May 12,1923 Injury to Foliage by Arsenical Spray Mixtures 
509 
that were made, reaching into thousands. In orchard spraying, there¬ 
fore, a single branch 2 or 3 feet long was sprayed with each mixture, 
and it is probable that as accurate results were secured as though entire 
trees had been covered. Indeed, it was possible to cover these single 
limbs more evenly than whole trees could have been covered. 
It has been found most convenient to use two liters of spraying mixture 
for each application, and for this reason the strength of the chemical used 
is indicated as the number of grams in two liters. This method of expres¬ 
sion will be found in most of the tables in this paper. 
PRECAUTIONS 
Care was taken to avoid spraying on windy days, and if rain followed 
the application the results were rejected excepting as they could be used 
for data in relation to precipitation. For the regular work foliage was 
chosen that was normal in development and free from mechanical or 
other injuries. Both the upper and the under side of every leaf was 
drenched thoroughly, and except in special cases care was taken not to 
shake off the spray mixture before it dried upon the leaves. 
As the number of applications exceeded 6,000 in the orchard and plots 
and 4,000 in the greenhouse, it is evident that the greatest care was 
necessary to prevent errors in labeling, recording, etc., if the results 
were to be thoroughly reliable. This was realized from the first, and a 
complete and yet simple system of checking was adopted and scrupu¬ 
lously followed. From the nature of this system and from the fact that 
in the checking an error in the original record was very rarely found, we 
feel assured that the few erratic results that appeared were all due to 
other causes. 
In presenting these data comparisons are not made in the same table 
between tests run under different conditions, and the reader is warned 
not to make direct comparisons between injuries which resulted from 
spraying on different dates, in different places, or under other differing 
conditions, except for the purpose of studying the effects of these specific 
conditions, others being practically constant. 
DEVELOPMENT AND CHARACTER OF INJURY 
When a leaf is materially injured by arsenic, it shows visible symptoms 
in a day or two which become more and more pronounced until certain 
portions or, in severe cases, the whole leaf is dead, brown, and more or 
less shriveled. In severe cases the leaves drop in from one to four weeks 
from the time of treatment. Most commonly the first visible symptoms 
appear on the second day after treatment. At this stage there is very 
little change in color, but the surface of the leaf in the injured portion 
has lost its normal luster and becomes duller in appearance. The tissues 
under these duller areas have lost much of their turgidity and become 
more or less flabby. Very soon, perhaps the second day, a dull brown 
tinge is apparent, which at first is indefinite in outline and becomes more 
and more sharply defined. After 10 days of treatment no further 
change takes place except that the dead portion becomes frayed by the 
whipping of the leaves in the wind, or, if it is severely injured, the leaf 
drops off. No exact time can be given for this course of development, 
as it is hastened by hot, dry weather and retarded by cool, w^et weather. 
Most commonly the first visible injury may be detected on the first or 
the second day. The condition shown in Plate i, A may be seen on the 
