What is Phloem?
Phloem is the tree's food-delivery system. While xylem carries water upward from the roots, phloem carries sugars and nutrients downward from the leaves to every living part of the tree — roots, growing tips, flowers, and fruits.
How It Works
Key Components
- Sieve tubes: Long, connected cells that form the pipeline for sugar transport.
- Companion cells: Metabolic helpers that keep sieve tubes alive and functioning.
- Phloem parenchyma: Storage cells for starch and other nutrients.
- Phloem fibers: Structural support (bast fibers used in traditional cordage).
Transport Mechanism
Sugars are loaded into sieve tubes at the leaves, creating osmotic pressure that drives flow toward areas of lower sugar concentration (roots, growing points).
Location in the Trunk
Phloem sits just outside the cambium, forming the inner bark. This places it in a vulnerable position — any damage that rings the trunk (girdling) cuts the phloem and starves the roots.
Costa Rican Examples
Guapinol (Hymenaea courbaril)
Thick, resinous phloem produces copious amber-like resin when wounded.
Burío (Heliocarpus appendiculatus)
Strong phloem fibers (bast) are traditionally extracted for making rope and cordage.
Practical Significance
- Girdling kills trees: Removing bark in a ring around the trunk cuts the phloem and starves the roots.
- Latex production: Many tropical trees (rubber, chicle) produce latex in specialized phloem cells.
- Traditional fiber: Bast fibers from phloem have been used for millennia in rope-making.