Topic 7: Angiosperms, Flowers and Pollination Syndromes – Phylum Anthophyta (Chs. 30, 38)

I. Phylum Anthophyta – flowering plants (antho – flower)

A. also known as angiosperms (angeion – vessel or enclosure; sperma – seed)

B. ovules enclosed within carpel (parent diploid sporophytic tissue) at pollination

1. the “vessel” is the carpel, which is a modified leaf

2. carpels, especially their enlarged basal portion (the ovary), usually develop into fruit, which is unique to angiosperms

C. about 250,000 known living species (dominant photosynthetic organisms on land)

D. predominant source of human food

E. most widespread and diverse plant phylum

1. range from microscopic to plants with leaves over 6 m long

2. flowers show incredible variety from species to species

3. variety of lifestyles includes parasites (ex.: mistletoe, dodder, beechdrops); mycotrophs (derive nutrients from fungi; ex.: Indian Pipe, others); epiphytes (ex.: some orchids); “carnivorous” (ex. pitcher plants, sundews, Venus flytrap)

F. monophyletic group with seeds, refined xylem, double fertilization, and these synapomorphic characteristics:

1. seed contains endosperm

2. presence of flowers (modified stems and leaves)

3. true fruits

G. evolutionary history

1. monophyletic group

2. origin about 140 MYA

H. phylogeny

1. historically divided into two classes, dicots and monocots

· recent genetic analysis has shown that the traditional dicots are a paraphyletic group

· thus, the old classification scheme is being replaced

2. no conclusive cladogram has been produced for angiosperms

· studies are ongoing

· most modern cladograms have Amborella and water lilies as a sister group (or groups) to the rest of the angiosperms

· cladogram below from http://tolweb.org/tree?group=Angiosperms&contgroup=Spermatopsida

· various class-level groupings have been proposed, the overall naming and formal classification within Phylum Anthophyta is still in a state of flux

· nevertheless, by far most of the living angiosperm species are found within two monophyletic groups, eudicots and monocots

3. eudicots

· most have embryos have two cotyledons (seed leaves)

· leaves have netlike veins

· flower part typically in multiples of 4 or 5

· groups of vascular tissues in a ring

· pollen grains mostly with 3 or more apertures

· endosperm mostly used up in mature eudicot seeds

· about 175,000 living species; includes nearly all flowering trees and shrubs

· about a sixth are annuals (entire growth cycle in one year or less)

4. monocots

· embryos have one cotyledon

· leaves have essentially parallel veins

· flower part typically in multiples of 3

· groups of vascular tissues scattered

· pollen grains mostly with one aperture

· endosperm typically present in mature monocot seeds

· about 65,000 living species; no true wood, few annuals

II. Why were (and are) angiosperms successful?

A. 130 MYA two major continental masses

1. Laurasia = North America, Europe, Asia

2. Gondwanaland = South America, Africa, Australia, Antarctica, India, New Zealand)

B. angiosperms first appeared in Gondwanaland, in what was likely a drier interior region

C. advantages of flowering plants

1. transfer of pollen over great distances promotes outcrossing

2. efficient seed dispersal via fruit

3. endosperm gives seedlings a fast start

4. leaves appropriate for fast growth in hot, dry environment

D. coevolution with insects

1. dominant by ~80 MYA, second half of Cretaceous Period

2. all present angiosperm families represented by that time

3. many insect orders appeared or became more abundant at that time

III. Flowers

A. modified stems with modified leaves

B. develop as primordium bud at end of stalk called pedicel

C. pedicel widens at tip to form receptacle

D. other flower parts attached to receptacle in four whorls; from outside in:

1. calyx – sepals; usually green, leaf-like, and protect immature flower

2. corolla – petals; usually colorful, attract pollinators; together with calyx called perianth

3. androecium – stamens; male reproductive structures

· filament + anther

· microspores produced within anther, shed as pollen

4. gynoecium – female reproductive structure

· center location is most protected

· formed from leaf-like structure with ovules along margin

· edges fold inwards around ovules, forming carpels

§ primitive: many separate carpels

§ advanced: carpels fused (called pistil)

· carpel/pistil segments

§ ovary – swollen base with 1 to hundreds of ovules; develops into fruit

§ stigma – tip; sticky and/or feathery to catch pollen

§ style – usually present; separates stigma from ovary

· nectaries may be present at base of pistil; secrete sugar, amino acids, and other compounds to attract pollinators

E. know the structures of a flower [Figure 38.2] and their functions

IV. typical Angiosperm life cycle

A. female gametophyte

1. single diploid megaspore mother cell in ovule undergoes meiosis while flower develops

2. of 4 haploid megaspores produced, usually 3 break down

3. remaining megaspore expands and replicates and divides until there are 8 haploid nuclei in two groups of 4

4. one nuclei from each group migrates toward center; these are polar nuclei

5. polar nuclei usually fuse to make a diploid nucleus, but may remain separate – in either case, they wind up in a single cell

6. cell walls form around other nuclei, creating the 7-celled, 8-nucleate embryo sac or megagametophyte (female gametophyte)

7. meanwhile, two layers (integuments) of ovule develop into seed coat with micropyle (small opening)

8. in the megagametophyte, one of the cells closest to the micropyle becomes the egg; the other two there are synergids

9. the three cells on the other end (the antipodals) eventually break down

B. male gametophyte

1. anthers with patches of tissue that become chambers lined with nutritive cells

2. each patch has many diploid microspore mother cells

3. microspore mother cell undergoes meiosis, making 4 haploid microspores that typically remain grouped in a tetrad

4. each microspore nucleus replicates and divides once (via mitosis) without cytokinesis (meaning they remain as one cell with two nuclei, a binucleate microspore)

5. usually, tetrad then breaks up

6. two-layered wall develops around each binucleate microspore, now called a pollen grain

· outer wall – sculptured, appearance usually species-specific, often has chemicals that can react with an appropriate stigma to stimulate pollen tube formation

· apertures in outer wall – where pollen tube may grow out; eudicots – usually 3; monocots – usually 1

C. pollination – transfer of pollen to a stigma

1. usually between flowers of separate plants

2. agents include wind, water, gravity, mammals, birds, insects

3. various reward systems for animal agents (pollen, nectar, etc.)

4. evolution of floral characteristics associated with pollination

5. some plants self-pollinate (inbreeding) – pollen to same plant

6. pollination followed by fertilization only if chemical signals are right

D. fertilization

1. pollen grain cytoplasm absorbs substances from stigma

2. bulge forms through an aperture in pollen grain; becomes pollen tube

3. pollen tube follows chemical gradient through style to micropyle

· chemicals diffuse from embryo sac

· micropyle usually reached within a few days (up to a year in some species)

4. pollen grain has two nuclei; one, the generative nucleus, lags behind

5. generative nucleus undergoes mitosis to make two non-flagellated sperm; this may occur in pollen grain or in pollen tube (male gametophyte now mature)

6. pollen tube enters embryo sac, destroying a synergid

7. double fertilization – essentially unique to angiosperms

· one sperm unites with egg, forming zygote

· other sperm unites with polar nuclei, forming 3N primary endosperm

· primary endosperm rapidly undergoes many cycles of mitosis, forming endosperm

· endosperm provides nutrients for embryo; in many seeds, it is gone by the time the seed is mature

· seed coat hardens

· remaining haploid cells degenerate

· now have seed with 2N embryo, 3N endosperm, and 2N seed coat (seed coat from parent female tissue)

V. Seeds

A. embryo – quickly forms all systems, then growth arrested (dormancy) – mature seed about 10% water, very low metabolic activity

B. typically, dormancy occurs just after first leaves (cotyledons, or seed leaves) form

C. stored food (in angiosperms, 3N endosperm and/or cotyledons)

D. seed coat – tough, relatively impermeable

1. protection from predators, pathogens

2. protection from desiccation, harsh conditions (crucial on land)

3. may allow seed to last hundreds of years

E. dormancy broken only when conditions are right (seed bank in soil)

F. germination = breaking dormancy = resuming metabolic activity, growing out of seed coat; occurs after water penetrates seed coat to embryo, bringing oxygen

VI. Fruits – mature ovaries

A. fleshy – pomes (apples), drupes (peaches), true berries (blueberries, peppers), hesperidiums (oranges), pepos (melons, gourds), aggregate fruits (strawberries, raspberries), multiple fruits (pineapple, fig)

B. dry – follicles (milkweed, magnolia), legumes (peas, beans), siliques and silicles (mustards), capsules (irises, lilies, orchids), caryopses (grasses), nuts (chestnuts, hazelnuts, acorns), achenes (sunflowers), samaras (maples, elms, ashes), schizocarps (parsleys)

C. dispersal

1. by wind

· wings – maples

· parachutes – dandelions, milkweeds

· dust-like seeds – orchids

2. by water – coconuts

3. by vertebrates

· fleshy, edible fruits (blue, black, red) – seeds often deposited in feces

· dry, edible – nuts, others – squirrels bury and forget about them

· dry, inedible – hooks to grasp hair, feathers (cockleburs, etc.)

4. by explosive dehiscence (jewelweed, others)

VII. many angiosperms also have asexual (or vegetative) reproduction

A. stolons – runners – long slender stems that grow along soil (ex.: strawberry)

B. rhizomes – underground stems – common in grasses; bulbs and tubers are rhizomes specialized for storage (ex.: potato)

C. suckers – roots produce sprouts that grow into new plants (ex.: apple, raspberry, banana)

D. adventitious leaves – numerous plantlets develop from tissue in notches along leaves

E. apomixes – embryos in seeds may be produced asexually

F. artificial: cuttings (for some species, can get roots to grow with appropriate environment)

VIII.Floral Evolution

A. first flowers

1. numerous spirally arranged sepals, petals, stamens, and carpels

2. petals and sepals similar in color and form

3. all parts free (not fused)

B. parts

1. complete – calyx + corolla + androecium + gynoecium

2. incomplete – one or more whorls absent

3. perfect – has both androecium and gynoecium

4. imperfect – missing either androecium or gynoecium

5. complete flowers are always perfect; incomplete flowers can be either perfect or imperfect

C. trends

1. separate floral parts grouped together or fused

· connation – fusing within a whorl

· adnation – fusing between whorls (for example, sepals and petals fused together)

2. reduction or loss of floral parts

3. bilateral symmetry instead of radial symmetry

· ancestral type: radial symmetry; example: buttercups

· derived type: bilateral symmetry; examples: snapdragons, orchids

· bilateral symmetry in some cases has arisen independently in different groups

IX. Pollination mechanisms (pollination syndromes)

A. wind – passive, primitive (oaks, cottonwoods, birches, grasses)

1. copious amounts of pollen

2. most pollen travels no more than 100 m

3. flowers small, greenish, odorless

4. corollas reduced or absent

5. often grouped in large numbers, may hang down with tassels that wave in wind and shed pollen freely

6. male and female parts often well-separated on plant to reduce chance of self-pollination

7. often flower before leaves grow – keeps leaves out of the way

B. animals – some cycads and gnetophytes also have this, so symplesiomorphic trait

1. bees – most numerous of insect-pollinated plants use bees

· find via odor

· orient via shape, color, and texture

· usually blue or yellow flowers, bee sees in ultraviolet

· many have stripes or lines of dots to indicate nectaries (nectar guides)

· nectar offered as food for bees (pollen also)

· often close association between a bee species and a plant species

§ flower only open when bees are active

§ pollen collecting apparatus specific for particular plant

2. other insects

· butterflies – flower usually has flat landing platform and long, slender floral tubes for long proboscis

· moths – flower usually white, yellow, or other pale color, heavily scented, typically need to be found at night

· flies – flower usually smells and somewhat appears like feces or rotting meat

· beetle – large flowers, copious pollen; beetle may eat other flower parts

3. birds

· large amounts of nectar

· red – bees can’t see red, less likely to feed on the copious nectar

· usually odorless – birds have a poor sense of smell

· often in long, thick tube

4. mammals (bats especially) – uncommon, but for some species is the only means of pollination; variety of appearances

C. self-pollination

1. small, inconspicuous flowers

2. shed pollen directly onto stigma (or falls there by gravity); often before bud opens

3. advantageous occasionally because no other plant is needed and no vector is needed – good when pollinators aren’t around (Artic, mountains)

4. if you are well-adapted, might as well produce clones

5. disadvantage of genetic load of bad mutations

X. Promoting outcrossing

A. staminate and pistillate flowers

B. dioecious – separate sexes

C. monoecious

1. dichogamous – stamens and pistils reach maturity at different times

2. stigma and stamens don’t touch (includes heterostyle)

3. genetic self-incompatibility – pollen tube arrested or never germinates