PARASITISM AND RESISTANCE 391 



are so modified that they revert to the embryonic condition and may even 

 divide. When the obhgate parasite enters the extremely susceptible 

 host, it causes so little disturbance that, at least in the first stages of 

 infection, the metabolism of the host is afTected but little or not at all. 

 Walker and Link (1935) caution against jumping at conclusions regard- 

 ing the importance of phenolic compounds as factors in resistance. They 

 point out that 



. . . the mere piesence of phenolic substances in a host plant does not warrant 

 the conclusion that they play a role in the resistance of that host to a given 

 parasite or parasites. Toxic phenolic substances might be present in concen- 

 trations so low that their inhibitory effects are negligible, or they might also be 

 present in concentrations that have a stimulative effect. When a phenolic sub- 

 stance with a specific toxicity toward a given organism is present in the host in an 

 appropriate concentration, it may be regarded as a part of the disease resisting 

 mechanism of that host. 



The four fungi studied by Walker and Link {Colletotrichum circinans, 

 Gibber ella saubinetii, Botrytis allii, and Aspergillus niger) reacted quite 

 differently to the various phenolic compounds. Protocatechuic acid 

 inhibited C. circinans at 1/800 and retarded growth at 1/12,800, while 

 it did not affect A. niger at 1/200. Colored onions containing this acid 

 are resistant to C. circinans but quite susceptible to A. niger. 



The immunity of monocotyledonous plants to Phymatotrichum omniv- 

 orum is due to certain unidentified toxic materials present in monocots 

 but apparently absent in most or all dicots (Ezekiel and Fudge, 1938). 

 Growth of the pathogen was prevented by the expressed juices from a 

 number of monocots but not by juices of susceptible dicots. Ether 

 fractions of monocot roots, or other underground parts, were highly 

 potent against the pathogen, while similar extracts from susceptible dicot 

 plants were uniformly nonpotent. 



In some other highly parasitic fungi the action of the fungus causes 

 the death of the surrounding cells, which then prevents the further spread 

 of the parasite. Leach (1923) found that in a highly resistant variety 

 of bean the hyphae of Colletotrichum lindemuthianum seldom attack more 

 than one or two cells of the host. Both the host cells and the fungus 

 hyphae then die, and the entire cell contents are stained a reddish brown. 

 In less resistant varieties the parasite attacks more host cells, but sooner 

 or later the mycelium disintegrates. Leach interprets this as "a nutri- 

 tional phenomenon," which results in death of the fungus by starvation, 

 and the products of autolysis then kill and stain the host cells. 



It has been pointed out previously that certain fungi are able to pene- 

 trate some plants but are then unable to establish themselves (Stakman, 

 1914; Jones, 1919; Salmon, 1905). These plants may be either closely 



