Peb. 19,1917 Arsenical Injury through the Bark of Fruit Trees 289 
coating on all parts except the leaves and the youngest stems and roots. 
This layer of cork is well formed on the young shoots when they go into 
their first winter and persists thereafter. It forms a layer that is smooth 
and for the most part fairly even in thickness until the limbs and trunks 
reach a considerable size. As the limb enlarges inside this layer, new 
cork cells are added from within. The older cork cells on the outside 
slough off to some extent, but, as a rule, no definite breaks are found 
until the natural formation of rough bark begins. This takes place when 
the trunk or the limb is from 2 to 6 inches in diameter, depending upon 
variety and environmental conditions. 
The well-known roughened condition of the bark is brought about 
by rifts or splits forming in the outer corky layer and the tissues lying 
just beneath it (PI. 81, A). These often begin at the intersection of 
branches, in crotches, atid about old wounds and other irregularities, 
and gradually extend over the entire trunk and the larger limbs (PI. 
81, B ). While these cracks form more or less gradually and cork is 
produced later at their inner extremities, there is often a period when 
the living cells of the bark, are wholly unprotected in places by the cork 
cells. Thus, it happens that while it might seem at first thought that 
the heavy, rough bark of trunks and crowns is a thicker and better 
protection than is found upon the smaller branches, such in reality is 
not the case; for while the cork of this rough bark is in some places very 
thick, in others it is quite broken through. Table I records a series of 
observations made in the late fall of 1910 upon the thickness of the 
cork on branches of different ages. Further examinations made in 
July, 1915, indicate that there is very little difference between the dor¬ 
mant and active periods. 
Table I .—Number of cells in periderm of smooth bark and in the furrows of rough bark 
of apple trees 
Year's 
growth. 
Character 
of bark. 
Diam¬ 
eter. 
Cells in 
cork 
layer. 
Character 
of bark. 
1 
Diam¬ 
eter. 
Cells in 
cork 
layer. 
Character 
of bark. 
Diam¬ 
eter. 
Cells in 
cork 
layer. 
Inches . 
Inches . 
Inches. 
1910. 
Smooth^.. 
% 
3 “S 
Smooths. 
tV 
1-3 
Smooth*.. 
'h 
- 3-5 
1909. 
.. .do. 
% 
3-5 
.. .do. 
* 
4-6 
.. .do. 
A 
2-5 
1908. 
.. .do. 
tV 
3-5 
...do. 
X 
A 
.. .do. 
X 
1907. 
.. .do. 
X 
4-6 
...do. 
O / 
3-7 
...do. 
X 
* 3 
3-7 
1906. 
.. .do. 
H 
4-8 
.. .do. 
Yt. 
4—7 
.. .do..... 
X 
1905. 
.. .do. 
X 
4-9 
.. .do. 
X 
3-7 
.. .do. 
X 
* 9 
4-8 
1904. 
.. .do. 
X 
4-8 
.. .do. 
X 
2-8 
.. .do. 
X 
3*7 
1903. 
;. .do. 
iX 
5-7 
1902. 
Smooth^.. 
X 
3-8 
.. .do. 
iX 
1-8 
1900. 
iX 
4-10 
1899. 
.. .do. 
1% 
4—9 
1896. 
4—9 
.. .do. 
4 
4-many 
Smooth d.. 
4 
4-many 
Smooth*.. 
A 
3—many 
Rough &... 
6 
2-many 
Roughs.. 
6 
4-many 
Rough/...! 
•r 
6 
6-many 
.. do. 
8 
5-many 
... doff. 
8 
3^many 
12 
i-many 
-«- 
a Okabena. c Hibernal, *Wealthy. Q Orange. 
& Transcendent crab, d Liebe. / Oldenburg. 
