The xylem tissue forms a ring around the pith.

Plants have two transport systems to move food, water and minerals through their roots, stems and leaves. These systems use continuous tubes called xylem and phloem, and together they are known as vascular bundles.

It is transported through the xylem vessels up the stem to the leaves.

It is calculated that sugars move up to
10 000 times more quickly
by mass flow than they could through diffusion alone.
The
pH
of
companion cells
is
higher
than that of surrounding cells.
The
concentration
of
sucrose
is
higher
in the
source
than in the sink.
Not all the solutes
in the
phloem sap
move at the
same rate
.
Sucrose
is moved to all parts of the plant at the
same rate
, rather than going more quickly to areas with a low concentration.
The
role of sieve plates
is
unclear
.
Xylem and Phloem


Xylem has an essential, but secondary, role in photosynthesis

Xylem is wood, one of the world's most abundant and valuable renewable raw materials.

Lin M‐K, Lee Y‐J, Lough TJ, Phinney BS and Lucas WJ (2008) Characterization of phloem‐sap transcription profile in melon plants. Molecular and Cellular Proteomics 8: 343–356.


What is xylem and its function? - Quora

Plants that load se–cc complexes through a symplasmic route translocate 20–80% of sugars in the form of raffinose-related compounds such as raffinose, stachyose and verbascose (Section 5.2.3(c)). Grusak et al. (1996) proposed a model for symplasmic phloem loading that accounts for the general characteristics stated above. According to this model (Figure 5.16), sucrose diffuses from mesophyll and bundle sheath cells into intermediary (companion) cells through plasmodesmata. Within companion cells, sucrose is thought to be enzymatically converted to oligosaccharides (raffinose or stachyose) maintaining a diffusion gradient for sucrose from mesophyll cells into se–cc complexes. The molecular-size-exclusion limit of plasmodesmata interconnecting mesophyll and companion cells is such that it prevents back diffusion of stachyose and raffinose molecules, which are larger than sucrose. These oligosaccharides are able to diffuse through plasmodesmata with larger diameters linking companion cells with sieve elements (van Bel 1993). This model accounts for selective loading of sugars to achieve high photoassimilate concentrations in phloem elements.

26/10/2015 · What is xylem and its function

Phloem loading with an apoplasmic step is an attractive model, explaining both how solutes become concentrated in se–cc complexes (energy-coupled membrane transport) and how they could be selected by specific membrane transporters (see van Bel 1993). Identifying transport mechanisms responsible for photoassimilate transport to and from the leaf apoplasm has proved challenging.

Enjoy learning about xylem and phloem in this ..

The above-described characteristics have been used to argue against loading of se–cc complexes through a symplasmic route on the grounds that plasmodesmata lack mechanisms for concentrating and selecting solutes. However, a contribution of plasmodesmata to concentrating and selecting solutes cannot be precluded from our current knowledge of plasmodesmal structure and function.