A.P. Biology Plant Notes
Basic Characteristics:
Multicellular Eukaryotes, photosynthetic autotrophs, cell walls contain cellulose,
food reserve that is starch stored in plastids, chloroplasts with photosynthetic pigments
(chlorophyll a, b, and carotenoids), gas exchange via stomata, waxy cuticle to prevent
desiccation
Overview of Groups
Nonvascular Plants: (Bryophytes) haploid gametophytes is dominant generation,
gametangia protect developing gametes, lack woody tissue
Division Bryophyta
Mosses
Division Hepatophyta
Liverworts
Division Anthocerophyta
Hornworts
Vascular Plants:
Seedless Vascular Plants: sporophyte (diploid) dominated life cycle, is the
familiar leafy part of the plant, small gametophytes that grow beneath surface of soil,
formed the coal forests of the carbinoferous period
Division Lycophyta
Lycophytes, club mosses, ground pines
Division Sphenophyta
Horsetails
Division Pterophyta
Ferns
Seed Plants: seeds replaced the spore as the main means of dispersing offspring;
pollen is vesicle for sperm cells,
Gymnosperms: lack enclosed chambers (ovaries) in which seeds develop
Division Coniferophyta
Conifers
Division Cycadophyta
Cycads
Division Ginkgophyta
Ginkgo
Division Gnetophyta
Gnetae
Angiosperms: most widespread and diverse, 250,000 species, refined vascular
tissue, flowers, fruit
Division Anthophyta
Flowering Plants
Dicotyledons
2 cotyledons (storage tissue), netted leaf veins,
flower parts in multiples of 4 or 5, circled vascular
bundle, taproot
Monocotyledons
1 cotyledon, parallel veins, flower parts in multiples
of 3, scattered vascular bundles, fibrous root system
Origin and Evolution
-Evolved from green algae
-Alternation of generations in plants may have originated by delayed meiosis
-Adaptation to shallow water pre-adapted plants for living on land (waxy cuticles,
protection of gametes, and protection of embryos)
-Eventually, accumulated adaptations (regional specialization of plant body (roots
and plant body), structural support, vascular system, pollen, seeds) allowed
terrestrial plants to live above water line on dry land which opened new adaptive
zone with:
-Sunlight unfiltered by water and algae
-Soil rich in minerals
-Absence of terrestrial herbivores
Vascular Seed Plants: Angiosperms and Gymnosperms
Plant tissues
Ground Tissues: the general cells of the plant cell wall, function in
photosynthesis, storage, and support
Parenchyma Cells: most common, least specialized, lack secondary walls
(thin walls), where photosynthesis takes place, food storage
Collenchyma Cells: thick flexible cell walls, lack secondary walls,
grouped in cylinders to support young growth, elongate as the stems and
leaves they support grow
Sclerenchyma Cells: main function is support, thick secondary cell walls
strengthened by lignin
Dermal Tissues: single layer of tightly packed cells covering and protecting the
young parts of the plant
Root hairs: specialized for water and mineral absorption near root tips
Cuticle: waxy, helps retain water
Vascular Tissues: xylem and phloem that function in transport and support, is
continuous throughout the plant, found in bundles
Xylem: conduction of water and minerals, primary and secondary cell wall
for strength, pits (absent 2o cell wall) for water movement, dead a
maturity, made up of two types of cells
Tracheids: long, thin, tapered, lignin hardened with pits, water
flows from cell to cell through pits, support
Vessel Elements: wide, short, thin walled, aligned end to end, end
of cell is pitted for end to end transmission of water, efficient,
more found in flowering plants
Phloem: conduction of sugars, living at maturity
Sieve Tube members: chains of phloem cells that transport
sucrose, organic compounds, and some minerals, in angiosperms:
sieve tube has pores and is called sieve plate to facilitate
movement between cells
The Seed
Embryo:
Epicotyl: top portion of embryo, becomes the shoot tip
Plummule: attached to epicotyl, young leaves
Hypocotyl: below epicotyl, attached to cotyledons, young shoot
Radicle: develops below the hypocotyl, develops into root
Coleoptile: in monocots, protects epicotyl
Seed Coat: formed from integuments of ovule, outer layer of seed
Endosperm or Cotyledons: cotyledons formed by digesting storage material in
endosperm
Monocots: most of storage tissue is endosperm, single cotyledon to
transfer nutrients from endosperm to embryo
Dicots: two fleshy cotyledons (most of what you see when you look at
two halves of pea seed), remainder is a small embryo
Germination and Development: after a seed reaches maturity, it remains dormant until
environmental cues such as water, fire, temperature, light cause germination to begin
Germination: begins with absorption of water, seed swells, seed coat cracks,
radicle produces roots, elongation of hypocotyl to form young shoot
Primary Growth: in young seedling, growth at roots and shoots called apical
meristems, meristematic cells (actively dividing) create the primary growth
Root Growth: Root cap protects apical meristem, dividing cells in apical
meristem from zone of cell division, behind is zone of maturation where cells
mature into xylem, phloem.
Primary Growth vs Secondary Growth: Primary growth occurs in monocots and
occurs in primary tissues (xylem and phloem) that originate from apical meristem
and occurs vertically. Secondary Growth (conifers and woody dicots) in addition
to primary growth extends laterally and is origin of woody tissue. Lateral
meristems, vascular cambium, and cork cambium, is origin of secondary growth
Vascular cambium produce secondary xylem and secondary phloem. Cork
cambium gives rise to periderm (protective material that lines outside of woody
plants.
Plant Morphology
Primary Structure of Roots: leads to formation of the following tissues:
Epidermis: lines outside surface of root, root hairs (from zone of maturation)
increase absorptive surface area, constantly grow
Cortex: bulk of root, storage of starch
Endodermis: ring of tightly packed cells that control water movement and
confine to vascular bundle, has casparian strip (water-impenetrable) strip on
outside of cells
Vascular Cylinder: (stele), tissue inside endodermis, pericycle (outer part of
vascular cylinder, form lateral roots)
Monocots: xylem cells alternate with phloem cells in groups in rings
around the center tissue area called the pith
Dicots: Xylem cells form a big X across center, Phloem occupy regions
between the lines of the X
Primary Structure of Stems: similar to root, except most lack endodermis and
casparian strip because they are designed for water absorption. Other differences from
roots:
Epidermis: with a waxy cutin to protect cuticle
Cortex: ground tissue that contains chloroplasts
Vascular Cylinder: differing arrangements of xylem and phloem
Secondary Structure of Roots and Stems:
Vascular cambium between xylem and phloem becomes a cylinder of
meristematic cells on inside and outside of cambium cylinder. Cells on inside
become secondary xylem (increases girth of stem and root as accumulates),
outside become secondary phloem. This growth pushes outside tissues as xylem
girth continues, they break apart as separate from root or stem.
Periderm: cells produced by cork cambium to cover epidermis
Wood: dead xylem tissue, sapwood is new xylem is for water transportation,
heartwood is old xylem for support
Annual Rings: alternation of growth and dormancy, number and size of rings is
related to age and amount of water
Structure of the Leaf:
Epidermis: protective covering, covered by cuticle (cutin) to reduce transpiration
Palisade mesophyll: parenchyma cells with chloroplasts for photosynthesis
tightly packed near upper surface
Spongy mesophyll: loosely arranged parenchyma cells below palisade cells,
spaces for CO2 to cells
Guard Cells: specialized epidermal cells that control stomata (gas exchange)
Vascular Bundles: xylem and phloem contained within bundle sheath
Plant Hormones: 5 classes of plant hormones:
Auxin (IAA): indoleacetic acid, promotes growth by elongation, influences
phototropism and geotropism, is also active in leaves, fruits and germinating
seeds
Gibberellins: gibberellic acid, 60 types, promote shoot growth, fruit
development, and seed germination, inhibits aging of leaves
Cytokinins: stimulate cyotkinesis, growth of lateral buds, and organ development,
produced in roots
Ethylene (CH2): gas promotes ripening of fruit, production of flowers, and leaf
abscission (aging and dropping of leaves)
Abscisic acid (ABA): growth inhibitor, maintains dormancy in winter, and seeds
Functions in the Plant Life:
Transport of Water: Enters root by osmosis in root hairs, then to center or root by
either:
Apoplast: path of water through nonliving cell walls without leaving cells
Symplast: path of water through living portion of cytoplasm through
plasmodesmata (tubes that connect cytoplasm of adjacent cells)
Mechanisms of Water and dissolved minerals in plants:
Osmosis: from soil through root into xylem due to concentration gradient
from water that is constantly leaving the root xylem up the plant, force is
called root pressure
Capillary Action: results from adhesion force of water
Cohesion-tension theory: explains most of water movement:
Transpiration causes a negative pressure (tension), to cause the cohesive
water in the column to bulk flow through the xylem cells as the water is
pulled by evaporation, so the sun is the driving force for water movement
Control of Stomata: controls gas exchange, transpiration, sap and photosynthesis,
controlled by guard cells as water diffuses into and out of cells controlling the shape.
Controlled by the some of the following:
Stomata: close when temperatures are high, reduces loss of water, no
photosynthesis
Stomata: open when CO2 are low inside leaf
Stomata: close at night, open during day in response to CO2 fluctuation due to
photosynthesis
Stomata: open when K+ ions diffuse into guard cells, this gradient causes water
to move into guard cell
Transport of Sugars: translocation is movement of carbohydrates through phloem from
source to sink, is described by a pressure flow hypothesis:
Sugars enter sieve-tube members: from palisade mesophyll by active transport,
creates a higher concentration of sugars at source
Water enters sieve-tube members: as a result of movement of solutes to move
water down concentration gradient
Pressure in sieve-tube members at source moves water and sugars to sieve-tube
members at the sink though sieve tubes: by bulk flow
Pressure is reduced in sieve-tube members at the sink as sugars are removed for
utilization by nearby cells: pressure relieved as sugars are used in sink, and water
is removed by diffusion
Sugars stored as starches (water insoluable)
Plant Responses to Stimuli:
Tropism: a growth pattern to an environmental stimuli
Phototropism: response to light, controlled by auxin
Gravitropism (Geotropism): response to gravity, controlled by auxin and
gibberellins
Thigmotropism: response to touch
Photoperiodism: changes in the plant to the length of light and darkness
controlled by circadian rhythm, controlled by phytochrome (Pt or 660 or Pfr or 730)
that is photoreversible depending on the red light (wavelength 660 nm) or far red
(wavelength 730 nm)
Pfr appears to reset the circadian-rhythm clock
Pr is the form of phytochrome synthesized in plant cells (in the leaves)
Pr and Pfr are in equilibrium during daylight:
Pr accumulates at night
At daybreak, light rapidly converts the accumulated Pr and Pfr
Night length is responsible for resetting the circadian-rhythm clock
Flowering plants initiate flowering according to changes in photoperiod as
follows:
Long-Day Plants: flower in the spring and early summer when daylight is
increasing
Short-Day Plants: flower in late summer and early fall when daylight is
decreasing
Day-Neutral Plants: flower in response to temperature, or water