176 CROWN-GALL OF PLANTS. 



The nonpathogenic wliite forms generally develop on agar plates 

 somewhat wliiter or creamier or denser colonies than the gall organ- 

 ism. They look more like the latter in early stages of growth than 

 after some days. But some resemble it so closely on agar that cul- 

 tures on other media are required. From old galls it is often difficult 

 to isolate the parasite, the tissues swarm with such a mass of secondary 

 and tertiary forms. So true is this that from such parts it is scarcely 

 worth while to attempt isolations. These are best made from the 

 youngest growing parts. 



Their fleshy nature also tempts parasitic fungi and bacteria, mites, 

 nematodes, and a variety of insects. 



Wlien the tumors are very fleshy decay sets in earUer than when 

 they are woody. 



EFFECTS OF THE DISEASE ON THE TISSUES NOT DIRECTLY 



INVOLVED. 



PHYSICAL EFFECTS. 



The necrosis of gall tissues already mentioned affords opportunity 

 for the entrance of rain water and many sorts of insects, bacteria, and 

 fungi, which bring about more or less destruction of supporting tis- 

 sues not involved in the original tumor. In tliis way the pear-blight 

 bacillus and facultative wood and bark parasites of various sorts 

 may enter, causing serious stem and root injuries. If the plant is an 

 orchard tree it may be weakened by this decay of the wood so as to 

 be easily broken off by animals or blown over by the wind. This 

 often occurs in the peach and almond; rarely in the ap})le. Plate 

 XXXI shows the bacterial apple blight {Bacillus amylovorus) orig- 

 inating in a hard gall, 



PHYSIOLOGICAL EFFECTS. 



The immediate and remote physiological effects of these tumors 

 vary from species to species and also within the species and are gen- 

 erally less pronounced and certainly less speedy than we might expect 

 from their size and vigor of growth. The plant, however, is less 

 specialized than the liigher animals, especially by absence of a nerv- 

 ous system, antl in this connection it might be interesting to speculate 

 on what would be the outcome of malignant animal tumors if the 

 depressing influence of pain were removed and the consequent greater 

 or less disturbance of all the functions of the body. 



In many instances the tree shows no material injury even after a 

 series of years. This is especially true of the apple, according to 

 Hedgcock, Stewart, and others. In other cases, and this is true 

 even of tlie apple, the attacked tree is dwarfed in comparison with 

 its unattacked fellows. Peaches and almonds show this dwarfing to 

 213 



