Phloem

Apart from the xylem and phloem in their vascular bundles, leaves are composed mainly of parenchyma cells.

As a result, tissues beyond the secondary phloem including the epidermis and cortex, in many cases tend to be pushed outward and are eventually "sloughed off" (shed).

By the formation of a layer of cambium between the bundles (interfascicular cambium), a complete ring is formed, and a regular periodical increase in thickness results from the development of xylem on the inside and phloem on the outside.

Diagram illustrating how Cuscuta uses haustoria to penetrate the vascular system of its host plant and remove sugars and nutrients from the host's phloem.

Exceptions include highly lignified cells, the sclerenchyma and xylem which are dead at maturity, and the phloem sieve tubes which lack nuclei.

In plants with secondary growth, the vascular cambium, originating between the xylem and the phloem, forms a cylinder of tissue along the stem and root.

Phloem cells mainly transport sucrose along pressure gradients generated by osmosis.

Phloem is a specialised tissue for food transport in higher plants.

The bryophytes lack phloem, but moss sporophytes have a simpler tissue with analogous function known as the leptome.

There is a protective outer layer (a peel or skin) with numerous long, thin strings (the phloem bundles ), which run lengthwise between the skin and the edible inner portion.

The soft phloem becomes crushed, but the hard wood persists and forms the bulk of the stem and branches of the woody perennial.

The vascular bundles of the stem are arranged such that the xylem and phloem form concentric rings.