244 



HOST PENETRATION 



species that produce appressoria or other means of attachment 

 that function in the same manner as appressoria. Here again evi- 

 dence and opinion are divided, since some workers maintain that 

 the penetration tube enters only in conjunction with the dissolu- 

 tion of the cuticle to prepare the way, and others that the cuticle 

 is pierced by a mechanical thrust. Brown (1915) made extracts 



Fig. 43. Haustorial tvpes. A. Branched haustorium of Peronospora calo- 

 thecae. (After de Barv.) B. Haustoria of Puccinia adoxae. A sheath partly 

 invests the haustorium. (After Guttenberg.) C. A bulbous haustorium of 

 Erysiphe communis. (After Smith.) D. Digitate, sheathed haustorium of 



Erysiphe graminis. (After Smith.) 



of Botrytis cinerea, as has been stated, which were capable of di- 

 gesting tissues very rapidly when injected into them, but which, 

 if placed at the surface of delicate rose petals, caused no injury 

 within a period of approximately 24 hours. His lack of evidence 

 of solvent effect to aid penetration is substantiated by the results 

 of Boyle ( 1921 ), Waterhouse (1921), Dey (1919, 1933) and many 

 others. Instead they adhere to the mechanical theory of penetra- 

 tion. In the experiments of Brown and Harvey (1927) cells of 

 Eucharis and other plants were readily penetrated if they had 

 previously been plasmolyzed, but no penetration took place if the 

 cells were turgid. It becomes difficult to understand how rigid- 

 ity of the host cells could inhibit the production of wall-dissolving 

 enzymes. 



