246 



HOST PENETRATION 



TABLE 23 



Relation of Resistance of Tomato Fruits to Puncture and to Penetration 



by Macros porium tomato 



eluded that species of Berberis which are resistant to puncture are 

 usually resistant to rust, but the converse is not usually true. 



Pioneer work on the correlation of structure of plant tissues and 

 inhibition of penetration by fungi into plant tissues was instituted 

 by Yalleau (1915). Thickness of the skin of plums was found 

 correlated with resistance to the brown-rot fundus. Yalleau also 

 found that the cells lining substomatal cavities possessed corky 

 walls and that the stomata were very commonly completely 

 occluded with corky cells. By and laroe, Curtis (1928) verified 

 Valleau's findings but believed that cuticular resistance to pene- 

 tration by the brown-rot pathogen was equally as important as 

 the presence of corky tissue in natural openings, if not more im- 

 portant. It might be expected that varieties of stone fruits lack- 

 ing stomata or lenticels would be immune. Curtis did not find 

 this to be true, however, since in the varieties which he investi- 

 gated the germ tubes entered through the stomata in plums, 

 through the cuticle in cherries, and down the hair sockets in 

 peaches, and penetrated cither through the cuticle or the stomata 

 in apricots. 



In varieties of tobacco resistant to invasion by the black root-rot 

 fungus, Thielaviopsis basicola, Conant (1927) found that resist- 

 ance to infection is correlated with the ability of the host rapidly 

 to develop a corky layer to inhibit the spread of the pathogen. 



The short period of time required for penetration of the cell 

 wall by Pythiitui de baryamtm [Hawkins and Harvev (1919)] 

 also constitutes evidence of mechanical puncture. They observed 



