Johnson — Further Observations on Powderij Potato-Scab. 169 



which the spore-balls are described as "globulis," the equivalent of Wallroth's 

 " polysporis," not " pseudosporis," as quoted by him. In both the diseases 

 described by Martins he sees a similar matrix or degenerated cell-sap. In 

 the scab this becomes transformed into a powder, but in the other disease it 

 gives rise to an "after organization," the mould or HyphomyceteFusisporium. 

 It is not surprising that at this date, four years before Berkeley's account 

 of PhyfojiJitliora infestans appeared, disease organisms should have been 

 confounded together. In the present case it is, I tliink, evident that 

 PI. III., fig. 20, of Martius (PI. XII., fig. 2, of this paper) represents the 

 same organism as Brunohorst's figure, here reproduced as PI. XII., fig. 1. 

 Beyond the one reference to the insignificant stalk occasionally seen attaching 

 the "grains" or spore-balls to the cell-wall, there is no suggestion in Martius's 

 account of the presence of hyphse in the scab organism. Berkeley, however, 

 without comment places it in the TJdilaginece. Wallroth in a few lines 

 accompanying his diagnosis calls the fungus a smut. Was this Berkeley's 

 reason for placing it in the smuts ? Were hyphse observed ? 



Frank (11) examined the organism and accepted it as a slime-fungus, but 

 considered it to be a saprophyte, not the injurious parasite it is. 



The general character of the Spongospora scab is evident from PI. XII., 

 fig. 3, showing a tuber with several scabby or wart-like patches. The 

 wart is a proliferation of the tissue of the tuber, and its disintegrating 

 cells are full of spore-balls. The plasmodial stage of the fungus may be 

 seen in the deeper-seated cells passing on to the ordinai-y starch-cells. In 

 PL XII., fig. 4, the plasmodium-occupied cells appear dark-coloured, and 

 lie between the cells with spore-balls and tiiose with starch in them. 

 PI. XII., fig. 5, taken from a scabby tuber, shows the plasmodial stage 

 only, beneath the cork which is recognizable to the left of the figure. 

 PI. XII., fig. 6, shows a little of this attacked tissue more highly magnified. 

 In some of the host-cells the starch and other cell-contents have been entirely 

 absorbed by the parasitic plasmodium. In others it is in course of absorp- 

 tion. The wall of the host-cell is mostly intact and unattacked. There are 

 no signs of fungal hyphse. In PI. XIII., fig. 6, the vacuolated plasmodium 

 is shown, just before spore- formation begins. 



There is abundant evidence that the Spongospora parasite is an intra- 

 cellular organism, and that it occurs both in its plasmodial and fruiting 

 stages within the host-cells. Plate XIII., fig. 5, shows a spore-ball lying 

 in the host-cell. At n the nucleus of this cell is still observable. The light 

 spots at s are unabsorbed starch-grains. The walls of the host-cells are 

 clearly present, and there are no fungal hyphse. In PI. XIII., fig. 8, a 

 host-cell is seen lialf occupied bj^ a spore-ball. In the other half are 



