DIFFERENT FORMS OF THE DISEASE. lis 
DESCRIPTION OF THE DIFFERENT FORMS OF THE DISEASE ON 
APPLE TREES. 
FORMS OF CROWN-GALL. 
The: term ‘“crown-gall”’ as used in this bulletin applies to the 
fleshy outgrowths, or galls, occurring rarely on the stems and fre- 
quently on the crown and the roots of apple trees. It is applied only 
to those galls that are not accompanied by an excessive formation of 
either roots or incipient roots and are not caused by insects. Two 
forms have been recognized by the writer—the soft and the hard (19). 
Soft crown-gall.—The term “soft crown-gall’’ has been applied to 
the form most frequently found on young seedling, grafted, and 
budded apple trees (Pl. II, figs. 1 and 2). These galls consist at first 
of a soft, succulent growth of young cells thrust out from the cam- 
bial layer of roots or shoots. They commonly originate in wounds 
and their initial growth is quite similar to that of an enlarged cailus 
(Pl. VIII, fig. 2), which they resemble in form, color, and structure. 
‘The surface of the galls, owing to an unequal growth of the elements 
in the tissues, soon becomes coarsely convoluted. Curled and dis- 
torted masses of wood cells and vessels are gradually formed through- 
out the softer tissues, often causing the galls to become hard and 
woody in the interior. At first the color of the surface is white, but 
later it either changes to the color of the adjacent healthy tissues or 
becomes brown or black from the decay of the surface cells, which at 
no time are protected by an epidermis. 
In size these galls usually vary from that of a pea to that of a man’s 
fist, but in exceptional cases they may grow to the size of a man’s 
head. They do not develop roots from their surface, and under 
natural conditions in the nursery they generally decay at the end of 
the growing season. In general appearance and structure they 
resemble the crown galls of the almond, apricot, blackberry, cherry, 
chestnut, grape, peach, pear, plum, raspberry, rose, and walnut, to 
which they are related (22). 
Hard crown-gall.—The term ‘“‘hard crown-gall’’ has been applied 
to the form occurring more frequently on older trees in nurseries and 
orchards (PI. II, figs. 3, 4, and 5). The earlier growth of these hard 
galls is similar to that of soft crown galls, but later they become cov- 
ered with bark and develop a woody interior. They finally have a 
texture intermediate between that of healthy wood and that of soft 
crown galls. Roots often spring from their tissues and they thus 
develop into a form of hairy-root (see p. 57). Unlike the soft galls, 
they do not decay, but continue their growth the following season. 
The hard galls finally attain the same size as the soft galls, but are 
much slower in their development. They usually have a more finely 
convoluted surface and are of the same color as the adjacent healthy 
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