Chapter 10: Plants

Question 1

seed plants

Answer 1

include gymnosperms (conifers) and angiosperms (flowering plants). Angiosperms are divided into two
groups: dicotyledons (dicots) and monocotyledons (monocots).

Question 2

plant tissues

Answer 2

3 distinct major groups

Question 3

1. ground tissue

Answer 3

1. Ground tissues: three kinds differ by nature of cell walls.
   a. Parenchyma: most common. Thin cell walls. Fxn: storage, photosynthesis, and secretion. (e.g. mesophyll cells in leaf)
   b. Collenchyma: thick but flexible cell walls, serve mechanical support functions.
   c. Sclerenchyma: thicker walls than collenchyma, also provide mechanical support

Question 4

2. Dermal tissue

Answer 4

: epidermis cells that cover outside of plant parts: guard cells that
surround stomata, hair cells, stinging cells, and glandular cells; in aerial portions
of plants the epidermal cells secrete waxy protective substance: cuticle.
Note: roots do not have cuticle – would prevent them from absorbing water!

Question 5

3. vascular tissue

Answer 5

consists of xylem and phloem => form vascular bundles.
a. Xylem: conduction of water and mineral and also fxns in mechanical support;
have 2
nd cell wall for additional strength; some places in walls of xylem cells
have pits (absence of 2nd cell wall). Cells are dead at maturity (no cellular
component – just cell walls). Two kinds of xylem cells:
- Tracheids: long and tapered where water passes from one to another
through pits.
- Vessel elements: shorter and wider, have less or no taper at ends. A
column of vessel elements (members) is called a vessel. Perforations are where
H2O passes through from one vessel member to the next (lack both 1st and 2nd
cell wall). Perforations are an advantage vs. tracheids – H2O moves more efficiently
b. Phloem: transport sugar. Made of cells called sieve-tube members
(elements) that form fluid-conducting columns (sieve tubes); cells are living at
maturity (but lack nuclei and ribosomes). Pores on end of member form sieve plates (areas where cytoplasm of one cell
makes contact with next cell). Sieve tubes are associated with companion cells (living parenchyma cells that lie adjacent to
each sieve-tube member) and connected by plasmodesmata to maintain physiological support due to lack of nuclei in the
sieve-tube members.

Question 6

the seed development

Answer 6

• Consists of embryo, seed coat, and some kind of storage material (endosperm or cotyledons-formed by digesting
  material in endosperm). There are two cotyledons in dicot (pea), 1 cotyledon in monocot (corn). In many monocots the
  endosperm is the primary storage tissue, cotyledon fxns to transfer nutrients from endosperm  embryo.
• Embryo:
  1. Epicotyl (top portion of embryo) becomes shoot tip.
  2. Plumule are young leaves often attached to epicotyl; plumule can refer to both together.
  3. Hypocotyl becomes young shoot (below epicotyl and attached to cotyledons).
  4. Radicles develops from below hypocotyls into root.
  5. A sheath called coleoptiles (in monocots) surrounds and protects
  epicotyl. In developing young plants, coleoptiles appear 1
  st as leaf,
  but the 1st true leaves are from the plumule within the coleoptiles.

Question 7

germination and development

Answer 7

Seed remain dormant at maturity until specific environment cues
(water, temp, light, seed coat damage), others may have required
dormancy period where germination won’t happen regardless.
- Germination begins with inhibition (absorption) of water 
enzymes activate  biochemical processes, respiration begin.
Absorbed water causes seed to swell and seed coat to crack 
growing tips of radicle produce roots that anchor seedling 
elongation of hypocotyl  young shoot formed.
- In young seedling/plants, growth occurs at tips of roots and shoots
(apical meristerms); areas of actively dividing (meristematic) cells.
This kind of growth is called primary growth (produces primary
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tissues-primary xylem and primary phloem  elongation). Most plants (incl. most monocots) just have this.
Root growth:
- Root cap: aka root tip, protects apical meristem behind it. Secretes polysaccharides that moisten soil, permitting root
growth.
- Zone of cell division: formed from dividing cells of apical meristem.
- Zone of elongation: newly formed cells absorb water and elongate. Responsible for our perception of growth.
- Zone of maturation: differentiation; cells mature into xylem, phloem, parenchyma, or epidermal cells (root hairs may grow
here). Note on root growth overall: the above is very similar for shoot tip growth, except there is no root cap present.
- Meristems: are areas in plants where active mitosis occurs, due to this cell division, it is also where growth occurs. Lateral meristems
can be at tip of lateral growth in plant. Apical meristems are responsible for vertical growth and found at root and shoot (apex) tips.

Question 8

primary growth vs secondary growth

Answer 8

• Conifers and woody dicots undergo secondary growth in addition to 1º growth (which extends length). 2º
  growth increase
  girth and is the origin of woody plant tissues; occurs at the two lateral meristems: the vascular cambium (2º xylem and 2º
  phloem) and the cork cambium (gives rise to periderm-protective material that lines outside of woody plant)

Question 9

primary structure of roots

Answer 9

1. Epidermis: lines outside surface of root. In zone of maturation, epidermal cells produce root hair). When zone of
   maturation ages, root hair die. New epidermal cells from zone of elongation becomes cell of new zone of maturation, forms
   new root hairs to continue absorption of water. Old epidermis fxns to protect root.
2. Cortex: makes up bulk of root, storage of starch, contain intercellular spaces to provide aeration of cells for respiration.
3. Endodermis: ring of tightly packed cells at inner most portion of cortex. A band of fatty material (suberin) impregnates
   endodermal cell walls to form encircling band called Casparian strip: creates water-impenetrable barrier between cells 
   All water passing through endodermis must pass through endodermal cells, not between  controls movement of water into
   center of root and prevents water from moving back out to cortex
4. Vascular cylinder (stele): makes up tissues inside endodermis (phloem, xylem, pericycle). Outer part consists of
   one/several layers of cells (pericycle-from which lateral root arise). Inside pericycle are vascular tissue.
   - Dicot: xylem cells fill center of vascular cylinder (shape X with phloem (sieve-tube members and companion
   cells) in the spaces of X).
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   - Monocot: groups of xylem and phloem alternate in a ring with the pith in the middle.

Question 10

primary structure of stems

Answer 10

• Lack endodermis and Casparian strips (not needed, these tissues specialized for water absorption).
  1. Epidermis: contain epidermal cells covered with waxy (fatty) cutin which forms protective layer called cuticle.
  2. Cortex: ground tissue types that lies between epidermis and vascular cylinder (many contain chloroplasts).
  3. Vascular cylinder: consists of xylem, phloem, and pith. In dicots + conifers, xylem and phloem are grouped in bundles
  (xylem inside, phloem outside) that ring a central pith area. A single layer of cells between the xylem and phloem may
  remain undifferentiated and later become the vascular cambium.

Question 11

Secondary structure of stems and roots

Answer 11

• Vascular cambium: becomes cylinder of tissue that extends the length of stem and
  root. Secondary growth in a stem illustrated above (cambium layer is meristematic,
  producing new cells on both inside and outside the cambium cylinder).
• Cells on the inside differentiate into 2º xylem, and those on the outside into 2º
  phloem. Over years, 2
  º
  xylem accumulate and increase girth of stem and root.
• Outside of cambium layer, new 2
  º
  phloem are added yearly. As a result, tisues
  beyond the 2º
  phloem are pushed outward as xylem increases its girth. These tissues
  include the primary tissue (epidermis and cortex) break apart and shed.
• In order to replace shed epidermis, cork cambium produces new cells on the outside (cork cells-impregnated with
  suberin). On the inside, phelloderm may be produced. Together, the cork/cork cambium/phelloderm are called Periderm.
  In stem of dicots/conifers, cork cambium originates from cortex just inside epidermis. In root, it originates from pericycle.
• Wood: formed from xylem tissues at maturity (dead), only the more recent 2º xylem produced from vascular cambium
  remain active to transport water (sapwood). Older xylem located at center (heartwood) functions only as support.
• Annual rings: alternation of growth (active vascular cambium divides) and dormancy due to season in secondary xylem
  tissue. Size of rings  rainfall history. Number of rings  age of tree.

Question 12

structure of the leaf

Answer 12

1. Epidermis: protective layer(s), covered with cuticle (protective layer
   containing waxy cutin) which reduces transpiration (water loss through
   evaporation); may bear trichomes (hair, scales, glands, etc. outgrowths).
2. Palisade mesophyll: consists of parenchyma cells with chloroplasts and large
   surface area (specialized for photosynthesis). Oriented and packed in at upper surface,
   but for dry habitat  both surfaces. (leaf photosynthesis occurs here primarily)
3. Spongy mesophyll: parenchyma cells loosely arranged below palisade
   mesophyll. Numerous intercellular spaces provide air chambers CO2 to
   photosynthesizing cells, O2 to respiring cells.
4. Guard cells: specialized epidermal cells control opening and closing of stomata (allow gas exchange).
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5. Vascular bundles: consist of xylem (water for photosynthesis) and phloem (transports sugar and by-products of
   photosynth to other parts of plants). Bundle sheath cell surrounds vascular bundle  no vascular tissue exposed to
   intercellular space  no air bubbles that can enter to impede movement of water; also provide anaerobic environment for
   CO2 fixation in C4 plant.

Question 13

transport of water

Answer 13

• Enter root through root hairs by osmosis. There are two pathways for water  center of root:
  a. Water move through cell walls and intercellular spaces from one to another without ever enter cells. This pathway is
  called apoplast (nonliving portion of cells).
  b. Water move through cytoplasm of one cells to another (symplast-living portion) through plasmodesmata (small tubes
  that connect cytoplasm of adjacent cells).
• Once H2O reaches endodermis, it can only enter by symplast (due to Casparian strips blocking) into the stele (vascular
  cylinder) and is selective permeable (K+
  pass, Na+
  is blocked - common in soil but unused in plants). Once through
  endodermis, apoplast pathway takes over to reach xylem (which is the major conduction pathway via tracheids and vessels)
  1. Osmosis: moves from soil through root and into xylem by gradient (continuous movement of water out of root by xylem,
  and high [mineral] inside stele). This osmotic force (root pressure) can be seen as guttation, formation of small droplets of
  sap (water and minerals) on ends of leaves in morning. But mostly, root pressure too small to have major effect on H2O transport
  2. Capillary action: rise of liquids in narrow tubes, contribute to movement of H2O up xylem; results from forces of
  adhesion (molecular attraction between unlike substances) between H2O and tube  meniscus is formed at top of water
  column. No meniscus in active xylem since water forms a continuous column; capillary effect minimal.
  3. Cohesion-tension theory: most water movement is explained by this; major contributor (above two minimal). Consists of:
  a. Transpiration: evaporation of water from plants, removes water from leaves => causing negative pressure (tension) to
  develop within leaves and xylem.
  b. Cohesion: attraction between like substances (water); so H2O within xylem cells behaves as a single, polymerlike
  column from roots to leaves
  c. Bulk flow: when a water molecule is lost from a leaf by transpiration, it pulls up behind an entire column of water
  molecules (generated by transpiration, which is itself caused by heat action of the sun, so technically sun drives sap ascent).

Question 14

control of stomata

Answer 14

affects gas exchange, transpiration, sap ascent, photosynthesis
- When stomata are closed  CO2 not available  cannot photosynthesize.
- When stomata are open  CO2 can enter leaf  photosynthesize but plant risks desiccation from transpiration.
- Guard cells: two surrounds the stomata. Cell walls of guard cells do not have the same thickness (thicker when border the
stomata). Guard cell expand when water diffuses in. Due to the irregular thickness and radial shape, the sides with thinner
cell walls expand more  creates opening (stoma). When water diffuses out  kidney shape collapses and stoma closes.
- Factors involved in mechanism of opening and closing:
1. High Temp -> Close. 2. Low [CO2] inside  Open  photosynthesis.
3. Close at night, Open during day. CO2 is low during daylight because used by photosynthesis. Could be response to CO2
levels: high at night because of respiration, low during day because used for photosynthesis.
4. Stomata opening accompanied by diffusion of K+
into guard cell  create gradient  more water moves in).
5. K
+
enter  unbalanced charge state. Clcan
come in or H+
(from ionization of cell’s organic substances) gets pumped out.
QVault: guard cells also have a blue light receptor on plasma membrane, blue light  H2O in  stomata opens

Question 15

transport of sugars

Answer 15

• Translocation: movement of carbohydrate through phloem from a source (e.g. leaves) to sink (site of carb utilization).
  Described by pressure-flow hypothesis:
  1. Sugars enter sieve-tube members: soluble carbs move from site of production (palisade mesophyll) to phloem sievetube
  members by active transport => higher [solute] at source than at sink [root].
  2. Water enters sieve-tube members: water diffuses into source by osmosis to balance the lower water cxn from step 1.
  3. Pressure in sieve-tube members at source moves water and sugars to sieve-tube members at sink through sieve tubes:
  when water enters the sieve-tube members, pressure build up since rigid cell walls do not expand. Result: water and sugar
  move by bulk flow through sieve tubes (through plates between sieve-tube members).
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  4. Pressure is reduced in sieve-tube members at sink as sugar are removed for utilization by nearby cells: pressure begins
  to build up at sink (from bulk flow source  sink). However, sink is where sugar are used sugars removed from sievetube
  members by active transport  increases [water] at sink  water diffuses out of cell  relieves pressure.
• Cells store energy as insoluble starch – benefit of this = any cell can act as a SINK and get the sugar and water transported there
  o Likewise, by breaking down starch, any cell can act as a source (e.g. plant roots at night break down starch when
  photosynthesis activity is low, they act as a sugar source)

Question 16

plant hormones

Answer 16

K. Plant Hormones
- Auxin (IAA-indoleacetic acid): promotes plant growth (elongation of cells) by increasing [H+
] in primary cell walls 
activates enzymes that loosen cellulose fiber (increase cell wall plasticity) thus turgor pressure expands cells to grow.
Produced at tips of shoots and roots (apical meristem). In concert w/ other hormones, influeces plant response to light
(phototropism) and gravity (geotropism). It is a modified tryptophan AA. After synthesis it is actively transported (ATP)
from cell to cell in a specific direction (polar transport) by means of chemiosmotic process. It inhibits lateral buds when it is
produced at terminal bud of growing tip. Moves unidirectional from shoot to root.
- Gibberellins (GA): group of hormones that promote cell growth (flower and stem elongation),synth’d in young
leaves/roots/seeds then transported to other parts of plant. Can act together w/ auxin to stim growth.Involved in inhibition of
aging in leaves, promote fruit development and seed germination (gibberellin is released from embryo, moves through
endosperm to aleurone layer. Aleurone then secretes digestive enzymes (amylase) to break down endosperm starch into
sugars  nourishment  germination commences. High cxn of gibberellins causes bolting (rapid elongation of stems).
- Cytokinins: stimulate cytokinesis [cell division], stimulate (and influence direction of) organogenesis; stimulate growth of
lateral buds (which weakens apical dominance-dominance growth of apical meristem); delay senescence (aging) of leaves.
Effects depend on target organ and presence/cxn of auxin. Structurally: variations of the nitrogen base adenine; include
naturally occurring zeratin and artificially produced kinetin.
- Ethylene (H2C=CH2): gas that promotes ripening of fruit; production of flowers; influences leaf abscission (aging
[senescence] and dropping of leaves); apoptosis. Together w/ auxin, can inhibit elongation of roots, stems, and leaves.
Stimulates ripening by enzymatic breakdown of cell walls. Ethylene is why ripe fruit in proximity to a spoiled one will als
cause it to spoil – remember, it is gaseous.
- Abscisic acid (ABA): growth inhibitor. In buds it delays growth and forms scales, maintains dormancy in seeds.
Dormancy can be broken by increase in gibberellins or mechanistic response to environmental cues (temp, light).

Question 17

plant responses to stimuli

Answer 17

• Tropism: growth pattern in response to an environmental stimulus
• Phototropism: response to light (achieved by hormone auxin). Auxin is produced in apical meristem  moves downward
  by active transport into zone of elongation  generate growth by stimming elongation.
• Stem grows straight when all sides of apical meristem are equally illuminated. But growth can be differential if…
• When not equally illuminated, auxin moves more toward shady side (grow more)  stem bends toward light.
• Gravitropism (geotropism): response to gravity by stems and roots (auxin and gibberellins involved).
• If stem is horizontal, auxin concentrates on lower side => stem bends upward.
• If root is horizontal, auxin is produced at apical meristem moves up in root and concentrates on lower side.
  However, auxin inhibits growth in roots due to higher [auxin] at root than stems  lower side grows less, root curls down
  Note: Dissolved ions, auxins, gibberellins, and other hormones do not directly respond to gravity (evenly distributed in solution regardless). BUT
  starch is insoluble in water and does respond to gravity. It’s believed that specialized starch-storing plastids called statoliths, which settle at the
  lower ends of cells, somehow influence the direction of auxin movement.
• Thigmotropism: response to touch (e.g. vines wrap around object they contact)

Question 18

photoperiodism

Answer 18

• Photoperiodism: response of plants to changes in photoperiod (relative length of daylight and night); plants maintain
  circadian rhythm (a clock that measures length of daylight and night); endogenous mechanism (internal clock that
  continues to keep time even if external cues are absent). External cues (dawn, dusk) reset clock for accuracy.
• Phytochrome: protein modified with light-absorbing chromophore. Two forms: Pr (P660-red) and Pfr (P730-far red). They
  are reversible. When exposed to red light, Pr  Pfr and vice versa.
  1. Pfr appears to reset circadian-rhythm clock: Pfr is active form of phytochrome; maintains accuracy by resetting clock
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  2. Pr is the form of phytochrome synthesized in plant cells: Pr is synthesized in leaves.
  3. Pr and Pfr are in equilibrium during daylight: red light is present as sunlight Pr  Pfr and far-red is also present (Pfr  Pr)
  4. Pr accumulates at night: cells keep making Pr at night, but no sunlight to convert Pr  Pfr; Pfr breaks down faster than Pr
  and is also converted back to Pr metabolically  Pr accumulates
  5. At daybreak, light rapidly converts accumulated Pr to Pfr: equilibrium is maintained.
  6. Night length is responsible for resetting clock: Interrupt daylight with brief dark period  no effect. But flashes of red
  and far-red during night period can reset the clock. Only the last flash effects the night length. Red  shorter night length.
  Far Red  Restores night length.
• Flash of red during night: Pr  Pfr  shorter night period measured  circadian rhythm reset
• Flash of far-red after red flash  effect of red light reversed  night length restored to before
• In series of alternating flashes, only last one affects perception of night length: red shortens, far-red restores
• Flowering Plants: initiate flowering in response to changes in photoperiod
  1. Long-day plants: flower in spring and early summer when daylight is increasing.
  2. Short-day plants: flower late summer and early fall when daylight is decreasing (need daylight < a critical length)
  3. Day-neutral plants: do not flower in response to daylight changes but temp or water.
• Florigen: when flowering is initiated, this flowering hormone is produced in leaves and travels to shoot tips
  Phytochrome involved in other light-related fxns:
   Many seeds require minimum light exposure before germinating. Phytochrome system detects changes in light amt  if
  critical exposure is exceeded (or other facts like water present)  giberellins produced (or ABA destroyed)  germination
   Red to far red ratio is measured by phytochrome to sense quality of light (i.e. if it is being shaded by other plants). If
  shaded it can stim growth if the plant is shade-intolerant.
   In C3 plants, CO2 levels are relatively low in leaves when photosynthesis is active during the DAY, when stomata are
  OPEN. At night stomata close and CO2 levels in leaves increase due to respiration.
   In CAM plants, stomata closed during day but photosynthesis proceeds because CO2 supplied by metabolic conversion of
  malic acid
  Note: Rhizomes are underground stems that can sprout to produce new shoots and roots for the plant.