268 Journal of Agricultural Research voi. iv, no. 3 



such changes might be the result of the direct action of the protoplasm 

 of the Plasmodium on the cellulose. 



As the pseudopodia push down between the cells the walls of the latter 

 seem to become somewhat gelatinous and the middle lamella is dissolved. 

 These tapering plasmodial projections sometimes extend down into the 

 sound tissue for a distance of five or six cell layers; often, however, they 

 do not go deeper than two cell layers. They are irregular in shape, as 

 might be expected from the way in which they crowd between the cells. 

 Through these pseudopodia the Plasmodium comes in contact with a large 

 number of cells. The cell walls seem to become more and more softened 

 and gelatinous as they thicken, and may even lose to a certain extent their 

 original shape, becoming wavy. Plate XXXIX, figure 4, shows the char- 

 acteristic way in which the plasmodium pushes down between the cells. 

 Deeply stained globular bodies are also conspicuous in it, two of which 

 are sometimes joined together. In certain portions of the plasmodium 

 nuclei can be distinguished, but they are poorly fixed. Plate XL shows 

 a small part of an infecting plasmodium. The cell containing the large 

 nucleus is not yet infected, but the one to the right of this has been pene- 

 trated. In Plate XXXIX, figure 3, is shown a vertical section through 

 the parenchyma of the potato and an infecting plasmodium which can 

 be seen spreading out over the healthy tissue and forcing the cells apart. 

 Certain cells have already become infected; many of them are beginning 

 to enlarge. They are stimulated to abnormal growth even before pene- 

 tration, which suggests that the stimulus may be due to a secretion from 

 the Plasmodium. 



Through the softened cellulose walls the plasmodium sends small 

 protoplasmic strands, which may for convenience be termed the "infect- 

 ing pseudopodia." These usually pass into the cell through openings 

 that are quite small, but occasionally the opening is rather large, as 

 shown in Plate XXXIX, figure 5. In this figure is shown a potato cell 

 that is being infected. The plasmodium has made an opening in the 

 wall and is flowing through and apparently into the protoplasm of the 

 cell. The nuclei of 5. subtcrranea are well stained. The large nucleoli 

 stain a bright red; the chromatin strands, blue. The host nucleus is 

 also clearly shown. A large nucleolus and chromatin strands can be 

 seen. In some manner which has not yet been determined the infecting 

 pseudopodia become separated from the remainder of the plasmodium, 

 and in this w^ay the individual cells receive each a small portion of pro- 

 toplasm from the plasmodium that is invading the tissue. The quan- 

 tity of infecting material received by different cells is quite variable. 

 The writer has not been able to decide what it is that determines the 

 amount received by a given cell. Within the cell the little plasmodium 

 often travels along the cell wall for a certain distance before actually 

 entering the protoplasm of the host. Just how the plasmodium actually 

 penetrates the protoplast of the host cell is a point that is not yet clear. 



