Sept. 16, 1918 Tissue Invasion by Plasmpdiophora brassicae 
553 
finally reaches the cambium. In the cambium it spreads around the 
stem and also up and down the stem. From the cambium it passes 
inward toward the pith and outward toward the epidermis. As is shown 
in the diagrams (fig. 1,2), most of the cortical tissues become infected 
from within. This was not to be predicted and is exactly opposite to 
the direction that previous workers have assumed. The greater part 
of the cortex becomes diseased through secondary infections rather than 
by primary infection, 
as has usually been 
supposed. 
As noted above a 
few of the outermost 
cells of [the cortex of 
stems that were fixed 
nine days after inocu¬ 
lation showed infec¬ 
tion. At this time 
there was no swelling 
whatever. In no case 
was an epidermal cell 
found to be iufected. 
A few young plasmo- 
dia were observed in 
the second, third, and 
deeper layers of cells. 
Usually these plasmo- 
dia contained from two 
to six nuclei, but in a 
few cases uninucleate 
ameba were seen. In 
all probability the pri¬ 
mary infections are 
brought about by such 
amebse. The writer 
has never been fortu- , ...... 
Fig. 2.—Diagram showing the course taken by the infecting plasmo- 
nate enough, however, dia in cabbage roots or stems that become infected after vascular 
to observe them pass- elements are differentiated. The arrows indicate the direction and 
. 1,1 .. extent of infection. 
ing through the walls 
of epidermal cells, and until such observations are made it is not possi¬ 
ble to say whether uninucleate or multinucleate bodies are concerned. 
The fact that epidermal cells are seldom infected permanently prob¬ 
ably accounts for the difficulty in observing the parasite in these cells. 
It is also not known whether the disease starts from one primary infec¬ 
tion or from the infection of several adjacent epidermal cells. In any 
case primary infections are local and are confined to very small areas. 
One might think that, if the spores of P. brassicae are closely packed 
