May 12.1923 Injury to Foliage by Arsenical Spray Mixtures 
515 
STEMS, FRUIT, AND FOLIAGE 
On all kinds of plants sprayed in this investigation the leaves were the 
most susceptible portion. Most of the herbaceous plants bore no fruit 
prior to the time of spraying, but beans, cucumbers, and squashes were, 
in a few cases, exceptions. On apple trees the leaves proved much more 
tender than either stems or fruits. This is true to such a marked degree 
that any arsenical treatment that caused even the slightest direct damage 
to fruit or bark nearly defoliated the sprayed branch. So far as this 
fruit is concerned, therefore, and probably in general, arsenical spray 
injury is a problem of foliage injury almost exclusively. The apple fruit 
proved slightly more susceptible to very severe treatment than the most 
tender portion of the bark on the stem. 
We have frequently noted that with plants of all kinds, whether 
herbaceous or woody, the youngest portion of the stem is the most easily 
injured. With leaves, however, this is not the case. Old leaves nearly 
ready to fall are damaged most, while young ones just expanding are 
most resistant. 
On the apple the foliage injury takes place almost exclusively through 
the lower epidermis. Gillette (10) sprayed the upper surface and then 
both surfaces by way of comparison and found that— 
wherever this was done, the damage sustained by the leaves that were wet on both 
sides was fully double that sustained by those wet only on the upper siuface. 
Woodworth (J5) tried painting the leaves with white arsenic, Paris 
green, and I^ondon purple, some on one surface, some on the other, and 
some on both. The entire surface was coated. He concluded that the 
lower side is more susceptible than the upper, the difference in his experi¬ 
ments being in the proportion of 47 to 22. 
In our own work the arsenical was applied individually to many leaves 
with a very soft brush, some to the upper side and others to the lower 
side. It was found that if the painted area extended to the margin it 
was practically impossible to keep a little of the liquid from extending^ 
over onto the opposite surface from that intended. Therefore, after a 
few preliminary experiments the practice was followed by painting a 
strip about K hy i X inches down one side of the midrib. This strip 
never approached the leaf margin. In Table V each report represents 
the injury of 10 painted leaves on one shoot. Both sodium arsenite and 
calcium arsenite were used in this experiment, one being a soluble and 
the other a nearly insoluble compound. Soap was added in some cases 
to cause the liquid to spread better and to penetrate the pubescence on 
the leaf more readily. 
The results shown in Table V are quite typical and indicate that the 
arsenical injury to apple foliage from spraying is brought about largely 
by absorption from the lower surface. Indeed, in some of our experi¬ 
ments when very toxic solutions were used no trace of injury occurred 
on leaves treated on the upper surface. We are led to believe that in 
those cases where injury did occur after treatment of the upper surface, 
there was some tiny abrasion, not easily visible to the naked eye. In 
view of our results it is difficult to account for so much injury from 
treating the upper surface as reported by Woodworth {33), The only 
essential difference in method was that he painted the whole surface 
instead of the central portion only. When we did the same we could 
not readily avoid getting some of the liquid over the margin onto the 
