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Transcript
Xylem and phloem form the vascular system of plants to transport
water and other substances throughout the plant.
LEARNING OBJECTIVE [ edit ]
Describe the functions of plant vascular tissue
KEY POINTS [ edit ]
Xylem transports and stores water and water-soluble nutrientsin vascular plants.
Phloem is responsible for transporting sugars, proteins, and other organic molecules in plants.
Vascular plants are able to grow higher than other plants due to the rigidity of xylem cells, which
support the plant.
TERMS [ edit ]
phloem
a vascular tissue in land plants primarily responsible for the distribution of sugars and nutrients
manufactured in the shoot
xylem
a vascular tissue in land plants primarily responsible for the distribution of water and minerals
taken up by the roots; also the primary component of wood
tracheid
elongated cells in the xylem of vascular plants that serve in the transport of water and mineral
salts
Give us feedback on this content: FULL TEXT [edit ]
Vascular Tissue: Xylem and Phloem
The first fossils that show the presence of vascular tissue date to the Silurian period, about
430 million years ago. The simplest
arrangement of conductive cells shows a
pattern of xylem at the center surrounded
by phloem. Together, xylem and phloem
tissues form the vascular system of plants .
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Xylem and phloem
Xylem and phloem tissue make up the transport cells of stems. The direction of water and sugar
transportation through each tissue is shown by the arrows.
Xylem is the tissue responsible for supporting the plant as well as for the storage and longdistance transport of water and nutrients, including the transfer of water-soluble growth
factorsfrom the organs of synthesis to the target organs. The tissue consists of vessel
elements, conducting cells, known astracheids, and supportive filler tissue,
called parenchyma . These cells are joined end-to-end to form long tubes. Vessels and
tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at
the ends. It is the thick walls of the tracheids that provide support for the plant and allow it to
achieve impressive heights. Tall plants have a selective advantage by being able to reach
unfiltered sunlight and disperse their spores or seeds further away, thus expanding their
range. By growing higher than other plants, tall trees cast their shadow on shorter plants and
limit competition for water and precious nutrients in the soil. The tracheids do not have end
openings like the vessels do, but their ends overlap with each other, with pairs of pits present.
The pit pairs allow water to pass horizontally from cell to cell.
Tracheids and vessel elements
Tracheids (top) and vessel elements (bottom) are the water conducting cells of xylem tissue.
Phloem tissue is responsible for translocation, which is the transport of soluble organic
substances, for example, sugar. The substances travel along sieve elements, but other types of
cells are also present: the companion cells, parenchyma cells, and fibers. The end walls,
unlike vessel members in xylem, do not have large openings. The end walls, however, are full
of small pores where cytoplasm extends from cell to cell. These porous connections are called
sieve plates. Despite the fact that their cytoplasm is actively involved in the conduction of
food materials, sieve-tube members do not have nuclei at maturity. The activity of the sieve
tubes is controlled by companion cells through plasmadesmata.