DIFFERENT FORMS OF THE DISEASE. ES 
healthy (PI. III, fig. 2). Frequently a broom-root springs from a 
root which has its origin in a hard crown-gall. Broom-roots resemble 
those of the woolly-knot in structure, and they develop in much the 
same way, except that they do not originate directly in swellings, but 
develop by the excessive branching of root ends. They usually 
grow upward from their point of origin toward the surface of the soil, 
in contrast with normal fibrous roots, which grow more nearly 
parallel to the surface. 
Aerial form.—A fourth form of hairy-root, which also has been 
called ‘‘stem tumors”’ (22), occurs on the trunks and limbs of apple 
trees, more commonly on older trees in orchards. This form con- 
sists at first of smooth, fleshy swellings (Pl. IV, figs. 1, 2, and 3) in 
which the cambial layer is much thickened. These swellings are 
more or less rounded, but are often irregular in outline, vary in width 
from half an inch to 2 or 3 inches, and project from a half inch to an 
inch beyond the surface of the tree. They develop, internally, incip- 
ient adventitious roots, which as the disease progresses break forth, 
throwing off the outer bark and forming a warty knot (PI. V, fig. 1). 
Cuttings containing the disease in its earlier stages readily throw out 
roots when placed in moist sand or soil in the springtime, developing 
directly into the woolly-knot form of the disease (PI. V, fig. 2). On 
orchard trees as the knot grows older the incipient roots in the center 
of the knot die, and in some cases new ones are formed in the adja- 
cent healthy tissues, increasing the diameter of the knots but not 
their projection. 
RELATIONS OF THE FORMS OF THE DISEASE TO EACH OTHER. 
RELATION OF SOFT CROWN-GALL TO HARD CROWN-GALL. 
The work of Smith and Townsend (54) in discovering and demon- 
strating that Bactervum tumefaciens is the cause of the crown-galls of 
the Paris daisy, the peach, etc., and the work of Townsend with the 
same or a similar organism from the galls and hairy-root of apple 
trees indicate that all of the forms of the disease may be caused by 
the same species of bacterium or some of its forms.” 
Since the soft and the hard forms of crown-gall are alike in their 
initial development and since the latter may develop from the former, 
the two thus differing only in the final form and in duration, it follows 
that they both may be due to the same cause. It may be that in 
the case of the hard form the apple tree partially resists the organism 
causing the disease by healing over the surface of the galls and con- 
fining the disease to limited areas, and the soft form may be a more 
virulent type of the disease. 
@ See also Smith, Erwin F., Science, n. s., vol. 30, no. 763, August 13, 1909, p. 223. 
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