Minnesota Plant Diseases. 87 



account of its vast importance economically. To realize this 

 importance one has but to think of the great losses sustained 

 yearly by the decay of mine timbers, house-foundation and 

 cellar timbers, of bridge-timbers, railroad ties, paving blocks, 

 fence posts and rails in fact, timbers wherever air and moisture 

 can reach them. The creosoting of pavement blocks and the 

 tarring of cedar posts are attempts to aid the wood in resisting 

 fungus attacks. Tar and creosote are substances in which the 

 fungi cannot live and their presence protects the wood. But 

 as soon as the substances are washed off the fungi commence 

 their attack. At present a considerable amount of money is 

 being expended to find a process or substance which will pro- 

 tect railroad ties from fungus rot. What is wanted is some 

 substance which when deposited in the wood will prevent the 

 entrance of fungi and which will not readily leach out into the 

 soil during heavy rains. 



Effects of parasites on anatomy of host. The effects of fun- 

 gus parasitism described above have to do with the destructive 

 attacks upon cells and tissues, particularly in those cases where 

 enlargement of parts is caused. One often finds other effects 

 in tissues, viz. : changes in quality and amount of certain kinds 

 of tissues. The covering layer is often affected by fungi which 

 live on the surface, and may also be ruptured by the spore-pro- 

 ducing hyphae of interior-dwelling fungi as in red rusts. Some 

 fungi excite in certain plants an abnormal growth of cork which 

 constitutes the cuter layers of the bark. In general, in the en- 

 larged parts of the hosts, the supporting or strengthening tis- 

 sues are not as well developed as in the normal host. Many tis- 

 sues, moreover, which are usually woody are not so in the dis- 

 eased and enlarged parts, although there are exceptions to this 

 generality. Sometimes the fungus attack stimulates the ex- 

 cessive production of resin in pines and their allies. ( Hher 

 products and tissues may undergo change, though no generali- 

 ties can be discovered in the action of fungi. It seems, how- 

 ever, that, in general, those changes take place which transform 

 the host part into a most suitable and profitable dwelling and 

 food store for the parasitic fungus without regard to the host's 

 needs, and often to the direct detriment of the host plant. 

 There is usuallv in this connection a trreat amount of tissue 



