Fig. 26.* 
REACTION OF THE PLANT. 
In many instances there is no perceptible evidence of 
defense or tissue-reaction in any part of the host, 7. e., the 
attacked plant succumbs quickly, offering no apparent 
obstacle to the advance of the bacteria. This is true of 
various soft rots, in virulent forms of pear-blight, in brown 
rot of young tobacco plants and tomato plants, and in the 
wilt of cucumbers. In less virulent forms of disease the 
plant often reacts by building a more or less impervious 
wall around the diseased parts, between them and the 
sound tissues, and thus ‘‘corks out”’ the intruder, e. g., 
potato-tubers attacked by Bacillus phytophthorus, various 
leaf-spots, and cankers. In some instances the presence 
of bacteria in the tissues leads to the premature develop- 
ment of organs—blossoms and side branches in the squash, 
male inflorescence in sweet corn, clusters of roots from 
other roots in hairy root of apple, aerial roots on tomato, 
daisy (fig. 26), and tobacco; in other cases retardation 
of development and atrophy occur. 
HYPERPLASIAS. 
In certain types of disease there is a very pronounced 
reaction of the host. This is manifested by rapid cell- 
division and enormous increase in the volume of tissues, the 
result being a tubercle or tumor which may continue to 
grow for months (plates 8 and 9) and exceptionally reach a 
diameter of a decimeter or more. The lowest stages of 
this hyperplasia may be seen in cankers of various sorts 
and in the effect of non-virulent cultures of Bact. solana- 
cearum on potatoes and tomatoes (fig. 27). The most 
striking examples are the crown-galls of peach, hop, daisy, 
sugar-beet, etc. (figs. 28, 29). These enormous swellings 
are the result of repeated cell-division under the stimulus 
of the presence of the micro-organisms in the tissues and 
as already stated inside of the rapidly dividing cells. Just 
what this stimulus is we do not yet know. It is probably 
a definite chemical substance derived from the bacteria, 
i. e., a by-product, or an endotoxin. The writer suspects 
ammonium or calcium acetate to be one of the stimulating 
substances. There is reason to suppose that acetic acid is 
formed by Bact. tumefaciens in tumors. Some substance 
liberated from the bacterial cells killed thereby may be the 
actual inciting cause. This whole subject is reserved for 
further consideration. Entomologists have maintained 
with respect to insect galls that if the egg is deposited 
anywhere but in the cambium layer, 7. e., too deep or too 
*Fic. 26.—Stem of Paris daisy showing incipient aerial roots at x induced by presence of tumor. Thiswas inocu- 
lated by needle-punctures as a check on virulence of a culture of Bacterium tumefaciens inoculated into sugar-beets. 
Plant inoculated Nov. 15 (or 18), 1907. 
90 
Photographed Feb. 20, 1908. 
