X)ixon—]\1 alio gun i/ . Recognition oj some Different Kinds. 441 



in the outer part of the growth-zone, and are more widely spaced towards its 

 inner boundary. This arrangement produces a strange reversal of what is 

 usually found in an annual ring, where the denser tissue forms the outer part 

 of each ring. Here the inner region of the ring is denser, and each growth- 

 zone (year-ring) becomes less dense on the outside owing to the greater 

 admixture of parenchyma in that region. Entandrophragma spp. furnish 

 examples of this phenomenon. Sometimes the laminae have no great exten- 

 sion in a tangential direction, and form vertical straps of tissue, more or less 

 narrow, instead of sheets of tissue (PI. XL., fig. 112). 



The contents of the vessels may also be used for purposes of identification. 

 Most mahoganies have a dark crimson or black substance more or less com- 

 pletely filling a number of the vessels ; others, such as Cuban, " Spanish," and 

 St. Domingo, often have a large number of the vessels filled with a white 

 substance ; in many others a few vessels contain a similar substance. This 

 appears to be an amorphous deposit of calcium carbonate, as it is soluble in 

 acetic acid with an evolution of gas. In Cuban mahogany, especially near the 

 pith, some of the vessels contain a translucent substance, tinged yellowish- 

 green. Then again the vessels of some mahoganies contain tyloses, e.g. 

 Cape Lopez (PI. XXXVIL, fig. 91); Colombian; Guatemalan (PL. XXVIIL, 

 fig. 40); Turiballi (PI. XXVII., fig. 36); "Cedrana" (PI. XXIX., fig. 43); 

 Panama mahogany (PI. XXIX., fig. 46) ; Gaboon (PI. XXXV., fig. 79), &c, 

 while the greater number are free from them. It should be noted that some 

 writers (10) call all deposits in the vessels tyloses, but it is better to retain 

 the name for the cellular ingrowths from the wood parenchyma cells into the 

 vessels, as it was originally used. 



The cells of the rays and of the circumvasal parenchyma may be empty 

 or tilled with contents. In the heart-wood this may be a valuable diagnostic, 

 but in the sap-wood this feature would evidently largely depend on the 

 conditions of metabolism in the tree, and should not be used in diagnosis. 

 Further the presence or absence of crystals in the marginal cells of the rays 

 seems a feature variable in the same wood, and does not consequently appear 

 suitable for distinguishing different woods. In a few mahoganies linear series 

 of crystals are found in septate fibres, and this feature seems to form a 

 constant characteristic of some woods, e.g. Colombian mahogany. 



The degree of uniformity in size of the rays seen in tangential section is 



often of great value in distinguishing various kinds of mahoganies. Generally 



speaking, there is no great disparity in the size of the rays of mahogany, such 



as there is, for example, in Oaks and Beeches. Turiballi may be mentioned 



as an exception to this general rule. 1 1 has occasional giant rays scattered 



widely among the smaller ones which form the vast majority. In this case 



the giant rays may be - 75 mm. high and 0"20 mm. wide, while the average 



4 a2 



