sept. 16,1918 Tissue Invasion by Plasmodiophora brassicae 549 
cells in this region are also somewhat abnormally enlarged, but their 
enlargement has not kept pace with the enlargement of the nuclei. 
These changes in the cabbage cells in advance of infection show that the 
growth stimulus* acts at a considerable distance from infected cells. 
This suggests that the stimulus may be some substance which diffuses 
slowly from infected cells into the surrounding tissues. The cells that 
contain the parasite are, as might be expected, the ones that make most 
rapid growth. 
The infection continues to spread in all directions during the twelfth 
and thirteenth days. Plate 65, A, shows a section through a stem 13 
days after inoculation. The parasite has passed deeper into the host 
tissues. Some of the plasmodia have already reached the cambium. 
Infection has also spread to the sides as well as downward, and the plug 
of diseased tissue is rapidly becoming larger. Nuclear and cell division 
as well as cell growth is greatly accelerated. The plasmodia have also 
increased in size and contain more nuclei and much more oil than they 
did two days earlier. The parasite seems to be a heavy feeder. Not 
only does it make rapid growth, but it begins to store up oil very soon 
after entering the host. 
Sections through stems 14 and 15 days after infection show a still 
further advance of the fungus. It is no longer confined to a small volume 
of tissue and is spreading rapidly in all directions from the original point 
of infection. Plate 65, B, shows a section through a stem 15 days after 
inoculation. Some of the plasmodia have passed beneath the cambium 
layer. Many of the host cells are very much enlarged, especially near 
the point of original infection. During the sixteenth and seventeenth 
days the parasite spreads still farther into the healthy tissues of the stem. 
It has not penetrated very much deeper, however, and does not seem to 
be able to attack the woody parts, at least not to any very considerable 
extent. Plate 66, A, represents a section of a stem 17 days after inocula¬ 
tion. Here the parasite may be seen spreading along the cambium. 
Plate 66, B, shows a section through a stem 19 days after inoculation. 
It will be seen that the infected cambium has been active and that growth 
in this region has contributed very materially to the swelling that is taking 
place. The outer cortical region has also grown until it is now more than 
twice as thick as it would normally be. Some of the cells in the cortex 
are greatly enlarged. Those in the cambial region remain small. While the 
thickening of the cortex is accomplished more by cell growth than by cell 
multiplication, the swelling in the cambium region is brought about largely 
by an increase in the number of cells. Plate 67, A, represents a portion 
of a section through a swelling on a stem 21 days after inoculation. The 
infected area is now too large to be included in a photograph of reasonable 
size. The plasmodia in the tissues of this section are much larger than 
those found in any of the earlier stages. The parasite has also spread 
