570 
Journal of Agricultural Research 
Vol. XIV, No. xa 
and Tetramyxa triglochin are morphologically much alike. In each case 
the greater portion of the infection is within the central cylinder. The 
cortex, or at least the outer part of it, is mostly free from disease. The 
points of similarity suggest that the method of infection may be the 
same for all of these parasites. The galls caused by Spongospora sub - 
terranea on the potato differ from those caused by each of the above- 
mentioned parasites on their respective hosts. This difference is probably 
correlated with differences in method of host tissue infection. 
SUMMARY 
(1) Cabbage plants of all ages up to one year are susceptible to 
clubroot. 
(2) Old plants are almost as susceptible as young ones, provided they 
are growing. 
(3) The typical club is a morphological unit and is usually the result 
of a single primary infection. 
(4) Sometimes the swellings resulting from two or more primary 
infections may fuse together to produce a compound club. Such clubs 
are more irregular in outline and often have greater length than those 
resulting from a single primary infection. 
(5) The spread of the disease from points of primary infection is ac¬ 
complished through direct penetration of cells by infecting plasmodia. 
(6) Host cell divisions increase the number of infected cells but have 
a very small part in distributing the parasite throughout the tissues. 
(7) Infection by direct penetration may be divided into four different 
parts as follows: (1) primary infection of cortical tissues and penetration 
to the cambium; (2) infection of the cambium in all directions from the 
point of original penetration; (3) passage of the plasmodia out from the 
cambium into the cortex and in from the cambium toward the xylem 
region; and (4) infection of medullary rays. 
(8) The infection of a given cell may be either permanent or temporary. 
If it is temporary, it has no noticeable effect on the cell. If permanent, 
it stimulates the cell to abnormal growth and division. 
(9) The growth stimulus is diffuse—that is, it acts on the noninfected 
cells of diseased tissues as well as on the infected ones. 
(10) The stimulus seems to travel in advance of infection. This is 
easiest to observe in the early stages of infection and in the infection of 
medullary rays. 
(11) The disease stimulates the production of branch roots and shoots. 
These branches become infected by direct penetration of plasmodia 
from the diseased tissues out of which they arise. 
(12) Diseased shoots are frequently unable to react normally to gravity, 
as is shown by their horizontal or downward growth. 
(13) A single infection may give rise to many thousands of separate 
and distinct t( Krankheitsherde. ,> 
