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Transpiration stream

Vascular bundles

Xylem and phloem

Transpiration is the evaporation of water at the surfaces of
the spongy mesophyll cells in leaves, followed by loss of
water vapour through the stomata
...
The position of these
bundles varies in different parts of the plant
...


Plants have tissues to transport water, nutrients and minerals
...


Xylem vessels are tough and strong, so the vascular bundles are in the
centre of the root to resist forces that could pull the plant out of
the ground
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The
vascular bundles are arranged near the edge of the stem, with
the phloem on the outside and the xylem on the inside
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Water
molecules are cohesive so water is pulled up through the plant
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The transpiration stream has several functions
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They are long and thin so they can
penetrate between soil particles, and they have a large surface area for absorption of
water
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This happens because
the soil water has a higher water potential than the
root hair cell cytoplasm:

Solution

Water potential

Concentration of
dissolved solutes

Soil water

High

Low

Root hair cell cytoplasm

Low

Rate of transpiration
If the rate of transpiration increases, the rate of
absorption of water by the root increases too
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If water is scarce, or the roots are
damaged, a plant may wilt
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Unlike xylem, phloem vessels contain
cytoplasm, and this goes through holes from one cell to the next
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This is called translocation
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This means, for example, that sucrose is transported:
• from sources in the root to sinks in the leaves in spring time
• from sources in the leaves to sinks in the root in the summer
Applied chemicals, such as pesticides, also move through the plant by translocation
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