10 AMERICAN FOREST TREES 



that ft small amount of other foreign material may remain after water 

 has been driven off. Florida black ironwood is rated as the heaviest in 

 the United States, and it weighs 81.14 pounds per cubic foot, oven-dry. 

 The lightest in this country is the golden fig which is a native of Florida 

 also. It weighs 16.3 pounds per cubic foot, oven-dry. When weights 

 of wood are given, the specimen is understood to be oven-dry, unless it 

 is stated to be otherwise: it is a laboratory weight, calculated from 

 small cubes of the wood. Such weights are always a little less than that 

 of the dryest wood of the same kind that can be obtained in the lumber 

 market. 



Moisture in Wood The varying weights of the same wood indi- 

 cate that moisture plays an important part. No man ever saw 

 absolutely dry wood. If heated sufficiently to drive off all the moisture, 

 the wood is reduced to charcoal and other products of destructive 

 distillation. 



The pores and other cavities in green timber are more or less filled 

 with water or sap. This may amount to one-third, one-half, or even 

 more, of the dry weight of the wood. The water is in the hollow 

 vessels and cell walls. A living tree contains about the same quantity 

 of water in winter as in summer, though the common belief is otherwise. 

 It is misleading to say that the sap is "down" in one season and "up" in 

 another, although there is more activity at certain times than in others. 

 Strictly speaking, there is a difference between the water in a tree, and 

 the tree's sap; but in common parlance they are considered identical. 

 What takes place is this: water rises from the tree's roots, through the 

 wood, carrying certain minerals in solution. Some of it reaches the 

 leaves in summer where it mixes with certain gases from the air, and is 

 converted into sap proper. Most of the surplus water, after giving up 

 the mineral substance held in solution, is evaporated through the leaves 

 into the ah- ; but the sap, starting from the leaves which act as laborator- 

 ies for its manufacture, goes down through the newly-formed (and form- 

 ing), layer of wood just beneath the bark, and is converted into wood. 

 This newly-formed wood is colorless at first. It builds up the annual 

 ring, first the springwood very rapidly, and then the summerwood 

 more slowfy. 



The force which causes water to rise through the trunk of a tree is 

 not fully understood. It is one of nature's mysteries which is yet to be 

 solved. Forces known as root pressure, capillary attraction, and 

 osmosis, are believed to be active in the process, but there seems to be 

 something additional, and no man has yet been able to explain what it is. 



The seasoning of wood is the process of getting rid of some of the 

 water. As soon as lumber is exposed to air, the water begins to escape. 



