DIRECT PENETRATION 245 



The work of Link and his associates (1929) involving the in- 

 hibitory activity of specific chemicals is of special interest. They 

 noted that white onions are subject to attack by Collet otrichum 

 (Vermicalaria) circinans, which causes the disease known as 

 smudge, and that pigmented onions are disease-free. From these 

 onions they isolated protocatechuic acid, which was found to 

 inhibit the growth of the pathogen. This organic complex, fur- 

 thermore, does not occur in white-scaled onions. Inhibition of 

 penetration by the smudge fungus and disease resistance, there- 

 fore, are caused by protocatechuic acid. Presumably this is the 

 first chemical substance isolated that has been demonstrated to 

 render plants immune from infection. 



In connection with mechanical penetration it mav be recalled 

 that a mucilaginous matrix aids in sticking the spore, appressorium. 

 or germ tube to the cuticle, thus providing anchorage against 

 the force of the thrust required to pierce the cuticle. The small 

 diameter of the infection hypha minimizes this required force, 

 which attempts have been made to measure by mechanical de- 

 vices. Hawkins and Harvey (1919) studied penetration of potato 

 by the rot-producing fungus, Fythium de baryamnn. They em- 

 ployed a modified Joly balance with a needle having a point of 

 definite area to test resistance of potato tissues to puncture. Po- 

 tatoes of the McCormick variety, resistant to attack by this 

 organism, were found to require more pressure to puncture than 

 was required for Bliss Triumph or Green Mountain, varieties sus- 

 ceptible to decay. Rosenbaum and Sando (1920), using the same 

 appliance, correlated resistance of tomatoes to puncture with re- 

 sistance to penetration by Meter ospor'mm tomato. Certain of their 

 data are presented in Table 23. These data show that, as tomato 

 fruits increase in age, they also increase in ability to inhibit pene- 

 tration and consequent infection bv this fungus. Thickness of 

 the cuticular layer also increases with the age of the tomato fruit, 

 but, as Rosenbaum and Sando point out, these results do not prove 

 that inhibition of penetration is purely a matter of resistance to 

 mechanical pressure. 



Epidermal resistance of barberry to puncture was measured 

 with a mechanical device by Melander and Craigie (1927), and 

 they correlated their measurements with resistance to penetration 

 by germinating basidiospores of Puccinia grcmrinis. They con- 



