Download Plant Structure and Function

Plant Structure
and Growth
9.3
General Plant Structure
• All plants have roots, stems and leaves
– Roots:
• absorb water and mineral ions
• anchor plants to the ground
• provide food storage
– Stems:
• provide attachment for leaves (at nodes)
• transport nutrients (water and mineral ions from up from roots
through xylem; organic nutrients / sugars through phloem)
• provide storage and support
– Leaves:
• photosynthesis
• gas exchange and transpiration
Plant Classification
Kingdom Plantae
Non-vascular Plants
Vascular Plants
small; no conductive (vascular)
tissue – ex: bryophytes (mosses)
have conductive (vascular) tissue (xylem and
phloem)
Spermatophytes
Filicophytes
produce seeds
have alternating life cycles (haploid
and diploid) – ex: ferns
Angiospermophytes
Gymnospermophytes
(“angiosperms”)
flowering plants
(“gymnosperms”)
seeds are in cones (naked seeds)
Monocots
Dicots
ONE seed leaf ex: grasses, orchids
TWO seed leaves ex: roses
Angiosperms: Monocotyledonous plants
versus Dicotyledonous plants
Structure
MONOCOTS
DICOTS
Cotyledons (seed
leaves)
ONE
TWO
Leaf veins (venation)
Parallel
Netlike
Vascular bundles in
stems
Arranged throughout entire stem
(scattered)
Arranged in a ring/ circle around
edge of stem
Roots
Fibrous (groups of thin roots
spread out into the soil)
Taproot (one main vertical root
with small branches off of it)
Flowers
Flower parts in multiples of 3’s
Flower parts in multiples of 4 or 5
Dicot Tissues: Stems
Structure
Epidermis
Xylem
Phloem
Function
•Protection
•Transport (water and mineral ions from roots up to rest of plant)
•Structural/ mechanical support (lignin/ lignified in terrestrial plants)
•Transport (organic nutrients – sugars – from “source to sink”)
Cambium
•Region of stem cells that separate xylem and phloem; differentiate
into new xylem/ phloem cells
Cortex
•Support through turgid, fluid-filled cells (sometimes aids in storage)
Pith
A. Plan diagram
of dicot stem
tissue
•Support through turgid, fluid-filled cells
•Storage
B. Actual image
of dicot stem
tissue
Dicot Tissues: Leaves
Structure
Function
Relationship between distribution of structure in leaf and
its function
Cuticle (waxy layer) and
upper epidermis
Protection
Upper epidermis secretes waxy layer (cuticle) to protect leaf
tissues from heat/ light damage and prevent water loss
Palisade mesophyll
Photosynthesis
Cells are chloroplast-rich and tightly packed in upper half of
leaf for maximum light absorption
Vascular bundles
Transport (xylem = water
and mineral ions; phloem
= sugars)
Located in middle of leaf so all cells have access
Spongy mesophyll
Gas exchange
Cells are loosely packed with spaces between them in lower
half of leaf near stomata openings
Lower epidermis (guard
cells/ stomatal pores)
Transpiration (water
vapour loss) and gas
exchange
Lower temperatures/ less light on bottom of leaf so water
loss is minimized; guard cells open and close stomatal pores
for gas exchange and transpiration
Modifications of Plant Roots, Stems, and
Leaves
• Many plants have modified roots, stems, and leaves
in order to help them survive in different
environments
Modified Structure
Function and Example(s)
Storage roots
Underground root cells that are specialized/ modified to store
large quantities of water and carbohydrates
Examples: Carrots/ beets
Stem tubers
Underground, horizontally-growing stems that are modified to
store carbohydrates
Example: Potatoes
Bulbs
Underground, small, vertical-growing stems attached to enlarged/
swollen leaf bases that are modified to store food
Example: onions
Tendrils
Modified leaves that coil around objects (above ground) to aid in
support, attachment and climbing
Examples: Vines/ peas
Plant Growth: Meristems
• Plants ONLY grow at regions called meristems:
areas of undifferentiated cells (stem cells)
• Meristems can be apical (at
the apex/ tip of a root or stem
– to allow plants to grow taller
or allow roots to extend
throughout soil) or lateral (on
the side (cambium) – to allow
plants to grow outward/
become thicker)
Apical and Lateral Meristems in
Dicotyledonous Plants
• Dicotyledonous plants contain both apical and lateral
meristems
• Both apical and lateral meristems rely on totipotent cell
divisions for growth
Apical Meristems (1°
Growth)
Lateral Meristems (2° Growth)
Occur at tips of stems and
roots
Occur at cambium in woody trees/ shrubs:
•vascular cambium produces secondary xylem (wood) and
secondary phloem
•cork cambium (in bark) produces cork cells of outer bark
Add vertical growth to roots
and stems (increased light/
CO2 and nutrient absorption)
Add lateral growth to stems (widens/ thickens stem of plant)
Produces new leaves and
flowers
Produces bark and wood on trees/ woody shrubs
Develops into primary xylem
and phloem
Produces secondary xylem and phloem
Lateral Meristem Growth (Bark
and Wood)
Plant Growth: Tropisms
• A tropism is a directional response in a plant to an
external stimulus (light, chemicals, gravity, touch).
• Plant tropisms are either positive (toward stimulus)
or negative (away from stimulus)
• Phototropism is a positive tropism in plant stems –
they grow toward the light
Phototropism and Auxin
• Auxin is a plant hormone that promotes growth by lengthening plant cells
and altering gene expression (produced in coleoptiles = protective sheaths
around apical meristems)
• Normally, auxin is distributed evenly throughout plant cells in stems so
plant stems grow evenly
• If one side of a plant receives more light, auxin is redistributed so that the
shaded side of the plant has MORE of it. More auxin = more growth/ cell
elongation, so stem bends on shaded side (cells elongate) and plant stem
grows toward light.
– Auxin works by activating proton pumps in cells; H+ ions pumped to cell wall
from cytoplasm, decreasing pH, loosening cellulose for stretching/ growth