SYNOPSIS OF CONCLUSIONS. 199 



(18) The beginnings of the galls are visible in some cases as early 

 as the fourth day after inoculation by needle prick, and they often 

 reach a large size in one to two months, but frequently on woody 

 plants they continue to grow for several years. On the contrary, 

 sometimes they have been very slow to develop. 



(19) Some cross-inoculate less readily than others, but in general 

 the monotonous morphology, the cultural uniformities, and the ready 

 cross-inoculability (daisy, peach, hop, grape, poplar, alfalfa), point 

 to one polymorphic species rather than to several distinct species, 

 but further studies should be made. 



(20) The galls are often rapidly invaded by saprophytic bacteria, 

 especially the softer galls. On agar poured plates many of these 

 bacteria are readily distinguished from the parasite by differences in 

 form and color, but others are distinguished therefrom with great 

 difficulty, cultures on other media or inoculations being requisite. 



(21) The galls also invite various parasites — nematodes, fungous 

 root rot, fire blight of apple and pear, etc., and some of these are able 

 to cause great damage. 



(22) We have not been able to distinguish etiologically between 

 liard galls and soft galls. Even the hardest crown-galls are due to 

 bacteria which closely resemble those found in the softest. 



(23) Overfed plants are more subject to the disease than those 

 making a moderate growth. 



(24) The size of the tumor, other things being equal, depends on 

 how rapidly the plants are growing, i. e., the state of nutrition. 

 Actively growing plants usually developed large tumors when 

 inoculated, and slow-growing plants none at all or small ones; but, 

 as in apple, small slow-growing galls may finally become large. This 

 long-continued growth would not be possible if there were not a very 

 nearly even balance between the stimulus of the parasite and the 

 response of the host. 



(25) The apple hairy-root, hitherto a disease of unknown origm and 

 supposed to be noninfectious, has been shown to be due to bacteria 

 which culturally and morphologically differ, if at all, only shghtly 

 from the crown-gall organisms. 



(26) This causal organism is located not in the hairy roots them- 

 selves but in the flattened tumor from which such roots arise. 



(27) Typical hairy-root has been produced on sound apple seedlings 

 by pure-culture inoculations, and in the same way on sugar beet both 

 galls and hairy-roots have been obtained. 



(28) These abnormal growths which we have designated indiffer- 

 ently as tumors or galls are believed to be like malignant animal 

 tumors in various particulars : Permanent and very rapid new growth 

 containing all the tissues of the part attacked; enormous round- 



213 



