Page 94 - Biology Form Two

Biology for Secondary Schools

Question xylem vessel. When a plant is cut, sap
What caused the differences in the levels oozes out from the remaining stem or
of water observed in the three tubes? stump, as shown in Figure 4.16. This is
proof that root pressure drives the fluid
Root pressure upward through the xylem vessels.
FOR ONLINE READING ONLY
Root pressure pushes water and Fluid droplet
dissolved mineral salts upwards from Stump
the root. This happens because the
cells of the endodermis push mineral
salts into the xylem. This increases
osmotic pressure in the xylem thereby
creating a force that moves the water
and dissolved mineral salts up the

Figure 4.16: Sap oozing from stump

Transpiration
Transpiration is a process by which plants lose water through the stomata in the
leaves.
Water flows from the roots to the leaves through the xylem vessels. It enters the
spongy mesophyll of the leaf by osmosis. The spongy mesophyll has substomatal
air spaces in which water enters in the form of water vapour. As a result, the
concentration of water vapour in the substomatal air spaces becomes higher than
the concentration of water vapour in the air. This causes water to diffuse into the
atmosphere through the stomata (Figure 4.17).

Cuticle
Palisade
layer Upper epidermis

Xylem Vascular
Phloem bundle
Spongy
mesophyll Air space
Lower epidermis

Cuticle

Guard cell
Stomata pore
Figure 4.17: Movement of water through the leaves

88 Student’s Book - Form Two

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