Sept. 16, i 9 i8 Tissue Invasion by Plasmodiophora brassicae 545 
TIME AND NATURE OF INFECTION 
It has seemed desirable to study the early stages of infection. This 
was necessary in order to test the truth of the statement that only very 
young rootlets are liable to be invaded. We wish to know the exact age 
at which tissues become immune, whether the parasite can pass through 
the rootcap and infect the roottip, or whether it passes in through root 
hairs only. For this work young roots that had been exposed for various 
periods of time to soil infested with spores of the parasite were fixed in 
Flemming's weaker solution, embedded in paraffin in the usual way, 
sectioned on a microtome, and stained with Flemming's triple stain. If 
the organism enters through root hairs only, it should be possible to find 
a stage when the root-hair region would be infected and other parts of 
the young root would still be healthy. 
A careful study of many of these sections did not yield the results 
hoped for. In all cases observed the root was found to be either thor¬ 
oughly infected or quite free from infection. Stages in the process of 
infection were not to be observed, and the disease was never confined to 
any special zone of tissue. One thing that this study did make clear, 
however, was that the organism sometimes gets into the roottip. Cells 
in the region of most rapid growth were often found to contain small 
plasmodia, and the writer was able to confirm the observation made by 
Nawaschin (13) and others that the parasite is distributed to a certain 
extent by host cell divisions. 
Since the stained sections did not show the path of entrance of the 
organism nor indicate the age of tissue that is attacked, a somewhat differ¬ 
ent method was resorted to. The soil was carefully washed away from 
the young roots of some healthy plants. Then a small paper cylinder 
about 1 cm. long and 0.5 cm. in diameter was slipped over each root. 
The cylinders were placed at different distances from the roottip and 
always at a part of the root from which no branches-arose. Care was 
taken not to injure the young roots. They were then filled with moist 
earth containing spores of P. brassicae and the ends were sealed with 
melted paraffin. After attachment of the cylinders the roots were cov¬ 
ered with earth and left for future examination. They were usually 
examined after a period of approximately two weeks. Infection resulted 
in practically every instance. That part of the root contained within 
the cylinder became swollen, no matter whether the cylinder was placed 
near or far back from the roottip. Those portions of the root outside of 
the cylinder never showed infection. These experiments proved that the 
parasite is able to attack tissues far back of the root-hair region, and led 
to other tests that have yielded interesting results. 
It was soon found that the stems of young cabbage plants, as well as 
the older roots, are susceptible to the disease. The earth was removed 
from around the stems of plants of different ages growing in pots. 
