15() JOURNAL <H' FOKKSTR\- 



he gives the gross structure and some of the physical properties, and 

 then the miscroscopic structure of the cross section, tangential section^ 

 and radial section. 



Particular attention is called to a microscopic feature which is used 

 freely for distinguishing species or groups of woods from each other, 

 hut which has been found very unsafe to use in other woods, namely, 

 the height of the rays and the number per unit area. The size and 

 number of rays at any particular point in a tree trung Varies greatly 

 with the rate of growth and nourishment the tree received at that point, 

 and it is only after an exhaustive study of the height and distribution 

 of rays in different parts of the tree trunk and in trees growing under 

 different conditions that this feature can safely be used in identifying 

 species. This applies especially in distinguishing between two such 

 closely related species as Swietenia mahagoni and S. macrophylla 

 which he separated on this basis in the key. Whether the rays are 

 one, or two, or many cells wide is a more reliable diagnostic feature, 

 and is made use of in several places in the key. The very fine pitting 

 in the vessel walls of true mahogany and closely related species is also 

 a very distinctive characteristic and helps to distinguish these woods 

 from other families, but was not made use of in the key or mentioned 

 in the description. 



This paper together with Record's key form a good basis for fur- 

 ther investigations on a larger amount of authentic material than is 

 now available. 



The author makes the following explanation : "As a considerable 

 time must elapse before anything like a complete collection of mahoga- 

 nies can be examined, and as there appears to be an immediate need for 

 some method of identifying the more common kinds, I have put 

 together descriptions and illustrations of the structure of the mahog- 

 anies which up to the present have been available to me, in the hope 

 that they may be useful as a means of identification, and that they_ 

 may serve as a beginning of an examination of a more extensive list 

 of woods." 



Twenty references and 138 excellent photomicrographs showing 

 cross, radial, and tangential views are included. 



