DEVELOPMENT OF FORMS OF THE DISEASE. 29 
experiment in these plats were all made at one time in the same 
nursery, divided into eight equal lots, and planted in each of the 
plats. The location of each plat and a description of the soil are 
given on page 78. It was noted that more diseased trees occurred 
on the plats with the wettest, heaviest soils. The results from the two 
plats with the heavier, wetter soils and from the two plats with the 
lighter, better-drained soils are given in Table XIII, in the appendix. 
The wet, heavy soils gave from 8.1 to 11.1 per cent more diseased 
trees than the light, dry soils. The highest percentage of disease in 
a number of smaller experiments has invariably occurred on wet, 
heavy soils. It therefore appears that such soils are more favorable 
to the development of crown-gall. 
Period for the maximum development. of crown-gall and hairy-root umn 
the nursery.—After compiling the data from a large number of experi- 
ments, involving many trees dug at different ages, certain regular 
variations were noted in the amount of crown-gall present at different 
ages on root-grafted trees in the nursery aiden the Goud manus of 
experimentation. 
The results which follow are compiled from seven large sets of 
parallel experiments carried on in four of the cooperative plats— 
1, 2, 3, and 4 (see p. 78). The root grafts for each set were made 
from carefully selected healthy stocks of scions and seedlings at the 
same nursery, divided into four equal parts and planted in the four 
cooperative plats just named. Twenty-four varieties of apples were 
used, the same as are given in Table X XI, page 85, in the appendix. 
One-third of the trees in each plat, selected in a numerical series in 
each row, were dug each year, at the ages of 1, 2, and 3 years. The 
results are given in Table XIV, in the appendix. 
The results in Table XIV indicate plainly that in the nursery and 
under the conditions given root-grafted trees developed crown-gall 
chiefly during the first year, and that during the next two years many 
of the trees threw off the disease, which decreased the second year 
from 26.7 per cent to 9.1 per cent, and the third year from 9.1 to 4.7 
per cent. This table also shows that the development of hairy-root 
took place chiefly during the first year, but increased considerably 
the second year, 3.1 per cent, with little increase the third year, 0.1 
per cent. 
An objection to making such conclusions from average results may 
be offered for the reason that the average may be that of two widely 
differing extremes, either of which might include the results of a 
large number of the experiments, and that such extremes if taken 
separately would warrant the formation of entirely opposite conclu- 
sions. This objection can not hold here, since all of the sets of experi- 
ments gave approximately the same general results as the average, 
186 : 
