Pisum sativum, the garden pea
The pea stem vascular bundle shown here, has
undergone limited secondary growth. The primary phloem is capped by
primary phloem fibres (PF) which are still relatively thin-walled.
Beneath the fibres, you can see a broad band of phloem tissue, with some
of the smaller protophloem elements visible, between the larger
parenchymatous elements near to the fibres.. metaphloem sieve tubes
(ST) contain conspicuous protein aggregates and occupy 5-6 rows of
cells, immediately exarch to the cambial zone (CZ). The
cambial zone has produced a few, as yet, unlignified secondary xylem
elements but little evidence of secondary phloem
differentiation can be seen in this photomicrograph.
The primary xylem consists of broader diameter metaxylem vessels and narrower tracheary elements including lignified parenchymatous elements. Internal to this are several rows of narrow protoxylem vessels (PX).
The pea plant, like many other herbaceous plants, wastes little energy in the production of lignified secondary xylem, and mechanical support for this stem is provided by the primary phloem fibres as well as the functional xylem elements.