DEVELOPMENT OF FORMS OF THE DISEASE. it 
wounded on the root just below the surface of the soil with a sterile 
knife. In each case the cut extended upward and through the 
cambial layer. The blade of the knife was sterilized in a 1-to-1,000 
solution of mercuric chlorid before each cut was made. Four seed- 
lings became diseased with small, hard galls, which formed from out- 
growths from the cambial layer on the upper edge of the wound. All 
other wounds healed normally. No formation of hairy-root occurred. 
Nursery experiments with young seedlings.—In the nursery it is 
quite impossible to grow seedlings entirely free from injuries. To 
ascertain to what extent wounds favor the develapment of crown- 
gall and hairy-root, 100 consecutive trees in a nursery row were 
wounded by a knife cut in the cambial layer, just below the surface 
of the soil. These trees were about 2 months old when wounded. 
For a control, the same number of consecutive trees in an adjacent 
row were selected, in which no wounds were made. At the end of 
the growing season the trees in both rows were dug. Of the wounded 
trees, 3 died. Of the remaining wounded trees, 8 were diseased with 
soft crown-gall, 2 with the simple form of hairy-root, and 2 with the 
woolly-knot form. Of the control trees, 1 was diseased with soft 
crown-gall and 2 with the simple form of hairy-root. Thus, four 
times as many wounded as unwounded trees became diseased. 
The percentage of diseased trees in apple-seedling nurseries is 
usually small. In exceptional cases only is it considerable, no doubt 
due to injuries received by the young trees during the first two or 
three months of their growth. ‘This is certainly true in the case of 
the development of crown-gall, and may apply also in that of hairy- 
root. 
In order to ascertain the relation of wounds to the various forms 
of crown-gall and hairy-root, 300 diseased yearling apple seedlings 
were selected, as follows: 100 with soft crown-gall, 50 with hard 
crown-gall, 100 with the simple form of hairy-root, and 50 with the 
woolly-knot form of hairy-root. These were dissected to ascertain 
the presence of wounds in the roots at the time that the disease 
attacked the seedlings. Such injuries are indicated by the presence 
of a small discolored rift, showing a break in the tissues near the 
heart of the tree beneath the outgrowth of the diseased portion. 
Certain indications of wounds were found under 90 of the soft galls, 
47 of the hard galls, and 22 of the woolly-knots. In the case of most 
of the trees diseased with simple hairy-root, there were discolored 
areas which were not considered rifts or breaks in the tissues. On 
the whole, there was decided proof that the first three forms just 
named are quite closely connected with injuries to the young roots. 
Nursery experiments with older seedlings.—In a field experiment at 
Louisiana, Mo., 350 apple seedlings were transplanted and grown 
50414°—Bul. 186—10——2 3 
