568 
Journal of Agricultural Research 
Vol. XIV, No. 12 
farther and farther into the organ that is being attacked. The period of 
infection is indefinite; it continues up to the time the host plant dies or 
stops growing. The infected cells of the gall do not lie adjacent to each 
other, but are distributed in small groups throughout the diseased tis¬ 
sues. If one were to cut out of a typical gall all of the groups of dis¬ 
eased cells, he would get thousands of distinct and separate pieces of dis¬ 
eased tissue. Instead of one large “Krankheitsherde” there are many 
small ones. The mature gall is seldom or never wholly on one side of 
root or stem. Usually it surrounds the stem. From the very nature of 
the infection it is easy to see that P. brassicae is a much more 
dangerous disease than £. subterranea . If the writer knew absolutely 
nothing of the damage done by these two parasites, but had before him 
the account of the way in which each infects its host tissue, he would be 
able to predict that the one is a much more serious disease than the other. 
In the one case the disease spreads indefinitely; in the other it does not* 
The tumor produced by P. brassicae is malignant; that produced by 
5 . subterranea is benign. 
OTHER GENERA OF THE PLASMODIOPHORACEAE 
We have seen that the method of infection for P. brassicae is very dif¬ 
ferent from that found for 5 . subterranea. This raises the question of how 
other genera of the Plasmodiophoraceae accomplish the infection of host 
tissues. In order to answer finally the question, it will be necessary to 
make a careful study of the method of infection for each genus. This 
the writer has not done. It may, nevertheless, be of interest in the light 
of what we now know regarding the nature of the galls caused by P. 
brassicae and S. subterranea to compare these overgrowths with those 
caused by some of the parasites in related genera. 
Sorosphaera veronica. —This parasite causes swellings on the above¬ 
ground portions of a number of species of Veronica. The galls have been 
described in detail by Lagerheim and are pictured by Winge (13). They 
are elongated, more or less tapering swellings which resemble in general 
outline the galls of P. brassicae. Stained sections show that these galls 
are like those of the clubroot in a number of important respects. The 
outer layeis of the cortex are usually free of the parasite. This suggests 
that infection may proceed from within. The apparent age of the 
plasmodia in different parts of the tissues also points in the same direction. 
In one important respect the distribution differs from that of P. brassicae. 
Instead of it being confined to small groups of cells scattered about 
through noninfected tissues it invades large numbers of cells that are 
adjacent to each other. On the whole the galls caused by this parasite 
are so much like those caused by P. brassicae that the writer feels justified 
in predicting that when the method of infection is worked out for Soro¬ 
sphaera veronica , it will be found to resemble closely that described for 
clubroot. 
