host, it spreads rapidly. 

 There is no remedy except to kill the tree and burn it, so as to burn up the 

 spores. Many fine trees, sound except for a slight bruise or other injury, 

 are annuallj'" infected and eventually killed. In cities thousands of trees 

 become infected through careless hitching of horses so that the horse may 

 gnaw or crib on the tree, thus exposing a fresh surface for the growth of 

 spores. 



Field Work. — A field trip to a park or grove near home will show the 

 great destruction of timber by this means. Count the number of perfect 

 trees in a given area. Compare with the number of trees attacked by the 

 fungus. Does the fungus appear to be transmit; ted from one tree to a tree 

 near at hand ? In how many instances can you discover the point where 

 the fungus first attacked the tree ? 



Parasitic Fungi. — Of even more importance are the fungi that 

 attack a living host. The most important of such plants from 

 an economic standpoint are the rusts, smuts, and mildews which 



prey upon grain, corn, and 

 other cultivated plants. 

 Some of these are also 

 parasitic upon fruit and 

 shade trees. Damage ex- 

 tending to hundreds of 

 millions of dollars is annu- 



Corn smut, a fungus parasitic on corn; the black „ii„ Afxnc^ hir f Itaqa -nlanf « 



mass consists almost entirely of ripe spores. ^^V ^^'^^ ^y Lnese pidms. 



Wheat Rust. — Wheat rust is probably the most destructive parasite in 

 the world. For hundreds of years wheat rust has been one of the most 

 dreaded of plant diseases, because it destroys the one harvest upon which 

 the civilized world is most dependent. For a long time past the ap- 

 pearance of rust has been associated with the presence of barberry bushes 

 in the neighborhood of the wheat fields. Although laws were enacted nearly 

 two hundred years ago in New England to provide for the destruction of bar- 

 berry bushes near infected wheat fields, nothing was actually known of the 

 relation existing between the rust and the barberry until recently. It was 

 then proved beyond doubt that the wheat rust passed part of its life as a 

 parasite on the barberry and from it passed to the wheat plant. If a blade 

 of wheat infected with rust is examined early in the summer, the leaf 

 blade will show a collection of reddish brown spots or streaks. These 



