Il6 FUNGOUS DISEASES OF PLANTS 



the newly formed parenchyma. The centers of these growths ulti- 

 mately become most curiously twisted nodules of tracheides and 

 woody fibers." 



Galls upon relatively small roots may not attain more than 

 a centimeter in diameter, while ordinarily on nursery stock, 



raspberries, etc., they may be as 

 large as a walnut (Fig. 28). On 

 the crowns of large trees they 

 may be much larger. 



Cross-inoculation experiments. 

 It has cost no small amount of 

 - effort to determine the cause 

 of crown gall. Tourney found 

 a Myxomycete developing occa- 

 sionally upon the cut surfaces of 



galls in impure cultures. He 

 FIG. 28. CROWN GALL OF PEACH & 



further observed appearances of 



the protoplasm in certain cells of the parenchyma of young galls 

 suggesting stages in the development of the plasmodia. The evi- 

 dence was not strong, however, and many pathologists reserved 

 a final opinion regarding the nature of this disease. It was long 

 apparent that the disease is infectious, and many experiments 

 demonstrated that it could be conveyed from one susceptible 

 plant to another by inoculation of the roots with macerated galls 

 or by burying infected parts of diseased plants in the vicinity of 

 healthy roots. The results of rather recent and extensive inocula- 

 tion experiments by Hedgcock are summarized by him as follows : 



" The soft galls from the almond, apricot, blackberry, cherry, 

 peach, plum, prune, and raspberry have been transferred easily 

 to seedlings of the almond, apricot, peach, and raspberry ; less 

 readily to those of the blackberry, cherry, plum, prune, and pear ; 

 and with great difficulty to seedlings of the apple, chestnut, wal- 

 nut, and rose. 



" The soft galls of the apple, chestnut, walnut, rose, and pear, 

 as a rule, have not been transferred readily to any of the plants 

 mentioned. Evidence has been obtained of a wide range of suscep- 

 tibility in different varieties of the same plant. This has been noted 

 in varieties of the apple, blackberry, cherry, chestnut, pear, and rose. 



