250 HOST PENETRATION 



sugar-beet leaves by germ tubes of Cercospora beticola. They 

 observed that this organism is unable to penetrate at night when 

 stomata are closed but can do so during daytime when the open- 

 ing of the stomata permits the entrance of germ tubes into sub- 

 stomatal cavities. No doubt many fungi among those that pene- 

 trate through natural openings are able to do so only during day- 

 light hours. This factor must be borne in mind in tests involving 

 the pathogenicity of a given fungus. 



The germ tubes of aeciospores and urediniospores of rusts very 

 commonly enter through stomata, although the germ tubes of the 

 basidiospores of these same species may penetrate directly. Pady 

 ( 1935) noted that germinating aeciospores of Gymnoconia inter- 

 stitialis enter blackberry leaves not through the stomata but by 

 direct penetration. The urediniospores of many species are 

 known to produce a special appressorium, which functions in the 

 mechanism of entrance. The sequence of events in penetration 

 is as follows: When the tip of the germ tube comes to lie imme- 

 diately above a stoma, the protoplasm accumulates in the tip. 

 This apical region then swells, and the end cell is delimited by a 

 cross-septum to become the appressorium. By nuclear division 

 two or more pairs of nuclei form within the appressorium. Then 

 a hypha develops from the lower side of the appressorium and 

 forces its way between the guard cells into the substomatal cavity; 

 once inside, its tip swells to form a vesicle into which the proto- 

 plast of the appressorium migrates. Meanwhile more conjugate 

 nuclear divisions occur, and hvphae, whose cells contain paired 

 nuclei, grow radiately from this substomatal vesicle. These hy- 

 phae course between the host cells and establish intimate contact 

 with them by forming haustoria. 



Opinions differ as to whether the appressoria of rusts adhere 

 by means of a mucilaginous matrix. Rice (1927) saw no evidence 

 of such a matrix in Fuccinia sorghi. 



Study of penetration by Fuccinia graminis tritici into resistant 

 Khapli emmer by Allen (1926) indicates that the appressoria se- 

 crete a toxin upon the guard cells. This observation led her to 

 opine that ". . . the appressorial secretion is a survival from an 

 earlier period in the evolution of the fungus when it did dissolve 

 its way into the host." 



The time required for penetration is correlated with tempera- 

 ture, as has been capably shown by Peltier in studies with Fuccinia 



