Fungous diseases of plants . rtion of the oldpart but are entirely destroyed. Young galls may, however, springfrom the collar or roots near the margin of the gall previouslyformed, and thus the wounds and injurious effects are intensifiedfrom year to year. In time the functions of the conductingtissues are so interfered with that death of the parts above followsgradually. In the South and Southwest, galls which begin to growrather late in the season may continue their growth throughoutanother year. According to Toumey when the gall first begins its develop-ment, there is a pushing outward of a
Fungous diseases of plants . rtion of the oldpart but are entirely destroyed. Young galls may, however, springfrom the collar or roots near the margin of the gall previouslyformed, and thus the wounds and injurious effects are intensifiedfrom year to year. In time the functions of the conductingtissues are so interfered with that death of the parts above followsgradually. In the South and Southwest, galls which begin to growrather late in the season may continue their growth throughoutanother year. According to Toumey when the gall first begins its develop-ment, there is a pushing outward of a small area of the truecambium, which is transformed into large hypertrophied paren-chyma cells. ... In its youngest stages the tissue of the gallis a mass of parenchyma with numerous minute areas of rapidlydividing meristem scattered through it. The areas of meristematictissue are centers of growth. . As the galls become olderthese centers of growth increase in size and others originate in ii6 FUNGOUS DISEASES OF PLANTS. Fig. 28. Crown Gall of Peach the newly formed parenchyma. The centers of these growths ulti-mately become most curiously twisted nodules of tracheides andwoody fibers. Galls upon relatively small roots may not attain more thana centimeter in diameter, while ordinarily on nursery stock, raspberries, etc., they may be aslarge as a walnut (Fig. 28). Onthe crowns of large trees theymay be much larger. Cross-inoculation has cost no small amount ofeffort to determine the causeof crown gall. Toumey founda Myxomycete developing occa-sionally upon the cut surfaces ofgalls in impure cultures. Hefurther observed appearances ofthe protoplasm in certain cells of the parenchyma of young gallssuggesting stages in the development of the plasmodia. The evi-dence was not strong, however, and many pathologists reserveda final opinion regarding the nature of this disease. It was longapparent that the disease is infectious, and many experimentsdemonstrated that it c
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