10 CAUSE AND NATURE OF CROWN-GALL 



one's fist. When they appear upon nursery stock they are 

 usually limited to the crown, but occasionally are found deeper 

 in the earth, or even upon slender one-year-old roots. The 

 color of the gall is similar in its younger stages to that of the 

 sound root. Later a dark color appears, in consequence of a 

 deposit of dead matter, which forms the bark of the gall. If 

 o.ne examines the galls occurring upon the smaller roots, it will 

 be seen that they are generally located upon one side of the 

 root body. They have a softer tissue than the root, but their 

 color within is normal. The large galls are a series of hemi- 

 spherical growths superposed upon each other so that the surface 

 has an irregular or warty appearance. In the spring the more 

 prominent of these elevations have a light-brown appearance 

 and a perfectly herbaceous consistency. la cross-sections the 

 galls show an irregular fibrous mass." 



Although Sorauer observed that the disease appeared much 

 more frequently in certain nurseries than in others, and that 

 the prevalence of its attack and the season of its appearance 

 indicated a parasitic origin, he was led, on account of not being 

 able to find the parasite, to consider it not a true infection, but 

 as the probable result of mechanical injury. 



Although I have considered the disease described by Sorauer 

 and the various American authors as one and the same, it is pos- 

 sible from my experiments that we may have under considera- 

 tion a number of closely allied diseases affecting various host 

 plants and caused by true parasites, as will be seen later. It is 

 further possible that the swellings described by Sorauer are 

 caused, as he suggests, by mechanical injury. However, I sus- 

 pect, from his description of the excrescences and their close 

 comparison with the disease as it occurs in Arizona, that it also 

 is of parasitic origin. So far as I can learn no experiments 

 were made by Sorauer or other European investigators in order 

 to ascertain its communicability. 



OPINIONS REGARDING THE COMMUNICABILITV OF CROWN- 

 GALL. 



The general opinion of orchardists is that the disease is a 

 true infection and spreads from tree to tree. They have come 

 to this conclusion from observing its general behavior in the 

 field. Most investigators that have given attention to it, hav- 



