131 



studded with small dewy drops, apparently of precisely the same character 

 as the "callus." (See Fig. 23, page 135.) 



The host's cell-wall at and near the point of penetration, is markedly 

 altered chemically, as is shown by various stain-reactions. Thus, adjacent 

 to the point of infection several different regions giving different chem- 

 ical reactions may be distinguished, as is indicated in Fig. 19. Region 3 

 gives the usual chlor-zinc-iodide reaction and stains like normal cellulose. 

 None of the other regions do this. Region 4 stains darker with the usual 



FIG. 19. H. No. 1 : regions of a young diseased spot: 

 1, mycelium; 2, penetrating mycelium; 3, normal wheat 

 cell-Avail ; 4, region of darker staining ; 5, region of lighter 

 staining; 6, diseased inner lamella; 7, middle lamella; 

 8, "callus." 



stains, but not so dark as normal cell-wall. The "callus" and penetrating 

 mycelium stain faintly or not at all. The middle lamella stands out clearly 

 in all of the diseased region, and on each side of it the inner lamella is seen 

 to be thickened and of altered stain-reaction. Though penetration is some- 

 times directly through the wall it is much oftener into the middle lamella, 

 and the mycelium shows a strong tendency to follow along the line of 

 division between two cells, thus giving a gridiron effect to the mesh. This 

 is possibly due to chemotropic attraction by the middle lamella or, possibly, 

 because this is the weakest place in the cuticle. No case of stomatal 

 entrance was observed; indeed, on the sheaths of "Golden Chaff" wheat 

 stomata are seldom present. 



Once within the host cell the mycelium grows rapidly, soon nearly or 

 completely filling it (Fig. 20), and often forming a mass so dense that it 

 resembles a pseudoparenchyma. Both longitudinal and transverse sections 



