sept. i6,1918 Tissue Invasion by Plasmodiophora brassicae 
559 
number of transpiring cells. The cells of the infected tissues, instead of 
contributing to water conduction, use up water in their growth. Further¬ 
more, the large swellings themselves, especially when above ground, 
increase the transpiring surface. Finally on a warm, dry day the critical 
point is reached. The leaves are no longer able to obtain as much water 
as they transpire and the plant wilts or, as the gardeners say, “flags.” 
But not all plants are attacked while still young. It often happens 
that they escape infection in the seed bed, and contract the disease only 
after having reached a considerable size. These plants have well- 
developed vascular systems before they become infected. Can the 
parasite cause wilting in such plants and, if so, by what means? 
In the study of the infection of old stems we have seen that the 
plasmodia are able to attack the undifferentiated tissues on either side 
of the cambium, but that it can not penetrate far into the older portions 
of the bundles. It is unable to attack the woody cells of the xylem, 
but the undifferentiated cells of the medullary rays are still susceptible. 
The invader is able to penetrate the rays and to stimulate their cells, so 
that instead of remaining small and inactive they grow and divide 
and give rise to a pathological tissue. Whether the medullary cells are 
penetrated before they begin to grow and divide is not known. The 
observations of the writer indicate, however, that the growth stimulus 
travels somewhat in advance of infection. 
The growth of the medullary ray cells splits open the woody cylinder. 
The xylem tissues are forced apart and the bundles distorted in a variety 
of different ways. Sometimes the splitting up of these tissues is so 
complete that no two vascular strands can be found near together. 
In other cases the invasion of the medullary rays is not so complete, 
and the splitting of the woody cylinder is only partial. It frequently 
happens that the wood is split into two approximately equal halves. 
Plate 73, A, shows a large woody cylinder that is beginning to split 
apart. The parasite may be seen in some of the cells of the ray that 
has been stimulated most. Plate 73, B, shows the center of another 
rather old root. A wedge of diseased tissue has forced apart the two 
halves. Plate 74, A, shows a further development of the diseased ray. 
Here the split is complete, and the two halves of the woody cylinder are 
being forced apart. It will be seen that the right half of the cylinder 
is beginning to split up into still smaller parts. Plate 74, B, shows a 
wedge that has grown faster in the center than toward the two edges. 
It should be noted that the cells composing the wedge are much larger 
than the cells of the uninfected medullary ray. This is true of nonin- 
fected, as well as infected cells. Plate 75, A, shows the woody tissues 
separated still farther from each other. It is interesting to note that 
growth is quite uniform in different parts of this diseased ray. The 
edges of the rays remain almost parallel to each other. Figure B of this 
