DIRECT PENETRATION 231 



DIRECT PENETRATION 



Pioneer work on the initiation of attack by direct penetration 

 was done by de Bary (1886) in a study dealing with invasion by 

 Sclerotinia libertiana. He maintained that, when he applied asco- 

 spores in drops of nutrient solution to suitable intact plant tissues, 

 they were able to penetrate directly; whereas, if ascospores were 

 placed in drops of water, they formed organs of attachment by 

 means of which they intimately applied themselves to the surface 

 of the host. These organs of attachment secreted a principle 

 which killed the underlying cells, and in consequence nutrients 

 diffused from the dead cells. As a parasite, therefore, S". libertiana 

 was able to penetrate directly, and as a saprophyte it must first 

 kill the underlying host cells. When he prepared an extract from 

 infected tissues, he was able to demonstrate that this extract could 

 cause the cells to fall apart, that is, to rot, and could kill the proto- 

 plasts. Boiling destroyed the activity of the extract; from this 

 fact he concluded that rotting was caused by an enzyme, but he 

 was unable to determine the nature of the lethal substance. Al- 

 though he expressed the opinion that oxalic acid produced by 

 the fungus killed the cells, he did not know whether this acid was 

 solely responsible for the death of the tissues. Several reports 

 have subsequently appeared, the authors of which accepted the 

 de Bary hypothesis that parasitic fungi and bacteria secrete a fer- 

 ment that enables them to penetrate cell walls. Ward (1888) 

 expressed this opinion regarding Botrytis cinerea, the cause of a 

 disease of lilies. 



In regard to the cause of killing in advance of penetration, 

 Smith (1902) found in connection with Botrytis cinerea, parasitic 

 on lettuce, that it produced a thermostable toxic substance and 

 expressed the opinion that this substance was oxalic acid. Peltier 

 (1912), on the other hand, in a study involving presumably the 

 same fungus on pepper and lettuce, concluded that the toxic 

 thermostable substance was not oxalic acid but some other organic 

 acid or acids. Higgins (1927) demonstrated production by 

 Sclerotinm rolfsii of oxalic acid in certain nutrient solutions. 

 Considerable quantities of oxalates were also found in the dead 

 cells underlying the holdfasts of soybeans and peppers, but none 

 occurred in healthy cells of the same hosts. Moreover, the tox- 



