NOTES 841 



jnarkable strength comparatively and great elasticity. The cells, which 

 are parallel to the axis of the tree, are made up principally of woody 

 fiber; those which extend in a radial direction usually have a cellulose 

 structure with little woody fiber, and are defined as "medullary rays," 

 ■or pith cells. The annual concentric rings show very feebly. The dif- 

 ference between balsa wood and cork lies in the circumstance that the 

 first has interlacing fibers around the cellular structure ; the latter has 

 none, but the place of the fibers is taken by a resinous deposit which 

 ^ives it no structural strength. This resinous deposit can be collected 

 by means of pressure and heat to make what is known as cork board, 

 which is put on the market in blocks 2 or 3 inches thick and from 

 2 to 6 feet long, convenient for use as a lining in cold storage or other 

 structures. 



Tests show compressive strength of balsa wood to average 2,225 

 pounds per square inch, about one-half the strength of white pine or 

 spruce ; also "the modulus of rupture to be approximately one-half that 

 of good spruce. The uniformity of tests clearly shows that the ma- 

 terial may be relied upon both for direct compression and for trans- 

 verse loads." Its elasticity is demonstrated by the fact that beams 

 bent almost to the breaking point have resumed their original shape 

 upon release. It is practically impossible to split the wood by driving 

 nails into it. 



Heat transmission through balsa wood, though not much higher than 

 for cork, is considerably lower than through white pine, and very con- 

 siderably lower than through zinc. 



Balsa wood is a rapid grower, attaining a diameter of 12 to 14 inches 

 at an age of 4 to 5 years, when it may be from 40 to 60 feet high. The 

 leaves vary in length from 14 to 30 inches. Its seed pods yield a woolly 

 fiber suitable for pillows and mattresses. The tree is found in virgin 

 forest only as an isolated tree in clearings, it being most commonly 

 found as a second-growth tree. 



The Michigan Agricultural College Forester, the annual of the For- 

 estry Club for 1918. contains an interesting comparison bv Professor 

 Chittenden of the financial results of two woodlots belonging to the 

 college, the one being managed for fuel, the other for maple svrup. 



The fuel lot of 74 acres, with a growing stock of 2,369 cubic feet, or 

 26 standard cords per average acre, produces at the rate of nearly 2 

 cords per acre (178.5 cubic feet), an unusually large production. The 

 selling price being $12 for the standard cord, or $4 for the 16-inch cord 



