Fig. 15.3-9a and b. Transverse section of oak wood (Quercus). Oak wood is a complex dicot wood because its axial system has fibers, parenchyma, and several sizes of vessels. As is typical, the micrograph is oriented as if the vascular cambium were above the top of the computer screen, so the earlywood in the upper part of the picture is part of the annual ring that was produced the year after the latewood of the lower part. The earlywood vessels have a diameter six or seven times greater than the diameter of the narrow vessels of the latewood, and remember that a vessel’s conducting capacity is proportional to its radius taken to the fourth power (r⁴)(see page 113 in Plant Anatomy (Mauseth)): the conducting capacity of the large vessels is hundreds of times greater than that of the narrow ones, not just six or seven times greater.

            Axial parenchyma is visible in the low magnification image as many small white dots in the latewood, but they are easier to see in the high magnification image (small, vertical arrows) (there may also be some in the earlywood, but it would be very difficult to distinguish them because the earlywood fibers have such thin secondary walls). This wood does not contain any protoplasm: it might have been taken from the heartwood area or it might have come from a piece of lumber. When axial parenchyma has preserved, stained protoplasm, it is much easier to identify.