176 ❯ STEP 4. Review the Knowledge You Need to Score High
Stems (Shoots)
Structure:epidermis (cutin) →cortex (ground tissue) →vascular cylinder →xylem/phloem.
Vascular cambium:gives rise to secondary xylem/phloem; runs entire length of plant.
Cork cambium:produces protective covering that replaces epidermis during secondary growth.Leaves (Shoots)
Structure:epidermis (cuticle) → mesophyll (photosynthesis) → vascular bundles →
xylem/phloem.
C 4 plants:leaves contain another cell type, bundle sheath cells,which assist in respira-
tion in hot and dry regions.
Stomata:structure, controlled by guard cells,that when open allows CO 2 in, and H 2 O
and O 2 out.Plant hormones:abscisic acid(inhibits cell growth, helps close stomata), auxin(stem elon-
gation, gravitropism, phototropism), cytokinins(promotes cell division, leaf enlargement,
slows aging of leaves), ethylene(ripens fruit and causes leaves to fall), gibberellins(stem
elongation, induces growth in dormant seeds, buds, flowers).
Plant tropisms:gravitropism(a plant’s growth in response to gravity—auxin, gibberellins),
phototropism(plant’s growth in response to light—auxin), thigmotropism(plant’s growth
in response to touch).
Photoperiodism:response of a plant to the change in length of days; remember florigenand
phytochrome.
Driving force for H 2 O movement in plants:transpiration is the major driving force that draws
H 2 O up the xylem because of the cohesive nature of water molecules. Osmosis and capil-
lary action are minor contributors.
Driving force for sugar movement in plants:sugar, when created, is dumped into the phloem,
creating a concentration gradient that draws water in, increasing the pressure that drives the
sugar through the phloem.