10. Plants

Question 1

Seed Plants

Answer 1

Gymnosperms (conifers) and angiosperms (flowering plants)

Question 2

The two groups of Angiosperms are….

Answer 2

• Dicotyledons (dicots) and monocotyledons (monocots)

Question 3

Dicots vs. Monocots

Answer 3

• Dicots: 2 cotyledons, netted leaf venation, flower parts multiples of 4s or 5s, vascular bundle organized in a circle, taproot (large single root)
• Monocots: 1 cotyledon, parallel leaf venation, flower parts multiples of 3s, vascular bundles scattered, fibrous root (cluster of many fine roots)

** Vascular bundles is arrangement of bundles of vascular tissue (xylem and phloem)

Question 4

Plant Tissues

Answer 4

1. Ground tissue:
   a. Parenchyma cells - most common component of ground tissue, thin walls, storage, photosynthesis, and secretion
   b. Collenchyma cells - thick but flexible cell walls, mechanical support
   c. Sclerenchyma cells - thicker walls than collenchyma, also mechanical support
2. Dermal tissue: epidermis cells that cover outside of plant parts, guard cells that surround stomata. In aerial portions of the plant, epidermal cells secrete a waxy protective substance, the CUTICLE
3. Vascular tissue - xylem and phloem. The two usually occur together to form VASCULAR BUNDLES.
   a. xylem - conduction of water and minerals and mechanical support. In addition to primary cell wall that plants have, xylem cells have a secondary cell wall that gives them additional strength. Sometimes, walls of xylem cells have pits - places where secondary wall is absent. ***WX (water xylem)
   b. Phloem. Conduction of Sugars. Made of cells called sieve-tube members (or sieve-tube elements) that form fluid-conducting columns called sieve tubes. Unlike xylem, sieve-tube are living at maturity, although they lack nuclei and ribosomes. Pores on end walls of sieve-tube members form sieve plates, areas where the cytoplasm of one cell makes contact with that of the next cell. Sieve tubes are associated with companion cells, living parenchyma cells that lie adjacent to each sieve-tube member. Companion cells, connected to adjacent sieve-tube members by plasmodesmata, maintain physiological support to nuclei lacking sieve-tube members.

Question 5

Xylem are dead or alive?

Answer 5

• dead at maturity.
• They are essentially cell walls, completely lacking cellular components, and contain only the material being transported. Two kinds of xylem cells: Tracheids and vessel elements (or members). Movement of water through column of vessel members (vessel) is more efficient because of PERFORATIONS, literally holes between cells (devoid of prim and secon cell walls). —> Vessels are considered a more evolutionary advanced feature –> found mostly among flowering plants.

Question 6

The Seed

Answer 6

• consists of embryo, a seed coat, and some kind of storage material. The major storage material may be endosperm or cotyledons. Cotyledons formed by digesting storage material in endosperm.
• Dicots: peas
• Monocots: corn

Question 7

Embryo parts:

Answer 7

1. Epicotyl - top or embryo –> shoot tip
2. Plumule - first true leaves. attached to epicotyl
3. Hypocotyl - below epicotyl and attached to cotyledons. Becomes young shoot.
4. Radicle. In some embryos, develops below hypocotyl. Dvelops into the root.
5. Coleoptile. Monocots only. Surounds and protects the epicotyl.

***First true leaves emerge from plumule within the coleoptile.

Question 8

Germination and Development

Answer 8

• Seed reaches maturity —> remain dormant until cues (water, temp., light). In some cases, dormancy period required.
• Germination begins with imbibition (absorption) of water. H2O initiates activity of various enzymes, and caused the seed to swell and seed coat to crack.
• Tips of radicle —> roots that anchor seedling. Elongation of hypocotyl follows, producing a young shoot.
• In young seedling, growth occurs at tips of roots and shoots, called apical meristems. These are areas of actively dividing, or meristematic, cells. This is PRIMARY GROWTH.
• Growth of a root:
  a. Root tip or root cap protects apical meristem behind it.
  b. ZONE OF CELL DIVISION - dividing cells of apical meristem.
  b. ZONE OF ELONGATION. Newly formed cells absorb H2O and elongate.
  c. ZONE OF MATURATION. or Differentiation. Here, cells mature into xylem, phloem, parenchyma, or epidermal cells. Root hairs may extend fro epidermal cells.
• **except for absence of a root cap, similar regions of growth occur at growing tips of shoots.

Question 9

Primary vs. Secondary Growth

Answer 9

• Primary Growth: actively dividing cells at apical meristems, producing growth that increases length of shoot/root. Tissues formed are primary tissues. Thus, primary xylem and primary phloem refer to vascular tissues originating from apical meristem growth. Occurs in most plants, including monocots.
• Secondary Growth: Other plants, like conifers and the woody dicots undergo secondary in addition to primary growth. Secondary growth increases girth or lateral dimension. Origin of woody plant tissue. Two lateral meristems:
  a. Vascular cambium: produces secondary xylem and secondary phloem.
  b. Cork cambium: produces the periderm, protective material that lines outside of woody plants.

Question 10

Primary Structures of Roots:

Answer 10

• The following formed from primary growth: ordered from outside of root to center:
  1. Epidermis. Lines outside surface of root. In zone of maturation, epidermal cells produce root hairs (increase absorptive surface of roots). As zone of maturation ages –> root hairs die –> epidermal cells from zone of elongation become new zone of maturation —> root hairs. Thus, roots must constantly grow to provide new root hairs.

2. Cortex. Makes up bulk of root. Storage of starch
3. Endodermis. Ring of tightly packed cells at innnermost portion of cortex (Suberin (fatty) attaches them). Encircling band creates CASPARIAN STRIP —> water-impenetrable barrer –> water must pass through cells and not between. In this way, endodermal cells control movement of H2O into center of root (vascular tissue resides) and prevent backflow of H2O
4. Vascular Cylinder (stele). Make up tissue inside endodermis. Outer part of cylinder consists pericycle cells —> give rise to lateral roots. Inside Pericycle is the vascular tissue.

Question 11

Primary Structures of Stems

Answer 11

• many of same characteristics as that in the root.
• In most cases, however, endodermis and casparian strips are lacking, as these tissues are specialized for H2O absorption.
• Other differences:
  1. Epidermis contains epidermal cells covered with waxy (fatty) substance called cutin –> forms cuticle (protective layer). Also, other specialized cells such as guard cells and stinging cells

2. Cortex consists of various ground tissue types that lie between epidermis and vascular cylinder. Many of these contain chloroplasts.
3. Vascular cylinder consists of xylem, phloem, and pith (central tissue area). Arrangement varies with species. Conifers and dicots: form ring; Monocots: scattered.

Question 12

Secondary Structures of Stems and Roots

Answer 12

• Vascular cambium originates between xylem and phloem and becomes a cylinder of tissue that extends the length of the stem and root. Cambium layer is meristematic, producing new cells on inside (secondary xylem) and outside (secondary phloem). Expansion pushes tissue beyond secondary phloem outward –> outside tissues (primary (ex. epidermis, cortex) break apart and eventually shed.
• In order to replace shed epidermis, new cells are produced by cork cambium.
• Each year, new layers of secondary xylem are produed by vascular cambium.
• *Xylem tissue, which is actual wood of a plant is dead at maturity. However, only xylem produced during more recent years remains active in transport of H2O. This xylem is referred to as SAPWOOD. Older xylem, located toward center of stem, is called HEARTWOOD and functions only as support.
• SAPWOOD: outside, active
• HEAERTWOOD: inside, dead

Question 13

Annual Rings?

Answer 13

• Environmental conditions create seasons of growth and and dormancy.
• Alternation of growth and dormancy produces ANNUAL RINGS in the secondary xylem tissue.
• These rings can be used to determine age of tree and provide a rainfall history of the region.

Question 14

Structure of Leaf

Answer 14

1. Epidermis. Protective covering. As in other regions, epidermis covered by waxy material cuticle (cutin). Cuticle reduces TRANSPIRATION (evaporative loss of H2O)
2. Palisade mesophyll. Consists of parenchyma cells (has numerous chloroplasts)–> specialized for photosynthesis.
3. Spongy mesophyll. Consists of parenchyma cells loosely arranged below palisade mesophyll. The numerous intrecellular spaces provide air chambers that provide CO2 to photosynthesizing cells.
4. Guard cells. Specialized epidermal cells that control the opening and closing of stomata (gas exchange).
5. Vascular bundles. Consist of xylem and phloem tissues. Xylem delivers H2O for photosynthesis, while phloem transports sugars and other carbohydrate by-products of photosynthesis to other areas of the plant. Bundle sheath cells provide anaerobic environment for CO2 fixation in C4 plants.

Question 15

Two pathways by which water moves toward center of root:

Answer 15

**Water and dissolved minerals enter roots through root hairs by osmosis.

1. Water moves through cell walls and intercellular spaces w/out ever entering cells. This is called APOPLAST and consists of nonliving portion of cells.
2. Water moves through SYMPLAST, or living portion of cells. Moves from cytoplasm to cytoplasm through plasmodesmata, small tubes connecting cytoplasm of adjacent cells.

**When water reaches endodermis, it can continue into the vascular cylinder only through the symplast pathway. Apoplast is blocked by SUBERIN that permeates the casparian strips. Endodermal cells allow water to enter but are mineral selective (K+ okay, Na+ no). Once through endodermis, continue by apoplast to xylem. The xylem tissue, consisting of tracheids and vessels is the major conducting mechanism of the plant.

Question 16

Three mechanisms involved in movement of water and dissolved minerals in plants:

Answer 16

1. Osmosis. Water moves from soil through root into xylem cells by osmosis. A concentration gradient is maintained by continuous movement of water out of root by xylem and by higher mineral conc inside STELE. Root pressue –> GUTTATION, formation of small droplets of sap (water and minerals) on ends of leaves of grasses in early morning. Root pressure usually not enough for plants, especially large plants such as trees.
2. Capillary action. Rise of liquids in narrow tubes. Also contributes of movement of water up xylem. Results fro adhesion forces (attraction of unlike). A meniscus forms on top of column as a result. In active xylem cells however, no meniscus –> effect of capillary action is minimal.
3. Cohesion-tension theory. Most water movement through xylem is explained by cohesion-tension theory. Major concepts:
   a. Transpiration - evaporation of water from leaves –> negative pressure (tension) develops within leaves and xylem tissue
   b. Cohesion - (attraction btw like substances), water molecules within a series of xylem cells behave as a single polymerlike molecule. (results from H bonds)
   c. Bulk flow of water through xylem occurs as water molecules evaporate from leaf surface. Water molecule lost –> pulls entire column of water behind it. In other words, water moves by pulling action generated by transpiration. Since transpiration is caused by sun, the sun, then is the driving force of ascent of sap (water and minerals) through plants.

Question 17

Control of Stomata

Answer 17

• opening and closing of stomata influence gas exchange, transpiration, ascent of sap and photosynthesis. When stomata are closed, CO2 is not available and photosynthesis cannot occur. Stomata open –> CO2 can enter but plant risk excessive transpiration. Opening regulated to balace these two states and optimized photosynthesis and minimize transpiration.
• Each stomata is surrounded by two guard cells. When water diffuses into a guard cell, the cell expands outward —> opening stomata. When water diffuses out of guard cells —> shape collapses and stomata closes.

**thus, opening and closing of stomata is controlled by movement of water into and out of guard cells.

Question 18

Factors involved in opening and closing of stomata:

Answer 18

1. Stomata close when temperatures are high. –>reduce water lose but shut down photosynthesis
2. Stoamta open when CO2 concentrations are low inside the leaf.
3. Stomata close at night and open during the day. May be in response to CO2 fluctuations caused by photosynthesis. During day, CO2 is low because it is used by photosynthesis, but at night, CO2 are high because of respiration.
4. Stomatal opening is accompanied by a diffusion of K+ into guard cells. An increase in K+ creates a gradient for movement of water into guard cell –> guard cell expand –> stomata open
5. When K+ enter a guard cell, imbalance in charge occurs. In some plants, Cl- enters guard cell along with K+. In other plants, H+ are pumped out of cell.

Question 19

Transport of Sugars

Answer 19

• Translocation: movement of carbohydrates through phloem from a source (leaves) to a sink.
• Translocation is described by pressure-flow hypothesis, as follows:
  1. Sugars enter sieve-tube membranes. Soluble carbohydrates move from production site to phloem sieve-tube member by active transport. –> concentration of solutes in sieve-tube members at source higher than at sink (ex. root).

2. Water enters sieve-tube members (diffusion).
3. Because of pressure build-up inside sieve-tube members at source, water and sugars move by bulk flow through sieve tubes to sink.
4. Pressure begins to build at sink, but since sink is area where carbohydrtes are being utilized, sugars are removed, sugars are removed by active transport–> water conc increases—> water diffuses out—> pressure relieved.

Question 20

Physiological importance of starch storage?

Answer 20

• Starch is insoluble in water, thus any cell can act as source or sink.
• A cell can act as sink by converting soluble sugars into starch (same affect as breaking them down)
• A cell can act as source if it breaks down starch into soluble glucose molecules. ex. when photosynthetic activity is low (night/cold), roots in plants act as a sugar source when stored starches are broken down to sugars.

Question 21

Plant Hormones

Answer 21

• Hormones: substances produced in one part of an organism that have influence elsewhere. Small amounts can alter physiology.
• Five classes:
  1. AUXIN (or IAA, Indoleacetic acid). Promotes plant growth by facilitating elongation of developing cells. Auxin increases H+ in primary cell walls–> H+ activates enzymes that increase cell wall plasticit –> turgor pressure causes cell wall to expand, generating growth. Auxin is produced at tips of shoots and roots. Auxin is a modified tryptophan amino acid. After synthesis from trp, it is actively transported from cell to cell in a specific direction (polar transport by means of a chemiosmotic process.

2. Gibberellins. Promote growth. Synthesized in young leaves, roots, and seed but often transported to other parts. ex. gibberellins produced in the roots and transported to shoot tips interact with auxins to stmulate shoot growth.
3. Cytokinins. group of hormones that stimulate cytokinesis (cell division). Structurally, they are variations of the nitrogen base adenine.
4. Ethylene. Gas that promotes ripening of fruit. Involved in stimulating production of flowers.
5. Abscisic Acid (ABA). Growth inhibitor. In many species of plants, ABA maintains dormancy in seeds. Dormancy broken by increase in gibberellins or by other mechanisms that respond to environmnetal cues such as temperature or light. In some desert species, dormancy is broken by leaching of ABA from seeds by rains.

Question 22

Plant Responses to Stimuli

Answer 22

• Plants are anchored by roots, therefore they can’t move in response to environmental stimuli. Instead, they change their growth pattern. A growth pattern in response to an environmental stimulus is called a TROPISM. Three Tropisms are described below:
  1. Phototropism. Response to light. Achieved by action of hormone auxin. Auxin produced in apical meristem—-> move downward by active transport into zone of elongation —> generate growth by stimulating elongation. When all sides of meristem equally illuminated –> growth of stem uniform. Stem unequally illuminated –> auxin concentrates in zone of elongation of shady side of stem–> shady side grows more than sunny side—> stem bends toward the light.
  2. Gravitropism (or Geotropism). Response to gravity. Not well understood.
  3. Thigmotropism. Response to touch. When vines and other climbing plants contact some object, they respond by wrapping around it. Not well understood.

Question 23

Photoperiodism

Answer 23

• response of plants to changes in the photoperiod, or the relative length of daylight and night. To respond to changes, plants maintain a circadian rhythm, a clock that measures the length of daylight and night. The mechanism is endogenous (internal clock).
• Mechanism for maintaining the circadian rhythm is not well understood. Phytochrome, a protein modified with a light-absorbing chromophore, seems to be involved.

Question 24

Three groups of flowering plants:

Answer 24

• Long-day: flower in spring/summer when daylight is increasing.
• Short-day: flower in late summer and early fall when daylight is decreasing
• Day-neutral: do not flower in response to daylight changes. Some other cue, such as temperature or water, triggers flowering.
• When the photoperiod is such that flowering is initiated, it is believed that a flowering hormone is produced. There is evidence that this hormone, called florigen, is a protein produced in leaves that travels to shoot tips.