28 



BULLETIN 127, U. S. DEPARTMENT OF AGRICULTURE. 



entry, or, at least, in conjunction with it, as De Bary (2), Busgen 

 (7), and Miyoshi (30) have concluded for certain parasitic fungi. 

 There is no evidence of a cavity in the cell wall, indicating dis- 

 tinct solvent action, such as Hasselbring 

 found in the barberry (26). Nor was 

 any indentation and rupture of the cu- 

 ticle discovered, such as Blackman and 

 Welsford describe. However, the germ 

 pore is very small, and swelling of the 

 contents of the appressorium could ex- 

 ert considerable pressure on the cuti- 

 cle at that 

 point lying 

 directly be- 

 neath the 



Fig. 8.— Cross section of the outer wall 

 of a leaf epidermal cell, showing 

 swelling under an appressorium ac- 

 companied by retention of safranin 

 stain, 65 hours after inoculation. No 

 penetration tube is visible. (Camera- 

 lucida drawing; magnified about 1,300 

 times.) 



Fig. 9.— Cross section of the up- 

 per end of a cucumber-fruit 

 epidermal cell, showing pene- 

 tration from an appressorium, 

 two weeks after inoculation. 

 Retention of the safranin stain 

 is shown in the inner wall 

 layer under the appressorium. 

 (Camera-lucida drawing; mag- 

 nified about 540 times.) 



germ pore. 

 This is sug- 

 gested in 

 figure 10, 

 a case in 

 which the 

 appresso- 

 rium has apparently forced itself loose. In figure 6 the palisade 

 cell wall is clearly seen to be indented by the pressure of the 

 hyphal tip. On the other hand, the swelling of the wall under an 

 apparently unbroken cuticle has been noted (fig. 8), and swelling of 

 an area of the palisade cell wall about the point 

 in contact with the fungus is also evident in 

 figure 6. 



To summarize, penetration takes place di- 

 rectly through the cuticle from appressoria in 

 close contact with the latter and provided with 

 a small round germ pore. The exact method 

 of cuticle penetration has not been determined. 



Fig. 10. — Cross section of the 

 outer wall of a leaf epidermal 

 cell, showing an appressorium 

 apparently forced off by pres- 

 sure exerted by the penetrat ion 

 tube, 121 hours after inocula- 

 tion. (Camera-lucida drawing; 

 magnified about 1,300 times.) 



EFFECT ON INVADED CELLS. 



In the leaf sections from material fixed 121 

 hours after inoculation incipient lesions are 

 found to be characterized by marked shrinkage and collapse of the 

 epidermal and palisade cells. These cells stain deeply with the hsema- 

 toxylin. The mycelium is almost entirely intracellular (fig. 11). In 

 some cases the appressorium from which infection occurred is still to 

 be seen (fig. 11). In sections of a stem lesion collapse of the collen- 

 chyma cells is plainly visible. In razor sections of diseased stems and 



