34 Research Bulletin No. o 



2. Growth habit. — Jt was observed in nearly all cultures that 

 /•'. oxysporum n<>t only made a greater initial growth at ordinary 

 temperatures, Imt that it was at all temperatures much mere of 

 a surface grower than F. trichothecioid.es, making a superficial 

 spreading growth, rather than the penetrating restricted intensive 

 growth of the latter. Early sporulation was associated with the re- 

 stricted growth habit of the latter (fig. 13). These habits were espe- 

 cially clearly marked on solid substrata, but even in liquid media F. 

 oxysporum made a much less compact growth than the other species. 

 It may be that the more spreading and extensive growth habit of 

 F. oxysporum at all temperatures and its more rapid initial growth 

 at temperatures above 10-15 C. are associated with a greater oxy- 

 gen requirement than that possessed by F. trichothecioides. This 

 would explain in part the frequenting of intercellular spaces and 

 xylem elements by the former, and its consequent greater efficiency 

 in causing vascular mycosis and wilt, as well as its tendency to cause 

 bundle discoloration. 'Idle xylem elements of the stem end are 

 undoubtedly infected while the tuber is yet in the soil, where 

 temperature conditions are such as to favor the growth of / ; . oxy- 

 sporum. Storage temperatures check the growth of this organism 

 and (he cells honk-ring the infected vascular elements shut the 

 infected area off by suberizing their walls. Cultural experiments 

 and microscopical studies show that cork is not absolutely impene- 

 trable to these organisms, although it provides under normal con- 

 ditions an effective barrier to the progress of both of these species. 

 Because of the slower growth of F. trichothecioides at higher tem- 

 peratures, the potato plant undoubtedly has a much better opportun- 

 ity to guard itself by corf: formation against this organism than 

 against the other. 



3. The carbon sources of the two organisms. — A differ- 

 ence in the metabolic requirements of two organisms, a difference 

 in their ability to utilize various substances, or a difference in their 

 ability to tolerate the presence of substances may be factors of 

 critical importance in determining which of the two will attack a 

 given tissue or a given plant. These factors may determine also 

 the modes of attack of an organism upon a tissue or a plant. Thus 

 an organism that can digest pectinaceous material and not cellulose 



