Page 92 - Biology Form Two

Biology for Secondary Schools

Increasing concentration Ascent of water in root xylem

Cell vacuole
Soil particles

Xylem vessel
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Direction of water
Cytoplasm
Epidermis Cortex Endodermis Pericycle

Figure 4.13: Movement of water from a root hair to the xylem

Once in the xylem, water and the Stoma
mineral salts dissolved in it move up
the xylem vessel by transpiration pull,
capillarity, and root pressure.

Transpiration pull
Transpiration occurs when water Water vapour
evaporates from the plant through
the stomata in the leaves. As water is
lost, the mesophyll cells draw water
from the xylem in the leaf which in Direction
of water
turn draws water from the xylem in
the stem. This creates a tension called
transpiration pull, which draws water
from the roots. Thus, transpiration pull Root hair
is the biological process by which plants
draw water in the upward direction as Figure 4.14: Transpiration stream
a result of transpiration. This results in Capillarity
a continuous column of water through Capillarity is the ability of water to
the xylem from the roots to the stem and flow through narrow tubes. Xylem
leaves. This column of water is called vessels are structurally adapted for
the transpiration stream (Figure 4.14). capillarity because they are long and

86 Student’s Book - Form Two

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