PARASITISM AND RESISTANCE 373 



arise from the intercellular mycelium, penetrate the host cells and absorb 

 food. Water vapor has been suggested as the stimulus which causes the 

 germ tube to turn inward and enter a stoma. This, however, cannot be 

 the case with zoospores which are immersed in water and which have been 

 noted to cluster around stomata. The fungi which normally enter the 

 host plant through stomata include the cereal rusts (aeciospore and 

 urediospore stages), Cercospora heticola, Phytophthora infestans (zoospore 

 stage), the Peronosporales, Albugo Candida, and others. 



The cereal rusts have received a great deal of attention in resistance 

 studies. It has been reported (Hart, 1929) that Puccinia graminis 

 apparentl}^ requires the open stomata of wheat plant for penetration. 

 On the other hand, Caldwell and Stone (1936) have shown that the germ 

 tubes of Puccinia triticina are able to force their way between the guard 

 cells of closed stomata of wheat leaves. A germ tube from a urediospore 

 may start to enter an open stoma, but as it forms an appressorium, the 

 stoma closes. Further penetration is accomplished between the guard 

 cells by a slender hypha. Allen (192G) believes that the appressorium 

 probably secretes some toxin which harms or even kills the guard cells, 

 causing the stoma to close. Caldwell and Stone, however, do not believe 

 that this injury to the guard cells is necessary for entry of germ tubes. 

 The appressorium seems to function as a special organ to apply the pres- 

 sure needed for the forced entry between the closed guard cells. 



Penetration through lenticels more often occurs in the underground 

 parts of the host which are in a more or less moist situation. Potato 

 tubers may become infected by Actinomyces scabies and by germ tubes 

 from sporangia of Phytophthora infestans, chiefly through the lenticels. 



Direct penetration. A large number of fungi are capable of penetrating 

 the unbroken epidermis of a plant, directly through the cutinized outer 

 walls. The spore may germinate on the surface of the plant in a drop of 

 water. The germ tube grows over the epidermis and by some stimulus 

 is caused to turn inward and penetrate the cell. Brown (1922) demon- 

 strated that there is a certain amount of exosmosis of materials from host 

 tissue into a drop of liquid on the surface. In some cases this may lead 

 to a chemotropic response by the fungus. However, in most cases the 

 stimulus of contact is believed to initiate appressorial formation and 

 penetration. The formation of appressoria is common among many fungi 

 when the germ tubes come in contact with the epidermal cells. The 

 fact that the appressoria are often formed on a glass slide is further 

 evidence that their formation is in response to contact with a solid sur- 

 face. Appressoria are usually bulb-like or disk-like in shape and are 

 believed to serve as an adhesive disk against which the slender infection 

 hypha may push in penetrating the cell wall. Brown (1915, 1922) 

 presents evidence that the host cells are not killed by Botrytis cinerea, 



