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BULLETIN 727, U. S. DEPARTMENT OF AGRICULTURE. 



Incipient and advanced stages of penetration, as seen in surface 

 view, are illustrated in figure 1. The appressoria are seen to be 

 quite uniform in size, shape, and. size and shape of germ pore. Al- 

 though this material was subjected to several hours' washing in 

 running water, the appressoria remained adherent to the cuticle. 

 From a study of these preparations it appears that penetration 

 never occurs except from an appressorium and that penetration 

 occurs directly through the cuticle. From this, it seems safe to con- 

 clude that the appressoria have a definite function in connection 

 with host penetration. No cases of stomatal entrance were found. 

 A case was found in which a spore lying directly over a stomatal 

 opening formed an appressorium at one side. 



To better understand the mode of penetration, material similar to 

 that above described was fixed in 10 per cent HC1, acetic alcohol, or 

 Gilson's fixative, embedded, sectioned, and stained in the triple 

 stain or Haidenhain's iron alum hematoxylin. Numerous penetra- 

 tions were found in sections from fixations made 65 and 121 hours 



Fig. 3.— Cross section of leaf epidermis, shewing 

 penetration of epidermal cell from an appres- 

 sorium, 65 hours after inoculation. The tip of 

 the penetration tube is swollen. (Camera-lucida 

 drawing; magnified about 575 times.) 



Fig. 4.— Cross section of the outer wall of a 

 leaf epidermal cell, showing an incipient 

 stage of penetration from an appressorium, 

 65 hours after inoculation. (Camera-lucida 

 drawing; magnified about 1,300 times. ) 



after inoculation, and types are illustrated in figures 3 and 4. The 

 latter represents an incipient stage. Here, again, it is seen that 

 direct penetration of the cuticle and outer wall takes place. Stomatal 

 entry does not occur. In one case (fig. 5) an appressorium has been 

 formed in the stomatal pore and penetration of a guard cell has fol- 

 lowed, rather than a direct invasion of the substomatal chamber. 

 In figure 6, showing an apparent stomatal penetration, close exam- 

 ination reveals that the lower guard cell was pierced by the penetra- 

 tion tube, which then emerged into the substomatal chamber. 



Appressoria may be distinctly flattened on the side next to the 

 cuticle (fig. 6), and all are in very close contact with the latter. The 

 penetration tube varies in shape, but is usually more or less club 

 shaped, owing to the fact that it becomes swollen after gaining 

 access to the lumen of the cell (fig. 3). In figure 7 it is quite evident 

 that the contents of the appressorium have entered the penetration 

 tube. In no case has it been possible to trace the course of the 

 penetration tube through the cuticle and underlying wall layers. 



