WHERE AND WHAT IS SAP? 29 



III —MOISTURE IN WOOD. 



Water may occur in wood in three conditions: (1) It forms the 

 greater part (over 90 per cent) of the protoplasmic contents of the 

 living cells; (2) it saturates the walls of all cells, ami (3) it entirely or 

 at least partly fills the cavities of the lifeless cells, fibers, and vessels. 

 Id the sapwood of pine it occurs in all three forms; in the heartwood 

 only in the second form, it merely saturates the walls. Of 100 pounds 

 of water associated with 100 pounds of dry wood substance in 200 

 pounds of fresh sapwood of white pine, about 35 pounds are needed to 

 saturate the cell walls, less than 5 pounds are contained in living cells, 

 and the remaining 00 pounds partly fill the cavities of the wood fibers. 

 This latter forms the sap as ordinarily understood. It is water brought 

 from the soil, containing small quantities of mineral salts, and in cer- 

 tain species (maple, birch, etc.) it also contains at certain times a small 

 percentage of sugar and other organic matter. These organic sub- 

 stances are the dissolved reserve food, stored during winter in the pith 

 rays, etc., of the wood and bark; generally but a mere trace of them is 

 to be found. From this it appears that the solids contained in the sap, 

 such as albumen, gum, sugar, etc., can not exercise the influence on 

 the strength of the wood which is so commonly claimed for them. 



The wood next to the bark contains the most water. In the species 

 which do not form heartwood the decrease toward the pith is gradual, 

 but where this is formed, the change from a more moist to a drier con- 

 dition is usually quite abrupt at the sapwood limit. In longleaf pine, 

 the wood of the outer 1 inch of a disk may contain 50 per cent of water, 

 that of the next, or second inch, only 35 per cent, and that of the heart- 

 wood only 20 per cent. In such a tree the amount of water in any one 

 section varies with the amount of sapwood, and is therefore greater 

 for the upper than the lower cuts, greater for limbs than stems, and 

 greatest of all in the roots. 



Different trees, even of the same kind and from the same place, differ as 

 to the amount of water they contain. A thrifty tree contains more water 

 than a stunted one, and a young tree more than an old one, while the wood 

 of all trees varies in its moisture relations with the season of the year. 



Contrary to the general belief a tree contains about as much water 

 in winter as in summer. The fact that the bark peels easily in the 

 spring depends on the presence of incomplete, soft tissue found between 

 wood and bark during this season and has little to do with the total 

 amount of water contained in the wood of the stem. 



Even in the living tree a flow of sap from a cut occurs only in cer- 

 tain kinds of trees and under special circumstances; from boards, tim- 

 ber, etc., the water does not flow out, as is sometimes believed, but must 

 be evaporated. 1 



'The seeming exceptions to this rule are mostly referable to two causes, namely: 

 (a) Clefts or "shakes" will allow water contained in them to flow out. (b) From 

 sound wood, if very sappy, water is forced out whenever the wood is warmed, just 

 as water flows from green wood in the stove. 



