570 BACTERIAL DISEASES OF PLANTS 



differences in culture media, growing well in one medium and 

 refusing to grow in another. See, for example, p. 401 dealing 

 with the olive tubercle. Here the organism causing the tumor 

 grows in an agar made with one peptone and refuses to grow in 

 that made with another peptone, or grows in the presence of the 

 latter only with much retardation. See also Fig. 160 (p. 217). 

 Many such cases are known to me, as they are to every working 

 bacteriologist. Second, the organism may have been isolated 

 and neglected for some more common form. How often the 

 wrong organism has been selected and studied, and by good 

 men too, sometimes for years, in case of other diseases both 

 of plants and animals! Nothing is more natural than to select 

 for study that which appears to be common and constant on the 

 poured plates and yet it may be the wrong thing. Third, cancer, 

 at least in the narrower meaning of the word, is a dyscrasia of 

 which the tumor is simply the end term and we may not hope 

 to reproduce it in animals by the inoculation of an organism until 

 the living substratum has become suited to it, that is until we 

 have reproduced the beginning and middle terms of the dyscra- 

 sia. These may depend on inherited tendencies (Slye) ; or on a 

 long-standing vicious physiological disturbance, a malnutrition, 

 for example; or on the long-continued action of some feeble 

 secondary parasite, a streptococcus, for example. Possibly the 

 cancer parasite itself is a streptococcus. Certain streptococci 

 have many of the necessary characters of such a parasite, viz., 

 low visibiUty in tissues, feeble parasitism and vitality, sensitive- 

 ness to slight changes in culture media so that they will not 

 grow, ^ ability to produce acids, ability to destroy blood, ability 

 to induce vegetations, etc. My assumption is that the carci- 

 noma parasite, if there is one, can act only in an organism which 

 has gradually become adapted to it, i.e., only in an abnormal 

 body. The normal animal body is amply protected against all 

 weak parasites by its leucocytes which might be expected to 

 pick up and destroy any injected bacterium of as feeble a 

 nature as a cancer parasite must be supposed to be, i.e., one 

 incapable of destroying the cells. For this reason plants, 



1 The writer was perhaps the first person to observe (1906) that the strepto- 

 coccus of endocarditis {S. viridans) is sensitive to sodium chlorid, and will not grow^ 

 in bouillon or nutrient agar made neutral to phenolphthalein by sodium hydroxid. 



