Dixon — Mahogany, Recognition of some Different Kinds. 439 



The number of cells forming the thickness of the medullary rays is often a 

 very useful diagnostic. For example, I have not found that rays 7 cells thick 

 occur in any West Indian or American mahogany, while such rays are 

 not uncommon in the Khayas of Africa. But, on the other hand, consider- 

 able variation may be found in the number of cells forming the thickness of 

 the rays in Cuban mahogany. Immediately round the pith the rays are seldom 

 more than 3 cells thick (3-ply); at some little distance from it, they are usually 

 4- or 5-ply. Some Cuban mahogany retains the embryonic condition much 

 later than others (PI. XXII, figs. 2 and 5). Also where the grain is locally 

 disturbed by the proximity of branches abnormally thick rays may be found. 

 The presence and absence of structural features are, perhaps, the most 

 satisfactory diagnostic characters. 



Foremost amongst these is the presence or absence of tangential laminae 

 of parenchyma (soft tissue) in the wood. The arrangement of this tissue calls 

 for a brief special notice. 



The parenchyma of the wood may be divided into three different categories 

 according to its distribution, viz.: — (1) Scattered among the fibres and tracheids, 

 as may be well seen in some of the soft and light cedars and mahoganies, 

 e.g. in Ceelrela odorata (Havana cedar) (PI. XXIIL, fig. 10) and Cape Lopez 

 mahogany (PI. XXXVIL, fig. 91). (2) Circumvasal surrounding the vessels 

 and in contact with them (PL XXXVL, fig. 88, PL XLIL, fig. 124, PL XLI1L, 

 fig. 130). (3) Laminar, forming thin sheets extending in a tangential, or 

 irregularly tangential, manner between the other wood elements. These 

 parenchymatous laminae are characteristic of the Swietenias and other West 

 Indian and some African mahoganies (PL XXII., fig. 1, PL XXV., fig. 22, 

 PL XXX., fig. 49, PL XXXIIL, fig. 67, PI. XXXIV., fig. 73). 



Where the parenchyma is distributed circumvasally it forms a thin layer, 

 mostly one cell thick and often interrupted, coating the vessels, and in cross- 

 sections is usually quite inconspicuous. In some cases at places round the 

 vessels it may be a few cells thick, and extend out among the surrounding 

 tissues in a tangential direction. But in none of the true mahoganies 

 (Swietenias) or those woods generally recognized as mahoganies does it form 

 a thick coating to the vessels, and a smooth cross-cut of the wood is readily 

 distinguished by lens examination from a section of those in which a thick 

 coating of circumvasal parenchyma marks out the vessels among the fibres with 

 a light and mealy-looking margin of soft tissue, giving the section a very 

 different appearance. For this reason it is advisable not to reckon such 

 timbers as Iroko wood {Chlorophora excelsa) and Locust (Hymenoca cour- 

 baril L.) &c, as mahoganies. 



The laminar parenchyma exhibits a great deal of variation in its develop- 

 ment in the mahoganies. Sometimes the laminae are thin (1 or 2 cells 



SCIENT. PROC. E.D.S., VOL. XV., NO. XXXIV. 4 A 



