Sept. 16, 1918 
Tissue Invasion by Plasmodiophora brassicae 
561 
late stages of the disease suggests the presence of such substances- 
Moreover, the large galls draw heavily on the food supply of the plant 
and must lead to profound changes in the metabolic processes going on 
in the cells of the different tissues. 
If roots are attacked while comparatively young, the medullary rays 
do not for some reason become infected and the woody cylinder remains 
intact. Most of the abnormal growth that takes place in these roots 
occurs in the region of the cambium and in the cortex. If, on the other 
hand, infection takes place after the root is quite old and considerable 
wood has been produced, the medullary rays, as has been seen, become 
diseased. The infected rays grow very rapidly, and in many cases give 
rise to most of the tissues that compose the clubs. The greater part of 
these tissues are parenchymatous, but weak vascular elements may also 
develop in them. Such elements are shown in Plates 73, B, and 74, B, 
and especially in Plate 75, B. They develop in the ray tissues between 
the bundles and probably aid in transporting water and food to the 
diseased medullary cells. The long axis of the cells in the tissues between 
the older portions of the wood are parallel to the direction of growth, 
while the long axes of the cells between the younger portions of the wood 
are at right angles to the direction of growth. This is very interesting, 
sincp all of these cells arise from the same medullary ray. 
The infection of old roots and stems logically falls into four parts, as 
follows: (1) Infection of the cortex from without (primary infection); 
(2) infection of the cambium in all directions from the point or points of 
original penetration; (3) infection of the undifferentiated cells on either 
side of the cambium and of the inner cortex; and finally (4) infection of 
the medullary rays. No better proof for direct penetration of tissues by 
the plasmodia could be wished for than that given by the different 
stages of medullary infection. It will be seen that direct penetration 
of the tissues by plasmodia plays a much larger rdle than has previously 
been supposed. 
Former students of the disease seem to have had very hazy notions as 
to the way in which the galls arise. This is strikingly brought out by 
Kiister’s (<?) reference to gall mother cells. He seems to believe that the 
cta.6xa. caused by Myxomycetes may result entirely from successive 
divisions of a single infected cell. We know that individual “Krank- 
heitsherde" increase in size through host cell divisions, but these divisions 
have a small part in distributing the parasite throughout the tissues. 
INFECTION OF YOUNG ROOTS 
The writer has not made a detailed study of the infection of very 
young rootlets. He has no doubt, however, that they become diseased 
through direct penetration. Even in such young tissues host-cell 
divisions probably play a minor part in the distribution of the parasite. 
