14 THE CROWN-GALL AND HAIRY-ROOT OF THE APPLE TREE. 
tissues. During each period of new growth the bark of the hard 
galls is usually ruptured, only to be re-formed at the close of the 
eTrowing season. 
FORMS OF HAIRY-ROOT. 
The term “‘hairy-root’’ as used in this bulletin is a general term 
applying to four closely related forms of the disease occurring on 
shoots and roots of apple trees. In these there is an increased and 
abnormal production of either roots or incipient roots. Four forms 
have been recognized: The simple, the woolly-knot, the broom-root, 
and the aerial. 
Simple form.—The term ‘‘simple hairy-root’”’ has been applied to 
the form of hairy-root originally described by Stewart, Rolfs, and 
Hall (64) (PI. III, fig. 1). This form is the common one on apple 
seedlings, but occurs less frequently on grafted and budded apple 
trees in the nursery and the orchard. It consists of numerous small 
roots growing out at nearly right angles, either singly or in tufts, from 
an older root or stem (PI. VI, figs. 2, 3, and 4). This root formation 
takes place from adventitious buds in the cambial layer, either directly 
from a root or stem or from swellings thereon. Such roots from the 
first are more succulent than normal ones. The cells of the par- 
enchyma are hypertrophied and the formation of wood cells is 
retarded. On exposure to air such roots dry up and become hair- 
like in appearance. 
Woolly-knot form.—A second form, which has been called ‘ woolly- 
knot,” is infrequent on young apple seedlings, but common on older 
seedlings and on grafted and budded trees in nurseries and orchards. 
This form originates as follows: A smooth, irregular swelling 
develops, usually in a larger root near the surface of the soil. This 
projects at first half an inch or so from the surface of the root. Mean- 
while, in the interior of the swelling an incipient root formation takes 
place. In a few months this usually develops to such an extent that 
it breaks through the epidermis, producing a warty knot. Under 
favorable growing conditions, during either the same or succeeding 
years, there is thrown out from each root center on the surface of the 
knot a rapid-growing, succulent root, which resembles in develop- 
ment and structure those of the simple form. Many roots usually 
develop from a single knot. These are often fasciated, and through 
intricate branching develop into a great mass of fine roots (PI. I, 
fig. 2). This form may develop from hard crown-galls. (See p. 57.) 
Broom-root form.—A third form of hairy-root occurs frequently on 
budded and grafted trees in nurseries in States bordering on the 
Missouri River. It consists of a broom-like formation of fine roots, 
occurring usually at the end of a side root which otherwise appears 
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