34 Research Bulletin No. g 



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

 F. oxysporum not only made a greater initial growth at ordinary 

 temperatures, but that it was. at all temperatures much more of 

 a surface grower than F. trichothecioides, making a superficial spread- 

 ing growth, rather than the penetrating restricted intensive growth 

 of the latter. Early sporulation was associated with the restricted 

 growth habit of the latter (fig. 13). These habits were especially 

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

 sporum 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. The 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 F. oxy- 

 sporum. Storage temperatures check the growth of this organism 

 and the cells bordering 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 tempera- 

 tures, the potato plant undoubtedly has a much better opportunity 

 to guard itself by cork 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 



