64 PHYSIOLOGICAL RELATIONS 



The obligate saprophytes include many of the mushrooms, 

 Basidiomycetes, and common molds of diverse families. 



Most of the important disease-producing fungi popularly known 

 as leaf spots, cankers, stem rots, etc., are capable of vigorous 

 growth in artificial culture, and some even produce normal fruit 

 bodies under such conditions. These fungi are commonly Asco- 

 mycetes and Basidiomycetes. In nature many organisms belong- 

 ing to these classes develop and mature their fruit bodies, especially 

 the perfect stages, only after the infested parts are dead and 

 fallen. 



The facultative parasites are found especially among the groups 

 mentioned in the last paragraph and also among the black molds, 

 Mucoraceae. Fungi which are ordinarily common upon decaying 

 logs, or destructive to timber, may occasionally develop as para- 

 sites on living trees. The black mold, Mucor mucedo, so common 

 in the household, may under certain conditions cause a serious rot 

 of sweet potatoes, and it has been known to injure some plants in 

 the seedling stage. The conditions under which such saprophytes 

 become parasitic are not always, clearly understood. In general it 

 is evident that some condition of the environment has operated to 

 render the host plant less resistant, or else the conditions have 

 been such as to develop exceptional vigor in the fungus. Almost 

 as long as fungous diseases have been known there has existed 

 the belief that such diseases in any given host plant are in some 

 way dependent upon a certain lack of vigor in the plant. Practical 

 growers and many plant pathologists have held that vigorous, well- 

 cultivated, and well-nourished plants mean plants resistant to disease. 

 No one would question the general desirability of resistant plants, 

 yet this attitude requires special comment and treatment. 



A very large number of the obligate parasites and some fungi 

 less obligatorily parasitic are, or seem to be, specially endowed with 

 the ability to enter relatively vigorous growing organs of the plant. 

 The majority of the fungi, moreover, do not kill immediately the 

 tissues which they invade. All sorts of deformities, including 

 witches' brooms, may appear as a result of the association ; but so 

 long as the fungus is rapidly growing, it seems generally to have 

 a well-established relation with the living cells, such that when the 

 invaded tissues die, the fungus spends itself in reproduction. 



