Fig. 8.1-5. Transverse section of mature stem of grape (Vitis). Once plants become woody, they produce secondary phloem (part of the bark) to the exterior of the secondary xylem (the wood) (this is true of dicots and gymnosperms only -- monocots, with their easy-to-see phloem, do not ever become woody the way that dicots and gymnosperms do). This micrograph of secondary phloem in grape shows a rather unusual dicot phloem -- the sieve tube members are large and recognizable, and they occur in clusters surrounded by phloem fibers. At this magnification it looks as if the companion cells are also easy to see -- the small red cells, but unfortunately, those are just tannin cells (notice there are some in the xylem too -- and companion cells never occur in xylem). The real companion cells are very tiny and inconspicuous, a few are just barely visible as flat green plates against the sides of some sieve tube members.

            An important lesson from this micrograph is that dicot phloem frequently contains one or two types of cells other than sieve tube members and companion cells: here there are fibers and tannin cells.