sept. i6 t 1918 Tissue Invasion by Plasmodiophora brassicae 
551 
a considerable distance from the point of entrance, the spindle will be 
long; if the infected cylinder is short, the spindle will be short. 
The mature spindle or club results largely from the abnormal growth 
of the infected cambium. By direct penetration of the parasite and 
through the division of infected cells the disease is spread in both direc¬ 
tions from the cambium. If the stem is quite old and the phloem ele¬ 
ments well differentiated a large part of the spread from the cambium 
is through repeated divisions of infected cambial cells or by the division 
of diseased cells that were split off from the cambium after it became 
infected. A cross section through such an old stem shows the wood and 
the outer portions of the bark entirely free of infection. Between this 
wood and outer bark there is a band of infected tissue. This condition 
is well shown in Plate 68, A. The inner portions of the wood are entirely 
free of infection, as are also the outer portions of the bark. For the 
most part only those tissues that have developed since the cambium be¬ 
came infected show the disease. Plate 68, B, shows a portion of a cross 
section of a somewhat older stem. It will be seen that the layer of non- 
infected bark is much thicker here than in A. That the infection shown 
in B is younger than that shown in A is indicated by the size of the plas- 
modia in the two sections. The band of infected tissue between wood 
and bark is very definite in each case. Plate 69, A, gives a portion of a 
longitudinal section through another stem that became infected after 
its wood and bark elements were well differentiated. Longitudinal 
sections through different portions of such a stem show that down near 
the point of original infection the band of diseased tissue is broad, while 
farther away it is narrower, and finally comes to a point at the place 
where the cambium is healthy. The plasmodia shown in Plate 69, A, 
are almost mature, and spore formation is beginning. The infection 
here is older than that seen in Plate 68, A, and much older than that 
shown in Plate 68, B. The three figures show three different stages of 
the development of the disease in old stem tissues. 
If the stem is attacked while the plant is young the greater part of 
its tissues are still very susceptible to infection The course of infection 
is much the same in these stems as in the older ones, except that here 
the fungus spreads more readily from the cambium out into the several 
layers of the bark and in toward the xylem. 
Sections through stems have been used to illustrate the above de¬ 
scription of infection. It has been easier to obtain the different stages 
in the stem tissues than in root tissues. This is probably because the 
infecting plasmodia travel more slowly through the hard stems than 
through the roots. It has, nevertheless, been possible to observe many 
of the different stages of infection in roots also. Although the series of 
stages is less complete for the root, enough of them have been observed 
to make certain that the general method of invasion is the same in the 
two organs. 
