allow the companion cells to regulate the content and activity of the sieve tube cytoplasm.
The companion cells also help load the sieve tube with sugar and the other metabolic prod-
ucts that they transport throughout the plant. This lowers the water potential of the sieve-tube
element, causing water to move in by osmosis, creating a pressure that pushes the sap down
the tube. The metabolising cells of the plant actively transport sugars out of sieve-tube ele-
ments, producing exactly the opposite effect. The diagram below illustrates how the overall
process works.
Figure 6.30: Diagram showing movement in the xylem and phloem vessels. Water movement is
upwards in the xylem and lateral into and out of the phloem. Lateral movement also occurs into and
out of the companion cells accompanying the phloem vessel.
6.5 Summary DUMMY
- Anatomy of dicotyledonous plants: Structures discussed in the previous chapter (col-
lenchyma, sclerenchyma, xylem and phloem) are important in carrying out the trans-
port functions. - Root anatomy:The root is important in absorption, anchorage and storage of food. It
is made up of the epidermis, cortex, endodermis, stele and pericycle. The epidermis
absorbs water and mineral salts. The cortex serves a storage role. The endodermis
allows water transport. The stele consists of a row of parenchyma cells which act as a
store for the vascular bundle. The pericycle is where secondary growth occurs. - Stem anatomy:Stems are important for support, transport, storage and production of
living tissue. - Secondary growth: The thickness of stems and roots is produced by mitosis. The
annual rings found in trees can be used to work out the age of a tree.
Chapter 6. Support and transport systems in plants 185