MORPHOLOGY OF WOOD 239 



which for the plant serve a double purpose ; one to act as a storehouse 

 for carbohydrates, oils, and the like, and the other to act as conduction 

 channels from the phloem toward the center of the tree. These bands 

 are mechanically important and will receive later consideration. 



In the dicotyledons the division of labor has progressed beyond this 

 more simple stage, and large specialized cells, known as vessels, have 

 been developed for conduction. For strength and support, the more 

 individual fibers have taken the place of the coniferous tracheids. The 

 other structural features have close analogies in the soft woods and 

 need not be mentioned. The vessels and fibers vitally affect the me- 

 chanics of timber and consequently bear a very intimate relation to 

 brashness. 



As these anatomical features are closely associated with brashness, 

 careful measurements of the following type were made in order to 

 clarify the affinity as between structure and weakness : 



1. The specific gravity bases on oven-dry volume was determined 

 in the usual way, by immersion. The purpose of such a test was to 

 serve as a check upon anatomical variations as well as to ascertain the 

 scientific grounds for its high regard among inspectors, 



2. The number of rings per inch or per centimeter was learned by 

 a direct count over as wide an area as the material at hand permitted. 



3. The percentage of summerwood was calculated by micrometer 

 measurements on prepared sections under low magnification. 



4. The percentage of ray tissue was calculated by methods developed 

 in this laboratory. The process by which these computations were 

 made will appear in another publication. 



5. The volumetric proportion of the fibrous tissues in the tulip 

 poplar was ascertained in the same way. 



6. The thickness of the fiber wall or tracheid wall represents an 

 average of several careful measurements on many cells under high 

 magnification. This test proved to be one of the most important made 

 since it determined the amount of woody substance which of course 

 is the ultimate proof of strength or weakness in wood. 



7. The diameter of the vertical elements was also computed by meas- 

 urements under high magnification and the technique by which this 

 was done may be used to compute the number of fibers per unit volume 

 after deductions for the ray and vessel content have been made. 



8. The length of the fibers and tracheids was determined by measur- 

 ing about sixty cells in each case. 



