The young Solanum Stem

The potato plant (Solanum tuberosum) is one of a small group of plants (approximately 27 families) that has both  external and internal phloem associated with its vascular bundles.

In this cross section of the stem, you can see that the phloem contains discrete strands with smaller diameter companion cells and larger diameter sieve tubes.

As is the case in many of the dicotyledonous stems, the cortex is separated from the stele by a starch sheath (SS).

The potato stem is also an example of a stem that undergoes limited secondary growth. This cross-section does not show any however, as all the visible vascular tissue  is primary.

Click here for more a detailed look at the internal phloem.