TRUE MAHOGANY. 19 



the actual quantity of wood laid on during one year, but that of a 

 much shorter or longer period. They may be due to drought, to 

 defoliation, or to excessive heat or cold which interrupts growth 

 activity for a short time. From this region northward the tree 

 forms more and more distinct and at the same time much narrower 

 annual rings of growth. The growth therefore is very slow, the ves- 

 sels or water carriers are small, the wood fibers also are small and 

 thick-walled, and as a result the wood is very heavy and hard. 



Mahogany wood is diffuse porous ; that is, the pores are distributed 

 more or less uniformly throughout the wood. They usually occur 

 singly, but may occur in radially disposed small groups of from two 

 to four. Most of them are filled with tylosis, usually dark brown or 

 reddish, but sometimes, as in Cuban and some Central American 

 mahogany, white or nearly so. 



The pith rays, from one to four cells wide, are the radially dis- 

 posed rows of cells (parenchyma elements) seen with the unaided 

 eye as faint lines in a smooth transverse section. They frequently 

 have a slightly wavy course and bend around the vessels. In a tan- 

 gential section they are often arranged in horizontal rows and are 

 known as storied rays. 



MINUTE CHARACTERS OF THE WOOD. 



The quality and often the source of true mahogany may be deter- 

 mined by its minute structure. The length of the wood fibers varies 

 according to the locality in which the tree grew. This variation in 

 the fiber length is shown in Table IV. Microscopically considered, 

 the wood of mahogany is made up of pores (or vessels), tracheids, 

 wood-parenchyma fibers, wood fibers, and pith-ray cells. The mi- 

 nute characters of these elements are as follows: 



Pores x in mahogany are rather large, with an average diameter of 

 0.15 millimeter, and together with accompanying parenchymatous ele- 

 ments often occupy the entire width of the wood ray; that is, the 

 portion lying between two pith rays. The small pores are located 

 near the periphery of the annual ring. These small vessels are com- 

 posed of segments which often resemble tracheids in form. These 

 segments, which can be seen in longitudinal sections or in macerated 

 material, are from one to three times as long as wide (fig. 4, A). 

 The openings in the horizontal partition walls of the larger vessels 

 are round and simple throughout (fig. 4, A, c), while the openings 

 on the smaller vessels are generally oval or elongated and placed 

 obliquely. The vessel walls are furnished with numerous lenticular 

 bordered pits (fig. 4, A, d), which occur in vertical rows where the 



1 These elements are known as pores in transverse sections, and as vessels in longi- 

 tudinal sections. 



