COMMUNICABILITY OF THE DISEASE. 49 
A third set of apple seedlings was also placed at the same time 
under the conditions of the experiment. This consisted of a number 
of bunches of apple trees badly affected with aphis galls but showing 
no hairy-root formation. It was thought that these seedlings with 
galls might develop hairy-root, but they behaved in their rooting no 
different from the set of healthy seedlings. 
The relation existing between the woolly aphis, however, and the 
aerial form (stem tumors) of hairy-root (Pl. V, fig. 2) is a subject for 
future investigation. The following observation gives the reason: 
In a seedling nursery in Indiana, mentioned on page 37, about forty 
trees were found diseased with this form of hairy-root, and they were 
also infested with the woolly aphis. It was noted that the trees 
adjacent to the badly affected trees were contracting the disease, 
indicating that this aphis may assist in the spread of the disease by 
carrying the organism causing it. 
No communication of the disease to healthy uninjured seedlings.—In 
an experiment with healthy apple seedlings, described on page 16, it 
was shown that healthy seedlings, uninjured as far as could be deter- 
mined, failed to become diseased even when the soil about their roots 
was mixed with finely chopped pieces of living soft and hard crown- 
galls from apple trees. Thus, without wounds, there was no com- 
munication of these forms of disease to apple seedlings. Where 
wounds were made, other conditions being the same as before men- 
tioned, 4 per cent of the trees became diseased with the hard form 
of crown-gall. 
The disease is communicable to wounded seedlings—A number of 
experiments have been conducted with healthy apple seedlings grown 
in the nursery and carefully washed and the tips of the roots cut off. 
They were then divided into lots of approximately equal numbers and 
prepared for the experiment as follows: The plants in one lot were 
prepared for a control by making a slanting incision into the root and 
then wrapping the wound with a thread in the same manner that 
grafts were wrapped. A second control set was similarly treated, 
except that a cross section of clean, healthy apple roots was inserted 
in the wound before wrapping. A third set was prepared like the 
first, except that a piece from the outside of a living apple crown-gall 
of the soft form was inserted in the wound. A fourth set was prepared 
similarly, except that a piece from the outside of a living apple crown- 
gall of the hard form was inserted. A fifth set, consisting of a smaller 
number of seedlings, was similarly prepared, except that a cross sec- 
tion of a piece of living apple seedling diseased with the simple form 
of hairy-root was inserted. After growing one season the results were 
as given in Table XXIII, in the appendix. 
50414°—Bul. 186—10 4 
