Sept. z6,1918 Tissue Invasion by Plasmodiophora brassicae 
563 
RELATION OF THE PARASITE TO THE HOST TISSUES 
Several previous investigators have given a certain amount of at¬ 
tention to a study of the relation of the parasite to the host cell in which 
it lives. From this work we know that the host cell is stimulated to 
abnormal growth and division, that its cytoplasm becomes vacuolate, 
that its nucleus enlarges and becomes malformed. We also know that 
this nucleus divides mitotically and that, on the whole, the cell lives and 
functions more or less normally. 
Previous workers seem not to have studied the relation of the parasite 
to the noninfected cells of the club and to the diseased tissues as a whole. 
They have looked upon the individual infected cells and groups of cells 
as separate and distinct pathological units, each independent of all the 
others. This was, of course, natural so long as it was believed that each 
diseased cell or group of cells was the result of a separate infection. As 
soon as it was recognized that the typical club is a pathological and 
morphological unit and usually the result of a single infection, the ques¬ 
tion of the relation of the parasite to the tissues as a whole at once pre¬ 
sented itself. It has been shown that the organism travels with great 
readiness through the cabbage tissues. Why is it, then, that some of 
the cells in each of these tissues always escape infection ? Is there any 
numerical relation between infected and noninfected cells in different 
dubs and in clubs from different plants? 
In order to study these problems, the writer selected at ramdom 60 
diseased plants growing in a field near Arlington, Va. The plants were 
inoculated on July 16 and were taken for study on September 14, just 
60 days after inoculation. They were all approximately 3 months old 
when inoculated and were in a vigorous growing condition. Small por¬ 
tions of tissue were cut from one or more of the clubs of each of the 60 
plants. These blocks of tissue were fixed in Flemming's stronger solu¬ 
tion, embedded in paraffin, sectioned on a microtome, and stained with 
the triple stain. They were usually Cut so as to obtain a cross section of 
the club, but longitudinal sections were also made. Sections from each 
of these plants have been observed under the microscope and have 
furnished material for a study of the distribution of the parasite in the 
host tissues. 
As might be expected, a good many different stages of the disease 
were obtained. In some of the clubs the plasmodia were still rather 
small. In others they were much larger and in still others spore form- 
tion had taken place. No very early stages of the disease were to be 
found in any of this material. The organism was found to be irregu¬ 
larly but rather evenly distributed throughout the tissues of all the 
different clubs from all the different plants. Only in rare instances was 
there to be found large numbers of infected cells adjacent to each other. 
Many of the cells of every club are free from infection. The diseased 
