Fig. 16.2-1a and b. Transverse section of linden (Tilia). The low power micrograph gives a general orientation, with secondary xylem at the bottom, phloem at the top, and the vascular cambium running horizontally across the center (indicated by arrows). This secondary phloem is more complex than that of pine. The bands that are stained dark red consist of secondary phloem fibers, seen more clearly in the high power view. Alternating with the bands of fibers are bands consisting predominantly of sieve tube members and companion cells. It is tempting to assume that these phloem bands correspond to the annual rings in xylem, but typically they are not annual: several sets can be produced per year.

            Two rays are visible in the phloem, and are a bit unusual in being narrower near the cambium (where they are young) and wider away from the cambium (where they are older; remember that phloem is pushed outward by formation of new phloem interior to pre-existing phloem). The width of the rays results because ray cells themselves divide (a process called dilatation and illustrated in micrographs below).