Sept. 16,1918 Tissue Invasion by Plasmodiophora brassicae 
565 
each of the 32 plants studied, and the area covered by spores is expressed 
in Table I as percentage of total area. 
Three of the pictures are reproduced in Plates 79, A, B, and 80, A, 
The spores cover 30.9 per cent of the surface of the photograph shown 
in Plate 79, A, 28.8 per cent of that shown in Plate 79, B, and 28.8 per cent 
of that shown in Plate 80, A. It should be noted that the shape and 
size of the infected cells vary considerably in the three illustrations. 
Figure B of Plate 79 shows fewer infected cells than either of the other 
figures. The infected cells are so large, however, that their spore masses 
occupy a space almost as great as that occupied by the spore masses 
shown in Plate 79, A. This figure shows a large number of infected 
cells, but these cells are small and the total spore mass is only slightly 
greater than in the two other tissues. In spite of differences in the 
distribution of the parasite, number of cells infected, size and shape of 
spore masses, and in spite of the differences in size and shape of non- 
inf ected cells, each of the three tissues contain approximately the same 
quantity of spores. The relation between quantity of spores and host 
tissues was found to hold with remarkable constancy in each of the 32 
clubs studied. This is well shown by Table I. 
Table I .—Quantity of spores of Plasmodiophora brassicae contained in average sections 
taken from 32 different cabbage plants 
Plant No. 
Weight of photographic 
paper— 
Part of 
photograph 
occupied 
by spore 
masses. 
Plant No. 
Weight of photographic 
paper— 
Part of 
photograph 
occupied 
by spore 
masses. 
Showing 
spore. 
masses. 
Showing 
no spore, 
masses. 
Showing 
spore 
masses. 
Showing 
no spore 
masses. 
Gm. 
Gm. 
Per cent. 
Gm. 
Gm. 
Per cent. 
I 
0. 272 
0 .598 
3 1 * 2 
18 
O. 198 
°* 759 
20. 6 
2 
.236 
. 642 
26.8 
J 9 
.256 
• 743 
25.6 
3 
•239 
• 645 
27. 0 
20 
.298 
. 665 
30*9 
4 
.281 
. 601 
31-8 
21 
.268 
• 55 1 
32.7 
S 
. 271 
• 592 
3 i -4 
22 
. 286 
• 516 
35-6 
6 
. 219 
. 619 
26. 1 
23 
.230 
. 607 
27.4 
7 
. 199 
. 647 
23 - 5 
24 
• 194 
.643 
23.1 
8 
. 180 
. 631 
22. 1 
25 
.250 
• 557 
3 °* 9 
9 
• 179 
• 663 
21. 2 
26 
.318 
. 520 
37-9 
10 
•332 
• 574 
36. 6 
27 
•255 
. 580 
3 °- 5 
11 
. 220 
.587 
27. 2 
28 
. 206 
• 605 
25-4 
12 
. 241 
.615 
28. 1 
29 
•235 
. 612 
27.7 
13 
•235 
. 667 
26. 0 
30 
•239 
*590 
28.8 
14 
.256 
. 610 
29-5 
3 i 
• 175 
.677 
20. 5 
15 
. 280 
. 690 
28.8 
32 
•245 
•675 
26.6 
16 
*239 
• 743 
24.3 
17 
. 211 
•774 
21.4 
Average 
27.7 
If the areas shown in the photographs be considered as of unit thickness 
then the percentages given in the table represent the space occupied by 
the spores of the parasite in each of the tissues studied. It will be seen 
that the quantity of spores per unit volume varies somewhat in the 
