Feb. 19,19x7 Arsenical Injury through the Bark of Fruit Trees 
291 
ing bark. This forms a tiny spot, with thelenticel in the center (PI. 83, C). 
If the affected tissues are cut into with a knife, they are found to be 
darkest near the surface; but the color grades imperceptibly into that of the 
healthy bark a millimeter or two below the surface. Each day now for 
about a week or two the spot becomes a little darker externally and inter¬ 
nally and increases a little in size. It then becomes sharp in outline, as a 
rule, and quite definitely sunken. Two or three weeks after treatment, if 
made during the growing season, most of the spots cease to grow in 
size. They are then very dark brown, almost black, externally and 
much sunken, and a crack begins to form at the boundary between the 
injured tissues and the healthy ones. These spots vary in size from one 
to several millimeters and often anastomose into irregular dark patches. 
As a rule, the spots are larger on the under side of a horizontal branch 
than on the upper. The difference, however, is somewhat less notice¬ 
able if the bandages are covered with a rubber dam and thereby kept 
evenly wet. In a spot 5 mm. in diameter the tissues are killed and 
colored a very dark brown to the cambium and the youngest wood is 
sometimes affected also. In a spot 10 mm. in diameter the bark is dis¬ 
colored to a depth of one to several millimeters and there is considerable 
injury to the underlying wood. The affected bark is not friable at this 
time, but tough and leathery. Not all the lenticels are affected, as a 
rule; sometimes only a few and sometimes many, from which we may 
judge that some of them are closed at times. 
If the action of the chemical is more severe, many lenticel spots are 
usually found anastomosed and the dead patch thus formed may girdle 
the limb under the bandage. From this dead patch one or more streaks 
may run up and down the limb. These vary in length and diameter, and 
their size may be used to indicate the severity of the attack. In extreme 
cases they are several feet long and kill some of the side branches. These 
streaks, like the* spots, are at first indefinite and later become nearly 
black and sunken. The wood under the dead bark is also discolored, 
varying from a light coffee-brown to nearly black. This discoloration 
may invade only the wood next to the cambium, but often it extends to 
the pith. 
When a wound is made through the outer corky layer just before the 
chemical is applied, the tendency to form streaks is much more marked 
than when the absorption is through lenticels. There will appear around 
the wound more or less discoloration and in mild cases the injury stops 
here. Sometimes near-by lenticels will show spots also. There is a 
strong tendency, however, to form a streak running up and down from 
the wound (PI. 84). As a rule, it extends farther upward than down¬ 
ward; but the reverse is sometimes true. Sometimes the whole branch 
above the bandage is killed, and only a streak runs down. Sometimes 
the streaks run a spiral course, and if there are two wounds on opposite 
sides of the branch the two streaks from them keep on opposite sides and 
