THE STRUCTURE OF WOOD. 



In the monocotyledons (endogens) these procambium strands 

 change completely into wood and bast, and so losing all their proto- 

 plasmic cambium, become incapable of further growth. This is why 

 palms can grow only lengthwise, or else by forming new fibers more 

 densely in the central mass. But in the conifers and broad-leaved 

 trees, the inner part of each strand becomes wood and the outer part 

 bast (bark). Between these bundles, connecting the pith in the cen- 

 ter with the cortex on the outside of the ring of bundles, are parts 

 of the original pith tissue of the stem. They are the primary pith 

 or medullary rays (Latin, medulla, pith). The number of medullary 

 rays depends upon the number of the bundles; and their form, on 

 the width of the bundles, so that they are often large and conspicu- 

 ous, as in oak, or small and indeed invisible, as in some of the coni- 

 fers. But they are present in all exogenous woods, and can readily 

 be seen with the microscope. Stretching across these pith rays from 

 the cambium layer in one procambium strand to that in the others, 

 the cambium formation extends, making a complete cylindrical sheath 

 from the bud downward over the whole stem. This is the cambium 

 sheath and is the living, growing part of the stem from which is 

 formed the wood on the inside and the rind (bark) on the outside. 



In the first year the wood 

 and the bast are formed di- 

 rectly by the growth and 

 change of the inner and outer 

 cells respectively of the pro- 

 cambium strand, and all such 

 material is called "primary;" 

 but in subsequent years all 

 wood, pith rays, and bast, 

 originate in the cambium,, and 

 these growths are called "sec- 

 ondary." 



(3) The wood of most 

 exogens is made up of two 

 parts, a lighter part called the 



sap-wood or splint-wood or alburnum, and a darker part called the 

 heart-wood or duramen, Fig. 7. Sap-wood is really immature heart- 

 wood. The difference in color between them is very marked in some 

 woods, as in lignum vitae and black walnut, and very slight in others. 



Fig-. 7. Sap-wood and 

 Heart-wood, Lignum Vitac. 



