May 12,1923 Injury to Foliage by Arsenical Spray Mixtures 
531 
Greenhouse conditions have been found best for determining the answer, 
and we have made innumerable trials on tomatoes, beans, and other 
plants. When slightly or moderately wilted plants were sprayed in 
comparison with turgid ones, the resulting injury has been invariably 
practically the same. Any differences that have been noted were within 
the experimental error. The only cases in which there was a marked 
difference was when the wilting was so extreme and so long continued 
that the leaves were practically dying. 
EFFECTS OF TEMPERATURE 
A factor that might be supposed to affect arsenical injury is the 
temperature of the leaves and the surrounding air. In an effort to 
determine the effect of temperature upon the extent of injury to plants 
sprayed with arsenicals several series were run in a differential ^ermostat. 
This consisted of a series of chambers in which were maintained tem¬ 
peratures varying from 5° to 40° C. Light was admitted through glass 
at the top. Sprayed tomato and cucumber plants were placed in this 
thermostat for two days and then transferred to an open bench in the 
greenhouse. 
The arsenicals used were diplumbic lead arsenate and Paris green. 
It was found that between 5® and 15° C. there w^as little difference in 
the injury. From 15° to 25® the injury materially increased. Above 
25® there was apparently a marked increase in the injury, but the 
unsprayed controls could not be kept in a healthy condition at these 
temperatures. Hence the records on the arsenical injury to sprayed 
plants at corresponding temperatures were of doubtful value. 
These experiments show that an elevation of temperatime increases 
the injury, but we wish to emphasize that under field and orchard con¬ 
ditions temperature is of minor importance as compared with humidity. 
EFFECTS OF WOUNDING 
In connection with studies made on the effects of arsenical compounds 
on the bark of fruit trees, we have shown (29, 30) that the all-important 
factor in determining whether or not injury will occur is the integrity of 
the outer corky layers of the bark, i. e., whether or not this has natural 
or artificial openings. Are wounds important in determining the 
extent of injury through the foliage ? We have often noticed that 
there is an excessive injury at the margins of the wounds made by hail 
stones, whipping by the wind, and other mechanical agencies. In some 
instances we have purposely made such wounds before spraying. In 
some of these cases the injury was strictly confined to the margins of 
such wounds, but this was only where the arsenical treatment was mild. 
In some instances where the injury was confined to a few scattered spots 
there was evidence of a tiny puncture in each spot, revealed only by the 
aid of a lens. Wherever the arsenical treatment was so severe as to 
injure the foliage badly, there was every evidence of absorption directly 
tlmough the unbroken epidermis. The injury at the margins of wounds 
is strictly local, usually making a brown strip about one-eighth of an 
inch wide and, except where such wounds were unusually extensive, this 
injury is of little practical importance. 
“ Both Baker’s. 
