566 
Journal of Agricultural Research 
Vol XIV, No. 21 
clubs of the different plants. In a few cases the spores occupy only 
about*20 per cent of the volume of the club, while in other instances 
they occupy a little more than 35 per cent of the total volume. On the 
whole, however, the quantity of spores contained in a given volume of 
diseased tissue is remarkably constant for the different plants studied. 
The average volume occupied by the spores in the diseased tissues of the 
different plants amounts to about 28 per cent. This means that approx¬ 
imately 28 unit volumes are occupied by the parasite in every 100 unit 
volumes of diseased tissue. In other words, the volume relation between 
parasite protoplasm and host protoplasm may be expressed by the 
ratio 28 to 72. The writer believes that in this ratio we have expressed 
numerically the balance which here exists between host and parasite. 
This balance is not between the individual host cell and the individual 
plasmodium within it, but between all of the plasmodia and all of the 
cells of the diseased tissue. The growth of the plasmodia in a given 
club is determined by the amount of tissue involved. Each club is as 
thoroughly diseased as is possible. The noninfected cells are apparently 
free of infection, not because they have accidentally escaped, but because 
of some influence which the host exercises over the parasite. There is 
a limit beyond which the parasite can not go in its growth in the cabbage 
tissues. How this limit is maintained is a problem that remains to be 
solved. It may be that the spread and growth of the parasite is held in 
check through the development of some protective substance in the 
host cells. The infected cells seem to have some means of controlling 
the growth of the plasmodia which they contain. If we assume that 
this control is exercised through the production of a protective substance 
or antitoxin, then it is easy to suppose that this substance might diffuse 
out into surrounding cells and thus render them immune to attack. 
Before the parasite would be able to establish itself again in a cell it 
would be necessary for it to pass beyond the region of immune cells. 
Such a theory would account for the distribution of the organism and 
for the balance maintained between it and the host tissues. Acquired 
immunity has not been observed in plants. Perhaps this is because 
plants do not have a circulatory system comparable to animals and 
because the protective substance, if such is developed, is not carried to 
parts distant from the foci of infection. 
Although the relation between quantity of spores and volume of host 
tissue is very constant, the relation between infected and noninfected 
cells varies greatly. The fact has been emphasized that many of the 
cells of most clubs remain free from infection. However, in rare instances, 
almost all the cells of a club may show infection. It is interesting to 
note that in such cases the plasmodia in most of the cells remain very 
small. These plasmodia do not grow and never give rise to large numbers 
of spores. They also fail to stimulate the host cells to abnormal growth. 
