Lecture 20: Flowering Plant Innovations

Understand the angiosperm life cycle major features

1. the megagametophyte is highly reduced, comprimising an embryo sac with just 7 cells and 8 nuclei

2. double fertilization, where one sperm fuses with the egg to give rise to a diploid zygote, and the second sperm fuses with the to polar nuclei of the polar cell to give rise to the triploid endosperm

3. the zygote divides by mitosis to form the embryo; this is the baby sporophyte plant that enters a dormant state after embryogenesis

4. the ovule is surrounded by an ovary (the base of the carpel), which develops into fruit after fertilization

the composition of the embryo sac includes:

1. three antipodal cells

2. two synergiid cells (that flank the egg and guide formation of the pollen tube)

3. the single egg cell (that gives rise to the diploid zygote when fertilized)

4. the single large polar cell (with two haploid polar nuclei, which gives rise to the triploid endosperm when fertilized)

double fertilization

• where one sperm fuses with the egg to give rise to a diploid zygote, and the second sperm fuses with the to polar nuclei of the polar cell to give rise to the triploid endosperm

  • the tube-cell nucleus is chemically guided by the synergic cells to form the pollen tube

  • the tube cell takes out one of the synergid cells when it reaches the embryo sac

  • on esperm nucleus fuses with the egg nucleus cell to give rise to the diploid zygote

  • the other sperm nucleus fuses with the two polar nuclei of the polar cell to give rise to the triploid endosperm

Flowering Plant Embryogenesis

1. the diploid, single-celled zygote divides by mitosis within the embryo sac to give rise to a suspensor (which is like an umbilical cord) and the embryo proper

2. which develops into a torpedo stage that includes the shoot apex (the primordial shoot apical meristem), the root apex (the primordial root apical meristem), and one or more cotyledons (“seed leaves”, which absorb nutrients from the surrounding endosperm)

3. Simultaneously, the triploid polar cell divides by mitosis to give rise to the endosperm that surrounds the embryo and completely fills the inner space within the integument of the ovule

The Angiosperm Seed includes 3 main components

1. the diploid embryo (the product of the first fertilization event)

2. the triploid endosperm (the product of the second fertilization event)

3. the diploid seed coat (derived from the diploid integument)

the seeds of all Seed Plants contain…

a diploid e,bryo (the baby sporophyte) and are encased in a seed coat (derived from the integument of the parent sporophyte plant).

the ploidy of the nutritive tissue surrounding the embryo in gymnosperms is

haploid from the megagametophyte

the ploidy of the nutritive tissue surrounding the embryo in angiosperms is

triploid from endosperm

innovations like non-motile sperm, vessel elements, and double fertilization exhibit…

homoplasy (these features evolved independently between angiosperm and gymnosperm

the flower is comprimised of of…

four concentric whorls (rings) of modified leaves

The flower is the organ of Flowering Plants that houses the…

Sporangia and therefore heterosporous

what does it mean that the flower is heterosporous

flowers are the organs where the microsporangia and megasporangia are located

the two outermost whorls of a flower are

• sterile (they do not bear spores)

• and collectively comprimise the perianth, which includes an outer whorl of sepals (collectively calyx), and an inner whorl of petals (collectively corolla)

the two inner whorl of the flower are

-sporophylls (spore-bearing leaves)

-there is an outer of stamens, which include a basal shift (the filaments) and apical microsporangia (anthers)

-within the anthers are microsporocytes; these are the single diploid cells that undergo meiosis to give rise to the male (micro)gametophyte, pollen

the pollen grain of a flowering plant is more/less reduced than a gymnosperm?

more reduced

what is the pollen grain of a Flowering Plant comprimised of?

only two cells: an outer tube cell that contains the second cell, the generative cell

the flowering plant pollen grain is enclosed in

a layer of sporopollenin

the innermost whorl of the flower consists of

carpels, which include the apical stigma, a neck-like style, and a basal ovary

what is the apical stigma

the pollen landing pad

what is the neck-like style

connects the stigma to the ovary and allows the stigma to extend out of the corolla

basal ovary

surround the ovule

what does the ovule contain

includes integuments and megasporangium within it

what does the megasporangium contain?

a megasporocyte

what is a megasporocyte

the single diploid cell that undergoes meiosis to give rise to the megaspore

how many of the megaspores survive?

only one of the four survive; the other three experience apoptosis

what happens to the megaspore after it’s created?

undergoes three mitotic divisions to give rise to a highly reduced female megagametophyte

what is a megagametophyte?

an embryo sac

how were the carpel and stamen thought to evolve?

from leaves that bore ovules or sporangia

how are angiosperm mainly pollinated

by animals because they are more efficient pollen-transfer vectors

what is the function of the flower

to attract animal pollinator by

-offering a reward for their services (such as nectar)

-tricking them into visiting the flower (eg, mimicking sexually receptive pollinators)

the xylem includes two new cell types (in addition to the tracheid cells)

• vessel elements increase the efficiency of water and mineral transport

• fiber cells are structural elements that aid in supporting the plant body

• note: eveolved independently for gymnosperm

flower types:

• perfect flower

• imperfect flower

  • monoecious

  • dioecious

perfect flower

has both stamens (organ with the microsporangia) and carpels (organ with the megasporangia)

imperfect flower

either has only stamens (called staminate flower) or only carpels (called carpellate flower)

monoecious

• “one-house” with seperate rooms

• an imperfect flower

• a single plant has both seperate imperfect staminate and imperfect carpellate flowers

dioecious

• “two-house”

• a single staminate plant either has imperfect staminate flowers or has imperfect carpellate flowers

• but not both

floral variations

flowers may be grouped together to produce an inflorescence

there are several types of inflorescences

• umbel

• compound umbel

• spike

• head

Inflorescence

a group of flowers borne on a branch

umbel

an inflorescence comprimised of flowers stemming from short stalks (pedicels) that radiate from a common point

ex: onions

compound umbel

an inflorescence comprimised of a cluster of umbels stemming from stalks (rays) that radiate from a common point

ex: carrots

spike

Inflorescence comprimised of a cluster of flowers along an unbranched axis

ex: bunny grass

head

inforescence comprimised of an outer ring of sterile ray florets and a central cluster of disc florets

ex: sunflowers

fruit develops…

by elaboration of the ovary (that surrounds the seed) after fertilization

fruit adaptive values

• it provides additional protection for the embryo

• it facilitates dispersal of the seed (by biotic and abiotic factors)

name the multiple kinds of fruits

1. drupe

2. berry

3. aggregate fruit

4. multiple fruits

5. accessory fruits

6. legumes

7. achenes

8. samara

drupe

a simple fruit that is derived from a single flower with one ovary that contains one ovule

ex: cherries, plums

berry

-a fleshy fruit that is derived from a single flower with multiple sepreate ovaries

-the fruit forms by glomming together the fruitlets from each of the seperate ovaries in a single flower

-as the fruit ripens, these ripened ovaries aggregate (fuse) together

-ex: blackberries

multiple fruits

develop from many seperate florets of an inflorescence

ex: pineapples

accessory fruits

develop from parts of the plant other than carpels and seeds

ex: strawberries

legumes

non-fleshy fruit that develops from one carpel with many ovules, where the fruit splits lengthwise into two halves; each ovule is a bean

ex: green beans and peanuts

achenes

dry fruit that contains a single seed

ex: sunflower “seeds”

samara

an achene with wings

ex: hops (beer) and maple “helicopter”