436 Scientific Proceedings, Royal Dublin Society. 



one another in the manner described, is further responsible for the resistance 

 of mahoganies to splitting, and also secures for it comparative immunity to 

 shakes and warping. Evidently owing to it a split or shake starting from 

 cleavage or shrinkage is directed in the zone in which it originates across the 

 direction of the fibres of the adjacent zones, and so its spread is prevented. 

 In a similar way the shrinkages of the different zones tend to nullify one 

 another, and so warping is reduced to a minimum. Thus by the peculiar 

 orientation of the fibres nature has produced in mahogany a substance with 

 properties similar to a " multiply " of other woods. When a radial cleavage 

 is effected the two surfaces produced have the form of a series of interlocking 

 ridges and grooves — a fact which gives a radial split surface of mahogany a 

 most characteristic appearance. 



Seeing, then, that this peculiarity in the arrangement of the elements of 

 the wood is fundamental to several of those properties which are generally 

 regarded as characteristic of mahogany, viz., figure, play of light, texture, 

 strength, and durability, it should certainly be required of any timber which 

 claims to be mahogany, and it is surprising that hitherto it. has not been 

 utilized diagnostically. 



With regard to the microscopic structure of the woods classed as maho- 

 ganies, it will be found that the following characters are almost always 

 observable, and it will be convenient to exclude the few woods which do not 

 possess them from the mahoganies : — 



In the cross-section the vessels are scattered, occurring singly or in small 

 groups of less than ten, which are for the most part in radial series. The 

 parenchyma is in the form of tangential sheets, cells scattered through the 

 fibres or in contact with the walls of the vessels ; but the soft tissue never 

 forms thick layers of circumvasal parenchyma. The medullary rays are fine, 

 and fairly uniform in size ; i.e., they are rarely 2 mm. high, and usually con- 

 siderably under 1 mm. in height. They vary from one to nine cells across. 

 The comparative uniformity and fineness of the rays cause a fine uniform 

 undulation in the fibres threading their way among them, which is apparently 

 largely responsible for the satin-like lustre of mahogany. 



These structural characteristics would exclude the following woods 

 described subsequently : —Panama mahogany, Mimusops globosa (Bullet- 

 wood) ; Lysiloma sabicu (Sabicu) ; Pterocaipus indicus (Indian Padouk) ; 

 Chlorophora excelsa (Iroko wood) ; Pterocarpus soyauxi (African Padouk). 



We may, then, define as mahogany all red or red- brown timbers in which 

 the fibres of the adjacent layers cross each other obliquely, and so give rise 

 to a play of light and shade on longitudinal surfaces ("roe"), greatly empha- 

 sizing and enhancing the figure. This causes the wood to be difficult to 



