Sept. x6,1918 Tissue Invasion by Plasmodiophora brassicae 
547 
spindle-shaped clubs that are so characteristic of this disease. If a 
number of clubs are brought together side by side, it will be seen that 
although they may differ greatly in size many of them are alike in 
shape. Some typical spindle-shaped clubs are illustrated in Plate 63. 
The clubs shown in this illustration were not produced artificially, but 
were taken from plants grown on an infested field. Those produced by 
artificial infection of a small circular bit of tissue and shown in Plates 
61 and 62 are essentially like the ones resulting from natural infection, 
in the field and shown in Plate 63. The one-sided knobs at X in the 
figure indicate the point of original infection in each case. Each club 
is a morphological unit and the result of one primary infection. This 
fact is very important to an understanding of the disease and lies at the 
basis of the explanation of the morphological changes which occur. 
It often happens, however, that several points that are not very 
distant from each other become infected. In that case the swellings 
may fuse together in such a way as to give rise to an irregular-shaped 
growth or compound spindle. Plate 64, A, shows a portion of two swell¬ 
ings that are about to fuse together. During the later stages of the 
disease branch roots arising from either the siftiple or compound spindle 
become swollen and serve to distort the original form. In this way 
badly diseased specimens often become quite irregular in shape, but 
even in these one sees that the overgrowth is made up of a large number 
of tapering elements. The spindle-shaped tumor so characteristic of the 
disease results from the reaction of the host to the stimulus produced 
by the parasite as it spreads gradually through the tissues from the 
point of original infection. 
STAGES IN CLUB FORMATION 
There are two general methods by which cabbage cells become in¬ 
fected. The first may be designated as the direct method and includes 
all cases of direct penetration. The second is by host cell divisions. 
This is an indirect method of cell infection. Both of these methods 
have been known to earlier workers; but the relative importance of the 
two methods has not been previously recognized. Distribution by host 
cell division was observed and described by Nawaschin. It will not be 
taken up in detail in this paper. The direct method of infection deserves 
further study. 
There are two good ways of determining cell-wall penetrations. One 
is by actual observation of stages in the passage of the parasite through 
cell walls. The other is by observing the advance of the plasmodia in 
successive stages of infection. Actual cell-wall penetration will be de¬ 
scribed later. Advance of the plasmodia in successive stages of infection 
will be considered here. 
The notion that the amebae of Plasmodiophora brassicm must enter the 
host through root hairs has gained a strong foothold among students of 
