sept. 16 , 1918 Tissue Invasion by Plasmodiophora brassicae 
567 
An example of this kind of infection is shown in Plate 80, B. Ordinarily 
the small cells between the groups of enlarged cells are free of the para¬ 
site. In this case, however, practically all of these small cells contain 
small plasmodium. The difference between this and the usual distribu¬ 
tion of the disease is strikingly brought out by comparing the two tissues 
shown in Plate 80. It might seem at first thought that such a tissue as 
that pictured in figure B would disprove the theory that cells immedi¬ 
ately surrounding those that first become infected are rendered immune. 
The figure shows that these cells are not always immune to attack. But 
the failure of the plasmodia to develop normally is the best indication of 
the influence the large plasmodia exert on surrounding cells. 
A COMPARISON OF THE GALLS OF PLASMODIOPHORA BRASSICAE 
WITH THOSE OF SPONGOSPORA SUBTERRANEA 
From the above account of the pathology of P. brassicae it will be 
seen that the disease caused by this parasite differs very materially from 
that caused by Spongospora subterranea (9). 
The typical overgrowth caused by P. brassicae is spindle-shaped, thick 
in the middle and tapering gradually toward either end. The overgrowth 
caused by Spongospora subterranea is a knot that protrudes abruptly 
out of the tissues from which it arises. The nature of the galls produced 
by these two parasites is in each case the result of a special method of 
infection. A definite number of cells adjacent to each other are invaded 
by the plasmodium of S. subterranea . The size of the gall produced by 
this infection depends on (1) the number of cells that are originally 
infected, and (2) the number of times these cells divide and the size to 
which they grow. In this way the size of the gall is limited, and it always 
remains small. It has never been observed that this parasite is able to 
pass from an infected to a noninfected cell, and there is no indication 
either direct or indirect that this can occur. It does not spread through 
the tissues except by means of the large infecting plasmodium. All of 
the cells of the sorus of 5 . subterranea are infected, and outside of this 
sorus all of the cells are healthy—that is to say, there is a very definite 
line between infected and noninfected tissue. If one were to cut out all 
of the diseased cells in a typical gall of 5 . subterranea he would get only 
one piece of tissue. This tissue might be called a large “ Krankheits - 
herdeT On the root and stem of the potato the gall is almost always 
on one side only; it never becomes a spindle-shaped swelling, but breaks 
through the epidermis, and exposes the rough surfaces of the infected 
tissue. 
P. brassicae behaves very differently in this regard. One or more 
small bodies of parasitic protoplasm enter the host tissue at some point. 
These bodies then grow and divide repeatedly. They enter into and pass 
through the living cells. Some of them become established here and 
there within the cells of the tissue, while others continue to penetrate 
