Leaves are the principal organs of photosynthesis in most plants.
The light reactions of photosynthesis take place most efficiently in the
palisade layer, and the light-independent (or "dark") reactions
in the spongy mesophyll.
Vascular tissues (xylem, phloem) help
transport materials within the plant.
- Leaves are made of:
- Upper epidermis, coated with a waxy cuticle.
- Palisade layer, containing the highest density of
chloroplasts. The "light" (light-dependent) reactions of
photosynthesis are most efficient here.
- Spongy mesophyll: The "dark" (light-independent)
photosynthesis reactions are most efficient here because
air spaces facilitate gas exchange.
- Veins (extensions of xylem + phloem) run through the
spongy mesophyll layer.
- Lower epidermis, coated with a waxy cuticle, contains
pores for gas exchange called stomates or stomata.
- Vascular tissues:
- Phloem transport: The phloem tissue transports photosynthetic
products from the leaves to other parts of the plant, principally
downward through the stem. The principal transport cells in phloem are
seive tube cells.
- Xylem and transpiration:
- Water and dissolved minerals
(ions, including K+, Ca2+, Mg2+,
NO3-, PO43-) ascend
from roots through stems to upper parts of the plant, traveling
through tube-like tracheids of the xylem.
- Loss of water from leaves is called transpiration.
- The ascent of sap seems to be governed largely by
transpiration pull, or reduced pressure from above,
a process requiring long, unbroken chains of fluid with no
air bubbles. Root pressure also helps a bit.
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