424 
Journal of Agricultural Research 
Vd. IX, No. » 
pit, once thoroughly established, effectively stops further progress of 
the fungus. No fungal filaments have ever been found penetrating this 
lining, and none have been found in the healthy tissues surrounding any 
of these pits. The evidence shows that the size of the pit is determined 
by the rate of suberization of cells sufficiently far enough in advance to 
effectively block further progress of the fungus. From very earliest 
infection, when the diseased area is scarcely more than an enlarged 
lenticel, the host cells always show more or less suberization three or four 
cells in advance of the deepest situated fungal filament. This being 
uniformly the case, together with the fact that the cells seem to die and 
lose their contents some distance ahead of the fungus, has suggested the 
possibility of a toxic substance being secreted by the hyphae. 
Microtome sections varying from 8 to 15 m in thickness have been 
made from all stages of the “dry core” from the earliest, showing only 
the infected lenticel, to the age when the diseased area has ceased to 
enlarge, some as great as 6 mm. in diameter. In every case Rhizoctonia 
mycelium has been found. These infected areas are remarkably free 
from secondary fungi. In characteristic cases there is no histological 
evidence whatever of the presence of any other fungus. This, coupled 
with the fact that pure cultures of Rhizoctonia have been obtained 
repeatedly from these infections, seems to present a very strong case 
against Rhizoctonia solani Kuhn (Corticium vagum B. and C.). 
What appeared oh the surface of a tuber as an enlarged lenticel is 
shown in section in Plate 30, £. After sectioning and staining this 
material, it was a matter of surprise to find Rhizoctonia hyphae so readily, 
and in all sections made of this area. It will be seen from the illustrations 
that the host cells have already begun to suberize four cells in advance 
of the fungal hyphae. No doubt this displacement and ragged edges of 
the normal corky cells of the cortex has been emphasized on account 
of their being washed and tom out in the process of fixing and cutting. 
However, it is believed that this will not prove misleading to anyone 
who has had experience in working with materials of this kind. The 
gradual progress of the fungus and the disintegration of the host cells, 
in what might be termed the second step, is shown in figure C (PI. 30). 
This affected area was slightly over 1 mm. in diameter. By comparison 
with figure A (PI. 30), which is a section of a normal portion of this same 
tuber, the results produced by the invading fungus may readily be seen. 
As the infection spreads and the fungal filaments become more and more 
abundant, this disintegration process goes on until the interior of the 
core is converted into a mass of broken-down cells, hyphae, and free 
starch, thus giving an appearance, when magnified, similar to Plate 30, D . 
The hyphae in this case will be seen to be of the sclerotia-forming type, 
as compared with the earlier infection stages or with the pure-culture 
hyphae of figure F (PI. 30). In a few cases the writer had the good for¬ 
tune to get sections in which the outer cortex had remained more or less 
