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43 Chapter 4 The Life and Growth of a Conifer Wood Conifers and broadleafs differ greatly in their internal organs, most noticeably their wood. In all trees, the trunk and branches contain tubes to carry liquids up and down the tree, from roots to leaves and back. The tubes leading upward, plus (in some trees) strengthening fibers, constitute the wood, technically, the xylem. The tubes leading downward , and carrying dissolved sugars from the leaves to the rest of the tree, constitute the phloem. Unlike xylem, there is no popular name for phloem. It will become clear later why xylem (that is, wood) and phloem, in spite of their being equally necessary for the growth of a tree, should rank so differently in public awareness. Everyone is familiar with wood, but only botanists and naturalists have ever heard of phloem. The primary reason for the contrast is that wood has commercial value and phloem has none. Consider the wood first. The structure of wood differs greatly between conifers and broadleafs. Wood contains the tubes that carry water and dissolved nutrients (sap) up the tree. In conifer wood there are no continuous “tubes”; there are merely chains of long, narrow cells, known as tracheids , linked to each other through small, porous membranes. In contrast, broadleaf wood contains continuous tubes known as vessels to carry the sap. A vessel consists of a chain of cylindrical cells linked end to end; when first formed, these cells have thin end walls that form partitions, but the 44 the world of northern evergreens end walls soon dissolve, leaving the vessels unobstructed, so sap can flow easily through them. Water travels upward in the tracheids only in daylight. Notwithstanding popular belief, the flow is not by capillary action, which is the way water rises a short way up a narrow glass tube, or in an absorbent sponge. It isn’t nearly strong enough to lift water right to the top of a tree. What happens is this: The cells in a germinating seed contain water that has been sucked a very short distance up, by capillary action, through the minute root hairs. After that, the water continues to flow upward through each of the growing, lengthening cells (tracheids) as an unbroken column of sap in a closed pipe. Each sap column is kept intact as the tree grows, by intermolecular attraction. This accounts for both “transpiration pull” and “cohesion,” two terms often used without definition. They mean what they say, and the scientific explanations are in the realm of molecular physics. Breakage of a column (cavitation) is rare—much rarer in conifers than in broadleafs because tracheids are narrower than vessels. Also, the enormous number of sap columns in a tree trunk ensures that breakage of a few of them does no harm. Figure 4.1 shows the contrast between tracheids and vessels. Most tracheids have diameters of less than one-twentieth of a millimeter while those of vessels are sometimes 20 times as great. Besides being exceedingly narrow, a tracheid is tapered at both ends. In a chain of connected tracheids, each one overlaps those above and below it so that the tapered ends are pressed against each other. Thus every chain of tracheids touches the chains on each side of it, and the side-by-side tracheids are linked to each other through small porous membranes through which sap can flow. The tracheids of conifers are far less efficient than the vessels in broadleafs . The rate of flow of sap in conifers is about half a meter (1.5 ft) per hour on average, a snail’s pace compared with that in some oaks in which it can be twenty times as great. Also, tracheids are less specialized than vessels in that they perform two distinct functions. Not only do they carry the sap, but also they give wood its mechanical strength. In the more highly evolved broadleafs, there is a division of labor among the cells in the wood. The two functions, sap conduction and mechanical support, are performed by two different kinds of cells: The vessels serve only as sap conductors; they haven’t the strength and rigidity to support a the life and growth of a conifer 45 tree. Strength is provided by wood fibers—long, narrow, thick-walled cells whose only function is to provide mechanical support. The importance of tracheids to coniferous trees should now be obvious. Coniferous wood consists almost entirely of tracheids. They are popularly known as “wood...


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