37.0 Introduction

1. Dominant Photosynthetic Organisms on Land fig 37.1
1. Flowers and Fruit Confer Reproductive Success
2. Involved in Exchange of Gametes and Dispersal

37.1 Many plants can clone themselves by asexual reproduction

1. Asexual Reproduction
1. Two Prominent Types of Reproduction in Plants
1. Sexual reproduction with alternation of generations
2. Asexual reproduction without alternation of generations

2. Vegetative Reproduction
1. New plants cloned from parts of adults fig 37.2
2. Stolons
1. Runners or stolons are long slender stems on surface of soil
2. Example: Cultivated strawberry
3. Leaves, flowers, roots produced at every other node on runner

3. Rhizomes
1. Underground stem
2. Examples include grasses and sedges
3. Each node can give rise to new plant
4. Corms, bulbs, tubers are specialized rhizomes
1. Have storage and reproductive functions
2. White potatoes propagated from tuber segments with "eyes"

4. Suckers
1. Roots of some plants produce sprouts called suckers
2. Examples: Cherry, apple, raspberry, blackberry, commercial banana, dandelions

5. Adventitious leaves
1. Example: Kalanchoe, commonly called maternity plant
2. Plantlets arise from meristematic tissue in notches of leaves
3. Plantlets drop to ground, take root
4. Walking fern forms plantlets when leaf tip touches ground

3. Apomixis
1. Include citruses, some grasses, dandelions
2. Embryos in seeds produced asexually from parent plant
1. Seeds produce individuals genetically identical to parent
2. Seeds have advantage of asexual reproduction with widespread seed dispersal

3. Advantages of asexual reproduction
1. Promote exact replication of individuals
2. Individuals well-suited to specific environment
3. More common in harsh or marginal habitats, little margin for variation

1. Rise of Flowering Plants
1. Most Common Plants Are Angiosperms
1. Great range in size, tiny to gigantic
2. Great variation in form, cactii to pondweeds
3. Near exclusive human food sources, direct or indirect

2. Why Were Angiosperms Successful?
1. Originated at time of only two major continental masses fig 37.3
1. Gondwanaland = Africa, South America, Antarctica, India, New Zealand
2. Laurasia = North America, Europe, Asia

2. Evolution occurred within hot, arid interior of Gondwanaland
3. Transport gametes over great distances, promote outcrossing
4. Efficient dispersal via fruit
5. Tough, water resistant leaves for survival in hostile environment
6. Produce natural insecticides

3. The Rise to Dominance
1. Became dominant 80 million years ago in second half of Cretaceous Period
2. Recognize members of present families 65 million years ago
3. Appearance of insects associated with flowers

2. Evolution of the Flower
1. Structure of Flower Related to Indirect Pollination
1. Life cycle overview
1. Pollen produced in and matures in anthers
2. Pollen tube grows through stigma to ovule
3. Double fertilization of ovule and endosperm nuclei
4. Seed ripens within fruit

2. Specialized pollination
1. Depend on insects and other animals to transport pollen
2. Flowers provide food reward, liquid nectar or pollen
3. Relationships evolved between pollinators and flowering plants
4. Gametes are dispersed as readily as in active animals

2. Characteristics of Floral Evolution
1. Determination of primitive versus specialized flowers
1. Correlate known primitive features of wood and pollen with flower type
2. Compare DNA base pair sequences
3. Earliest plants may have inhabited areas poor for fossil formation
4. Examine features of fossil flowers fig 37.4

2. Characteristics of primitive flowers
1. Numerous spirally arranged sepals, petals, stamens and carpels
2. Little difference in appearance of sepals and petals
3. Members of whorls are free, not fused together or with other flower parts
4. Earliest flowers may resemble pepper and sycamore families, not magnolias

3. Differentiation between floral and vegetative growth
1. Flowers are determinate, apical meristem does not continue to divide after flower is formed
2. Leafy shoots are indeterminate, continue to grow while progressively differentiating new leaves along the shoot

3. Calyx
1. Complete flower has four whorls of parts
2. Incomplete flower missing one or more whorls
3. Outer whorl of a complete flower, composed of sepals fig 37.5
4. Similarities in leaves and sepals, share common evolutionary origin
1. Pattern of veins
2. Coloration and form
3. Affected by some of the same genes
4. Sepals of many monocots are petaloid, with form and color like petals

4. Corolla
1. Composed of petals
2. Petals have two different evolutionary origins
3. Similarities between petals and stamens of most flowering plants
1. Structural similarities, petals became flattened
2. Affected by some of the same genes
3. May be homologous, sharing a common origin

4. Exceptions include water lilies
1. Petals originated as modified sepals
2. Transitional structures between sepals and petals are present

5. Functions to attract pollinators to flower

5. Androecium
1. Male flower parts
2. Composed of stamens: Specialized structures that bear microsporangia
3. Probable evolution from small branches containing microsporangia
4. Structure of most stamens includes slender filament and swollen anther
5. Primitive stamens are flattened and leaf-like

6. Gynoecium
1. Female flower parts
2. Composed of one or more pistils
1. May be single carpel or compound fused carpels
2. More primitive flowers have multiple pistils

3. Highly specialized organs unique to flowering plants
1. Ovules develop within lower portion, called ovary
2. Slender style between ovary and pollen-receptive stigma

4. Primitive plants have leaflike carpels
1. First carpels were leaves that folded longitudinally
2. Did not fuse until fruit developed
3. Had hairs on margins, hairs were interlocked and receptive to pollen
4. Hairs became stigma, style formed, carpel fusion produced carpel

5. Carpels of modern flowers are highly modified, recognized only when pistil cut open

7. Trends of Floral Specialization
1. Involve aggregation of and/or reduction in flower parts fig 37.6
1. Reduction in number of parts in each whorl
2. Spiral pattern evolves into single whorl at each level
3. Central axis shortens, whorls close together

2. Fusion of members of whorls, frequently joined into a tube
3. Fusion among whorls or loss of whole whorls

8. Trends in Floral Symmetry fig 37.7
1. Primitive flowers are radially symmetrical like buttercups
2. Advanced flowers are generally bilaterally symmetrical
1. Examples: Snapdragons, mints, orchids, violets, peas
2. Associated with advanced, precise pollination systems

3. Formation of Angiosperm Gametes
1. Review of Alternation of Generations
1. Diploid sporophyte produces haploid gametophyte
2. Gametophyte generation of angiosperms small and enclosed within parent
1. Male gametophytes = microgametophytes = pollen grains
2. Female gametophytes = megagametophyte = embryo sac

3. Each produced in specialized structures in angiosperm flower
4. Development different from that of animals
1. Male and female structures usually occur within same flower
2. Reproductive structures are not permanent part of adult plant

2. Pollen Formation
1. Form in two pollen sacs on anther
2. Each sac contains specialized chambers enclosing microspore mother cells
1. Microspore mother cells undergo meiosis
2. Produce four haploid microspores
3. Further mitotic divisions produce four pollen grains

3. Specialized shapes for different flower species
1. Pollen tube grows through stigma and style
2. Most pollen grains have tube from which pollen tube grows
3. Some grains have three furrows fig 37.8

3. Egg Formation
1. Eggs develop within ovaries
1. Each ovule contains megaspore mother cell
2. Megaspore mother cell undergoes meiosis to produce four haploid megaspores
3. One survives, three reabsorbed
4. Single megaspores divides mitotically to produce eight haploid nuclei

2. Nuclei enclosed within embryo sac in precise positions fig 37.9
1. Egg cell nucleus located near opening of embryo sac
2. Two nuclei in middle called polar nuclei
3. Two nuclei flank egg cell, called synergids
4. Remaining three nuclei are at opposite end of sac, called antipodals

1. Pollination in Flowering Plants
1. Pollination Occurs when Pollen Is Placed on Stigma
1. May result from action of wind or animals
2. May originate within flower: Self-pollination

2. Pollination in Early Seed Plants
1. Passive pollination by the wind
2. Requires production of great quantities of pollen
3. Individuals must grow relatively close together
4. A few gymnosperms are insect pollinated

3. Pollination by Animals
1. Important role in evolutionary success of angiosperms
2. Earliest angiosperms and perhaps ancestors were insect pollinated
3. Coevolution between plants and animals affects floral specialization

4. Bees
1. Bees are most common insect pollinators fig 37.10
2. Ability to locate flowers
1. Initially by odor
2. Secondarily orient by shape, color, texture
1. Bee pollinated flowers are generally blue or yellow
2. Nectaries identified by lines of dots or stripes

3. Food produced for bees by flowers
1. Few obtain nectar, primarily food source for adult forms
2. Generally obtain pollen, food source for larvae

4. Social structure of bee populations
1. Few are social or semisocial
1. Produce several generations during one year
2. Visit different kinds of flowers throughout a season
3. Utilize many different flowers at one time due to colony size

2. Most bees are solitary
1. Visit only a small group of generally related plants
2. Result in frequent evolutionary modifications

5. Insects Other than Bees
1. Characteristics of flowers visited by butterflies
1. Flat landing platforms
2. Long, tubular floral tubes filled with nectar
3. Specialized mouthparts of butterflies and moths

2. Characteristics of flowers visited by moths
1. Pale coloration, yellow or white
2. Heavily scented for locating at night

6. Birds
1. Hummingbirds are most common avian pollinators fig 37.11
2. Floral characteristics
1. Production of large quantities of nectar
2. Red coloration that is not conspicuous to insects
1. Contrary to carotenoid pigments in yellow flowers
2. Reflect in ultraviolet range, called "bee's purple" fig 37.12

3. Odorless because birds do not have well-developed olfactory senses
4. Floral tubes strong to withstand beak of birds

7. Wind-Pollinated Angiosperms
1. Examples: Oaks, birches, cottonwoods, grasses, sedges and nettles
2. Characteristics of flowers
1. Small, greenish, odorless fig 37.13,14
2. Corollas reduced or absent
3. Occur in large, close groupings
4. May hang down in tassels
5. May have separate male and female flowers on single plant or on separate plants
6. Usually flower in early spring before formation of leaves

2. Self-Pollination versus Outcrossing
1. Self-Pollination
1. Occurs relatively frequently
2. Flowers small and inconspicuous
3. Pollen shed directly onto stigma, often before flower opens
4. Rationale supporting self-pollination
1. Ecologically advantageous where animal pollinators are scarce
2. Advantageous to maintain genetic similarity in uniform habitats

5. Many weeds self-pollinate

2. Factors Promoting Outcrossing
1. Flower structure
1. Most flowers possess both stamens and carpels
2. Pistillate flowers possess pistils, lack stamens
3. Staminate flowers possess stamens, lack pistils

2. Presence of flowers on whole plants
1. Dioecious: Staminate and pistillate flowers on separate individuals
1. Example: Willow
2. Outcrossing is obvious

2. Monoecious: Staminate and pistillate flowers on the same individual
1. Example: Oaks and birches, corn, ragweed fig 37.13
2. Outcrossing enhanced by differential maturation of flowers

3. Dichogamous: Flowers possess both pistils and stamens, but they mature at different times
1. Either may mature first, flower may be staminate then pistillate fig 37.14,15
2. Significantly increases outcrossing rate

3. Physical separation of pistils and stamens
4. Genetic self-incompatibility
1. Pollen from an individual will not function on its own stigma
2. Embryos from self-fertilization abort soon after fertilization

1. Fertilization
1. Angiosperm Fertilization Is a Unique Process
1. Process called double fertilization utilizes two sperm cells fig 37.16
2. Results in two key developments
1. Fertilization of egg
2. Formation of nutritive endosperm

3. Embryo develops with numerous divisions
4. Protective tissues enclose embryo, form seed
5. Seed further enclosed within fruit
1. Aids in seed dispersal
2. Ensures genetic variability

2. How Fertilization Proceeds
1. Pollen grain arrives at stigma
1. Adheres to sticky, sugary substance on its surface
2. Begins to grow a pollen tube that pierces the style
3. Tube is nourished by sugary substance

2. Pollen tube grows through style to ovule in ovary
3. One cell in pollen grain divides to form two sperm cells
4. Pollen tube reaches embryo sac in ovule
1. Tip of pollen tube bursts through, releases sperm cells
2. Two synergid nuclei that flank egg cell disintegrate
3. One sperm cell fertilizes egg cell, forms zygote
4. Other sperm cell fuses with polar nuclei, forms triploid primary endosperm nucleus
5. Primary endosperm nucleus develops into endosperm

2. Seeds
1. Seed Contains an Embryo
1. Is a compact, drought-resistant package fig 37.17
1. Contains food
2. Enclosed in a protective seed coat

2. Embryonic development is temporarily arrested
3. Provides a convenient means for dispersal for an anchored organism
1. May be aided by wind, animals or water fig 37.18
2. Wind dispersed
1. Pine seeds, fruit of maple, elm, ash have wings
2. Dandelion fruitlets have plummules

3. Birds carry seeds
4. Seeds with hooks stick to animal fur
5. Coconuts float across water

4. Seeds can remain dormant for long time, till conditions favor germination
5. Adaptive importance
1. Protects embryonic plant from drying out
2. Protects embryo's food store from predators or parasites
3. Food storage analogous to yolk of egg

6. Embryo protected within coat of sporophyte tissue
1. Characterized by gymnosperms and angiosperms
2. First appeared 360 million years ago

7. Both male and female gametophytes reduced, dependent on sporophyte

2. Seed Formation
1. Plant tissues become more specialized during development
1. First stage in plants is division of zygote to form embryo
2. Differentiation begins almost immediately after fertilization
3. Principal tissue systems visible within five days
4. Root, shoot and apical meristems visible by sixth day

2. Significant event occurs when embryo stops developing and becomes dormant
1. Usually occurs after differentiation of apical meristems
2. First leaves, cotyledons, formed
3. Integuments develop into impermeable seed coat
4. Encloses embryo along with food source

3. Most metabolic activities cease after full development of seed coat
1. Seed contains only 10% water
2. Seed remains stable

4. Germination takes place when water and oxygen reach embryo
1. Metabolic activities resumed
2. May involve cracking of seed coat

5. Seeds may remain viable for hundreds of years
6. Environmental factors ensure germination will occur under appropriate conditions

3. Fruits
1. Fruit Are Mature Ovaries tbl 37.1
2. The Formation of Fruits fig 37.19
1. Pomes
1. Apples, pears, quinces

2. Drupes
1. Peaches, apricots, plums, cherries
2. Coconuts are seeds with fibrous flesh removed

3. True berries
1. Blueberries, cranberries, tomatoes, grapes, eggplant, peppers
2. Fruit named "berry" are usually not true berries, originate from more than one pistil

4. Hesperidiums
1. Oranges, tangerines, lemons, limes, kumquats

5. Pepos
1. Pumpkins, squash, cantaloupe, melons, cucumbers, gourds

6. Aggregate fruits
1. Strawberries, raspberries, blackberries

7. Multiple fruits
1. Mulberries, pineapples, osage oranges
2. Figs are a special type called synconium, an outside-in inflorescence

8. Follicles
1. Milkweed, larkspur
2. Individual fruitlets of magnolia

9. Legumes
1. Peas, beans, soybeans
2. Peanuts are atypical forms that fail to split naturally

10. Siliques/Silicles
1. Produced by members of mustard family
2. Cabbages, broccoli, radishes, shepherd's purse
3. Siliques are four times as long as they are wide
4. Silicles are shorter

11. Capsules
1. Irises, lilies, orchids, snapdragons
2. Some poppies have unique capsules with pores through which seeds are shed
3. Are most common dry fruit that splits at maturity

12. Caryopses
1. Corn, wheat, barley, rye, oats, rice, sugar cane, bamboo
2. Also known as grains

13. Nuts
1. Chestnuts, filberts, acorns
2. Walnuts, almonds, coconuts, peanuts are not true nuts

14. Achenes
1. Sunflowers, buttercups, buckwheat

15. Samaras
1. Maples, elms, ashes

16. Schizocarps
1. Produced by members of parsley family only
2. Two one-seeded mericarps that spilt at maturity
3. Parsley, dill, carrots, fennel, caraway, celery, anise

3. The Dispersal of Fruit
1. Fruit dispersed when used as food
1. Fleshy coverings
2. Shiny black, blue or bright red coloration fig 37.4c,20a
3. Red fruit signal abundant food supply

2. Parallel evolution of generalized adaptations
1. Hooks and spines attach seeds to passing animals fig 37.20b
2. Seeds buried for food, but never reclaimed
3. Many seeds are dispersed by the wind
1. Winged seeds of pines and maples
2. Fuzzy seeds of dandelion, milkweed, willow and cottonwood fig 37.21
3. Dustlike seeds of orchids

4. Dispersal by water fig 37.22
1. Example: Coconut
2. Important to island colonization