Some Concepts in Mycology. William B. Brierley. 211 



Now it may happen that organisms i and 2 under the influence 

 of Hke conditions {A) will produce growth forms having similar 

 essential descriptive characters. Under environmental factors 

 represented by B, C and D, however, the morphological charac- 

 ters of the growth forms may be very divergent. On the other 

 hand organisms 2 and 3 may converge under the influence of 

 D and the former may also exhibit a similar "ecad" when 

 stimulated by C. Thus lA and 2/4 would on the basis of 

 morphological comparison be regarded as a single species, as 

 would also 2C, 2D, and ^D. The constitutional distinctness 

 of the three organisms would however be clearly visible were 



the morphological response to two or more environmental 

 systems observed. As Thorn (45) states, "Two species closely 

 similar when grown parallel in one environment may differ 

 characteristically when transferred to a different medium or 

 a different set of conditions." 



Before continuing further the discussion of plasticity atten- 

 tion may briefly be drawn to a factor very seriously complicating 

 all systematic treatment of fungi, in which the criterion of 

 specific purity is morphological comparison. This is the 

 undoubted fact that very many accepted fungal species are 

 aggregates of two or more elementary species each of which 

 is plastic, the ecads or gro\Yth forms converging or diverging 

 in constant and definite relation to the adjustment of the 

 physico-chemical factors of the environment. 



Furthermore as stated by Dox(9), "While various species of 

 these two genera may be closely related morphologically, they 

 often shew wide chemical differences." To such chemical 

 species perhaps belong the so-called "biologic races" of 

 Pucciiiia gramiiiis, Erysiphe, Monilia, etc. 



The foregoing facts may be the more explicit if expressed 

 briefly in concrete terms. Let us take, by way of illustration, 



