Tin Trees of Texas 9 



stem, hence trees may he killed by cutting out a ring o f 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 

 cuts, 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 distinct 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 and 

 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 



