38 HOUGH'S AMERICAN WOODS. 



and few oblique primary reticulating veins uniting near the margin ; petioles 

 about l /4 in. long, enlarged at base. Flowers, appearing at all seasons of the 

 year, with bracts about as long as the lobes of the calyx. Fruit as described for 

 the genus, i-i l / 2 in. long. 



The Black Mangrove occasionally attains the height of 70 or 80 ft. 

 (25m.), with wide rounded top and a trunk rarely if ever mo-re than 

 2 ft. (o.6om.) in diameter. This is vested in a dark brown bark fissured 

 into very low flat ridges and these by narrow cross fissures, causing a 

 characteristic chequered appearance. It is often no more than a wide- 

 branched, bushy shrub or very low tree. 



It sends up from its horizontal roots numerous small knees, or 

 aerating roots as they are sometimes called, which bristle from the 

 mud beneath the trees somewhat like asparagus shoots. Whatever 

 may be the chief function of these curious growths they retain much 

 trash which might otherwise float off with the tides and they really 

 aid to make new land. 



PHYSICAL PROPERTIES. Wood very heavy, hard, strong and tough 

 arid of a rich dark brown color with ample creamy white sap-wood, 

 the latter usually soon assuming a pronounced bluish brown tint. 



The structure of this wood is very different from that of all the 

 woods with which we are familiar, indeed even being at variance with 

 the generally accepted ideas of wood structure and growth. It has no 

 medullary rays, at least which extend through more than a season's, 

 growth ; nor does it add its annual increment of new wood in unbroken 

 layers rings as seen in cross-sections. The new wood forms in strips, 

 and these are braided together, as it were, in a sort of basket work of 

 new growth each year around the older wood and beneath the bark. 

 This basket-work growth gives to the wood the radial or lateral 

 strength which is ordinarily given mainly ithe medullary rays. The 

 result is a wood which is practically non-splitable radially. 



\Ve have found it impossible to make transverse sections of this 

 wood, of the usual thickness adopted in our work, and keep them from 

 separating between ithe annual layers, though we have succeeded fairly 

 well with thinner sections. These are so fragile, however, that we are 

 obliged ,to protect them with celluloid or mica. It. has been impossible, 

 too, to make as perfect radial and -tangential sections as we would like 

 and some roughness and checks have to be tolerated. 



The government tests of physical properties are as follows : 

 Specific Gravity, 0.9138; Percentage of Ash, 2.51; Relative Approxi- 

 mate Fuel Value, 0.8909; Weight of a Cubic Foot in Pounds, 56.95. 



