298 Journal of Agricultural Research voi. vm, No. 8 
in amount of injury between the smaller branches and the larger until a 
diameter of 1 inch is reached. The portion above the point of treatment 
is more often entirely killed in the smaller branches than in the larger, 
but this is because there is less tissue to kill above the point of treat¬ 
ment. The largest two in this series showed somewhat less injury than 
the rest, but one much larger than the average was the worst injured of 
the 12. 
It is, of course, possible that had a milder chemical been applied 
than that used in this experiment greater differences due to size of 
branch would have developed; but with several opportunities during 
the course of the investigation to make isolated observations along this 
line we saw no reason to believe that size of branch is an important fac¬ 
tor in determining the extent of injury, except that lenticel injury 
through a thick bark extended less rapidly than through a thin bark. 
This statement applies only to limbs with a smooth bark. 
RELATIVE INJURY BY DIFFERENT ARSENICAL COMPOUNDS 
That equal amounts of different chemical compounds injure plant 
tissues in different degrees is to be expected. In the first place, many 
of these compounds are readily soluble in water, and others are only 
slightly so and are often called “insoluble.” All the arsenical insecti¬ 
cides used for spraying plants are of the latter character. As the solu¬ 
bles are more readily absorbed than the so-called insolubles when abun¬ 
dant moisture is present, it seems best to compare the solubles with 
each other and the insolubles with each other. In making such com¬ 
parisons it would be possible either to use equal weights of the differ¬ 
ent chemicals or such quantities as to give equal amounts of arsenic. 
Either basis would be logical for its own purposes, and the former—that 
is, equal weights of the chemicals—was chosen for this investigation. 
Considering, first, the soluble arsenicals, we have compared them on 
both wounded, and unwounded limbs. These chemicals were dissolved 
in water in proportions between 1 to 1,000 and 1 to 1,000,000 and the 
bandages were saturated with the solution, evaporation being practically 
prevented by wrapping with a dental rubber dam. 
Twelve unwounded branches from y 2 to H inch in diameter were 
treated with soluble compounds as follows: Two each with arsenic acid, 
ammonium arsenate, potassium arsenate, potassium arsenite, sodium 
arsenate, and sodium arsenite in the strengths of 1 to 1,000 and 1 to 5,000 
by weight. Arsenic acid in each strength caused a small lenticel spot. 
Potassium arsenate, 1 to 1,000, caused a few small lenticel spots. Sodium 
arsenite 1 to 1,000 caused a few small lenticel spots and a spot $4 inch 
in diameter about a latent bud. The others caused no injury. 
Thirty-six other branches received two wounds each, either by cutting 
off twigs or by cutting a small notch through the bark to the cambium. 
