4 BULLETIN 606, U. S. DEPARTMENT OF AGBICULTUEE. 



willow they are narrow as compared with those in sycamore, beech, 

 and oak. 



Wood prosenchyma (Wood fibers, etc.) — Wood prosenchyma, (in- 

 dicated by " X " in the plates, consists of thick-walled elongated cells 

 in the form of fibers or of spindle-shaped cells with pointed, inter- 

 locking ends. These specialized cells give the mechanical strength re- 

 quired in the tree. In the softwoods the fibers, or tracheids, serve 

 both for the conduction of liquids and for mechanical support. In 

 the hardwoods, however, these two functions are separated. The 

 pores or vessels act as a specialized conducting tissue, and the wood 

 prosenchyma, though it shares in sap conduction to a small degree, 

 acts chiefly as mechanical support. 



METHODS USED IN THE EXPERIMENTS. 



The presence of large vessels, the extent to which these are blocked 

 by tyloses and gums, and the presence of other infiltrating substances 

 in the cell'walls are known to be important factors in the impregna- 

 tions of hardwoods with creosote. A very careful study was, there- 

 fore, made of these factors. 



The resistance of the wood to treatment was studied in two kinds 

 of tests: (1) Applying the creosote to a small area on a specimen and 

 measuring the penetration in different directions, and (2) impreg- 

 nation in a treating cylinder, blocks from each of the species being 

 treated together in the same run. After treatment the specimens were 

 examined under the microscope to determine the influence of the wood 

 structure on penetration. 



APPARATUS. 



The penetrance apparatus in which the first series of tests was made 

 is illustrated in figure 1. A hole three-fourths of an inch deep was 

 bored in one face of the test specimen and the piece then clamped 

 with the hole directly over the open end of pipe A, which led to the 

 bottom of pressure tank B. The pipe and the lower portion of the 

 tank were filled with creosote. Pressure was obtained b,y means of 

 compressed air admitted through pipe C. A pressure regulator, K, 

 was used to maintain a uniform pressure during the test. The ap- 

 paratus was inclosed in a wooden oven having double glass windows 

 in the front and back. Steam coils, J, heated the specimens and 

 preservative to a constant temperature, which duplicated as nearly as 

 possible the temperature conditions of a treating cylinder. Pres- 

 sure and temperature were measured by gauge G and thermometer 

 H. The specimens were placed on shelves in the apparatus and 

 heated to a uniform temperature before testing. By the use of the 



