REACTION OF THE PLANT. 



In many instances there is no perceptible evidence of 

 defense or tissue-reaction in any part of the host, i. 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 phyto phthorus , 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 Bad. 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, i. e., too deep or too 



*FiG. 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. Photographed Feb. 20, 1908. 



90 



Fig. 26.* 



