The Trees of Texas 9 
stem, hence trees may be killed by cutting out a ring of this 
wood entirely around the stem, thus preventing the ascent of 
water. 
The medullary rays radiate from the pith to the bark, and are 
seen in cross cuts as fine lines of varying widths. In longitudinal 
euts, or when the wood is split lengthwise they are seen to be 
thin plates which differ in structure from the wood proper. The 
beautiful silver grain of quarter-sawed wood is due to these 
medullary rays. The rays transport material laterly in the stem 
and serve as a storehouse for reserve food substance principally 
in the form of starch. 
The cambium is a layer of living tissue which occurs at the 
outer boundary of the woody cylinder. It is fine and delicate 
and separates the wood from the bark. It is due to the growth 
of this tissue that the tree increases in diameter from year to 
year. The woody cylinder is made up of dead tissues, but the 
cambium is alive and active, forming new wood on the inside 
and bark on the outside. 
The bark is made up of two distinet layers, an inner and an 
outer bark. The inner bark is thin, white and fibrous, and has 
for its function the transport of plastic food substances from one 
part of the tree to another. If during the growing season the 
bark is removed just down to the wood, the tree remains alive 
for the remainder of the season, but dies the following spring. 
This is due to starvation of the roots, as there is then no way by 
which food from the leaves or stem can reach them. This prin- 
ciple is frequently made use of in the killing of willow trees an¢ 
others. 
The outer bark is made up of a layer of cork tissue which 
surrounds the stem and varies in thickness from a very thin 
papery layer on some trees to several inches in thickness on 
others. The bark develops from the inside out, and the outside 
layers are shed and fall away. The method of shedding the 
bark is quite variable among trees, but is relatively constant for 
any given species. In some species the bark is tight and smooth, 
while in others it becomes deeply furrowed or grooved and broken 
on the surface into smaller scales. The bark of the birch peels 
away in thin papery layers, the cedars become shreddy, while 
some of the hickories form large shaggy plates. An experienced 
