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Fig. 16.2-6a and b. Transverse section of grape vine bark (Vitis). The secondary phloem of grape is a little unusual in having very wide rays, well-demarcated masses of sieve tube members and companion cells and prominent bands of fibers. Because the sieve tube members form such a discrete mass, when they stop functioning and collapse, the entire mass collapses (high magnification micrograph) there are no non-conducting parenchyma cells present to retain any volume. You can see that after a few years, the accumulated secondary phloem provides good protection to the stem: the old phloem consists of dead, collapsed sieve tube members mixed with bands of dead thick-walled fibers. This would be quite a mess for any animal to chew through.