290 
Journal of Agricultural Research 
Vol. VIII, No. 8, 
OPENINGS THROUGH THE CORK LAYER 
Even on the smooth bark, where the layer of cork is for the most part 
fairly even, the protection offered is not perfect. At the time the peri¬ 
derm is forming, the lenticels are forming also. On apple bark these 
are quite numerous and of varying sizes (PI. 82, A , B). While they 
have not been studied with such thoroughness as those on some other 
woody plants, it is well established that they have numerous, though 
rather small, intercellular spaces and permit free access of air for sev¬ 
eral years. As the stem grows in diameter, they enlarge very much 
and on some varieties they divide into groups. From their appearance 
under the microscope (PI. 83, A, B) we would judge that some of them 
at least remain functional for several years, sometimes until nearly the 
time when the bark becomes furrowed and roughened. While the len¬ 
ticels are known to admit air, the extent to which they will admit liquids 
can be determined only by experiment. On the smooth bark the len¬ 
ticels are the only natural openings after the disappearance of the sto¬ 
mata, but artificial openings can easily be made and do frequently occur 
as a result of hail, insects, and various orcharding operations. The 
rough bark, while less easily broken by external agencies, is frequently 
cracked by the growth within, as described above. The roots also, 
after reaching a diameter of % to % inch, have lenticels in varying 
sizes and numbers, which for the purposes of this investigation are not 
essentially different from those on the aerial portions (PI. 82, C). 
ARSENICAL INJURY THROUGH THE SMOOTH BARK 
When arsenical compounds of almost any kind are applied to the 
branches by the methods described above, if the quantity is sufficient, 
more or less injury follows. In many cases the natural protection of 
the cork and the penetrability of the lenticels and wounds are clearly 
shown. 
CHARACTER OR INJURY 
Roughly speaking, we may say that the various compounds of arsenic 
produce the same symptoms of injury, varying only in degree.—that 
is, a given amount of injury by one compound is very much like the same 
amount produced by another. Calcium-arsenite injury in differing 
severity will therefore be described; and let it suffice for a general descrip¬ 
tion of arsenical injury through the smooth bark, keeping in mind only 
that with us some of the milder compounds like ortho zinc arsenite 
and iron arsenate have never produced the severest injury. 
When the bark is unwounded and the injury is only slight, it is not 
visible to the eye for several days, usually a week or more, depending 
somewhat upon the temperature, and perhaps other conditions. Then 
it may be noticed that close around certain lenticels is a small indefinite 
zone which is slightly darker in color on the surface than the surround- 
