16 THE CROWN-GALL AND HAIRY-ROOT OF THE APPLE TREE. 
RELATION OF HAIRY-ROOT TO CROWN-GALL. 
Smith, Townsend, and Brown have isolated from hairy-root a bac- 
terlum with which they have reproduced the disease on the sugar 
beet and on the apple. It has not yet been proved that the hairy- 
root forms of disease may be produced by inoculation of apple trees 
with Bacterium tumefaciens, unless the two organisms Smith, Town- 
send, and Brown have isolated are identical, but the production of 
these forms is not morphologically impossible. In the orchard 
experiments described on pages 38 and 57 it was noted that a con- 
siderable number of the trees diseased with hard crown-gall developed 
hairy-root from the galls. 
RELATIONS OF THE FORMS OF HAIRY-ROOT TO EACH OTHER. 
The forms of hairy-root are closely related to each other, the essen- 
tial feature of each consisting of an excessive development of roots, 
evidently the result of the same sort of stimulus acting upon dormant 
root centers or buds in the twigs and the trunk of the tree in the ease 
of the aerial form, upon root buds on the main roots in the case of the 
simple and woolly-knot forms, and upon the side roots in the case of 
the broom-root form. The woolly-knot form is directly related to 
the hard form of crown-gall (see p. 20). 
European investigators of these forms of disease have always con- 
sidered the crown-gall (Wurzelkropf, tuberculosis) a different dis- 
ease from the aerial tumors or the aerial form of hairy-root (Kropf- 
maser, broussin). As the identity is not completely established, these 
forms will be treated separately in the experimental data recorded, 
and all data will be given under.the separate forms as hairy-root and 
crown-gall. : 
DEVELOPMENT OF THE FORMS OF THE DISEASE. 
NO DEVELOPMENT OF THE DISEASE ON SEEDLINGS WITHOUT WOUNDS. 
Greenhouse experiments with uninjured seedlings—In experiments 
conducted in pots and beds in the greenhouse with uninjured seed- 
lings grown directly from seed (see p. 77), 102 apple seedlings kept 
free from wounds remained healthy, showig no traces of crown-gall 
or hairy-root. Of 48 apple seedlings grown under similar conditions, 
except that the surface layers of soil were mixed with finely chopped 
pieces of living hard and soft apple galls after the seed had germinated, 
none became diseased. 
Greenhouse experiments with wounded seedlings—In connection 
with the experiments mentioned in the preceding paragraph, 98 apple 
seedlings were grown under the same conditions as the first lot, with- 
out soil inoculation. When they were 2 months old each was 
is 
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