130 ECONOMIC WOODS OF THE UNITED STATES 



WOOD STRUCTURE 



Wood is a fibrous structure composed of cells which are for the 

 most part greatly elongated in a vertical or axial direction. Longi- 

 tudinal surfaces accordingly show the fibrous nature of wood to the 

 best advantage, while the cross section appears under the micro- 

 scope more or less like a fine honey-comb. Some wood cells are 

 large enough to be readily seen, others are at the limit of vision 

 and require a hand lens for distinctness, and a much larger number 

 are not individually visible without considerable magnification. 



All wood cells when first formed contain living protoplasm but 

 a large proportion of them apparently lose it very early. Such 

 cells provide channels for sap-flow from root to leaf, lend strength 

 and rigidity to the stem, and in some instances supply spaces for 

 storage of excess food and reservoirs for waste products. Since 

 these functions are in part physiological it seems unlikely that the 

 protoplasm has entirely disappeared from the elements concerned, 

 even if its presence cannot be directly demonstrated, since cells 

 without living protoplasm can only function mechanically. 



The wood cells which obviously retain hving protoplasm 

 throughout their functional period may be referred to as food cells 

 (parenchyma) since they are primarily concerned with the dis- 

 tribution and storage of plant food. This food is elaborated in 

 the leaves (and other green tissues) and is transported along the 

 stem chiefly through certain channels (sieve-tubes of the phloem) 

 in the inner bark. The cells (ray parenchyma) which divert por- 

 tions of the food current into the wood are typically elongated in a 

 horizontal or radial direction, while those (wood parenchyma) 

 which distribute it vertically in the stem are axially elongated. 

 Plant food assumes various forms, the principal ones being starch, 

 sugars and fats; the change from one form to another is brought 

 about by the action of certain ferments or enzymes. 



Structurally, a wood cell consists of a cell wall of ligno-cellulose, 

 inclosing a lumen or cavity (with or without visible contents), and 

 completely surrounded by a pectic layer called the middle lamella. 

 The lignified wall provides a strong and rigid framework. The 

 middle lamella limits the individual cells and cements them firmly 

 together to form the wood-mass. The cavity serves various pur- 

 poses such as the transportation of food and water, aeration, 

 storage, etc., and must accordingly be in communication with the 



