CE ee 
=r 
BEHAVIOR OF CHECK-PLANTS. 15 
diseased, then the experiments must be done over with more care and times enough 
to remove all possible chance of error. When check-plants become diseased, 
especially in any number, there is always room for grave suspicion. Either the 
experimenter has been grossly careless, assuming that he used the right organism in 
his inoculatio&-experiment, or else he is working in a locality where the cause of 
the disease is naturally abundant. In either case, however well convinced he him- 
self may be, his readers will generally have a lingering suspicion that even his inocu- 
lated plants succumbed not to what he inserted into them, but to some entirely differ- 
ent cause naturally present and overlooked by the investigator. ‘The remedy for the 
first is to learn to use infectious 
material with more caution, and 
for the7second is to make the in- 
oculation-experiments in localities 
“or under conditions where the 
plant shall be less subject to natu- 
ral infection. 
If the experiments must be per- 
formed in localities where the dis- 
ease is naturally present, then a 
large number of plants must be 
selected for inoculation and for 
control, and such a high percent- 
age of infections secured in the 
inoculated plants that the few 
cases occurring naturally in the 
control-plants may be neglected 
as not casting any doubt on the 
general result. For example, if, 
in a region subject to the given 
disease, 100 plants were reserved 
for control and roo similar plants 
were inoculated, and out of this 
number 50 of the latter and 4o 
of the former should contract the 
disease, it is manifest that no deductions of any value could be made from the 
experiment. All might be the result of some cause totally different from the 
*F ic. 9.—Cross-section of a small part of a cucumber stem, showing the parasitism of Bacillus 
tracheiphilus in one of the inner bundles. As yet there is no bacterial cavity, the bacilli being con- 
fined to the spiral vessels and a very few of tthe adjacent pitted vessels. Material taken from a field 
near Washington, D. C., in 1893. Sectioned from paraffin. Drawn from a photomicrograph. 
X 50. Introduced for comparison with plate 3. Beginning at the top, the tissues occur in the fol- 
lowing order: (1) Outer phloem, showing sieve plates; (2) cambium ; (3) immature xylem ; 
(4) mature xylem, consisting of pitted vessels and pitted connective tissues; (5) spiral vessels em- 
bedded in non-lignified living parenchyma, which is finally destroyed by the bacteria ; (6) pseudo- 
cambial layer; (7) inner phloem; (8) large-celled parenchyma to either side, separating this bundle 
from its neighbors. 
