Flowering Plants 4/13/00

Flowering plants are final major plant group we will discuss.

1. Flowering plants are a group of the seed plants.

2. Among the seed plants, flowering plants are most closely related to the Gnetales.

Earliest fossils of flowering plants date form early Cretaceous (135 mya)

1. Although they are presumed to have originated sometime before that, probably during the Jurassic period.

2. This hypothesis is based on extrapolation.

Flowering plants rapidly diversify during the Cretaceous

1. During early cretaceous we can recognize a few families of flowering plants that are still extant.

2. By the end of the Cretaceous many of the extant families of flowering plants have probably evolved.

Concomitant with the rise of flowering plant diversity we see decline in diversity of most other groups (although conifer diversification largely just levels off).

We'll start introducing you to attributes of flowering plants by introducing features of flowers.

Flower is specialized strobilus of flowering plants.

1. flower is a simple strobilus.

2. flowers never compound strobilus like those of Gnetales and female cones of conifers.

3. being simple strobilus and never compound means that flowers consist only of central axis with sporophylls

We can characterize a conventional flower as a strobilus with three regions:

1. Basal part of flower consists of a set of sterile leaves (also sometimes called bracts) that form region called perianth.

--the sporophylls of flower always form above the perianth in the strobilus.

2. Directly above the perianth along the strobilus of a bisexual (hermaphroditic) flower are the microsporophylls.

a. The microsporophylls of flowering plants are called stamens.

b. The region of the microsporophylls (i.e., stamens) is called the androecium.

3. Directly above the androecium along the strobilus of a bisexual flower are the megasporophylls.

a. The megasporophylls of flowering plants are called carpels.

b. The region of the megasporophylls (carpels) of flowering plants is called the gynoecium.

c. Some flowers unisexual-will lack either androecium or gynoecium.

Perianth composed of sterile appendages that serve to protect and enclose the flower as it develops.

1. The developing flower is called a flower bud.

2. When the flower bud nears maturity the appendages that compose the perianth expand greatly during a short interval and "open" outward.

3. This opening of flower is called "anthesis".

4. In an open flower, the perianth often has a display function.

5. Appendages of the perianth often large and brightly colored.

6. These appendages may also have structures that produced fragrance or structures that produce oils or sugars that serve as rewards to visiting pollinators.

The perianth may consist of appendages that all have the same basic size, shape, and appearance.

1. Such a perianth is said to be "undifferentiated".

2. The individual appendages of an undifferentiated perianth are called tepals.

Differentiated perianths usually consist of two sets of sterile appendages.

1. The lowermost (i.e., outermost) are called sepals (collectively sepals=calyx).

2. The uppermost (i.e., inside set of sepals) are called petals.(collectively petals=corolla).

Some flowers have lost their perianth during the evolutionary diversification of flowering plants.

1. This is often associated with a change from animal pollination vectors to wind or water pollination.

2. Little selection for an attractive perianth without animal pollination.

3. With wind and water pollination, the presence of perianth thought to actually hinder ability to pollen to reach carpels that contain the ovules.

The microsporophylls are called stamens and form a region of flower called androecium.

1. Stamens thus bear microsporangia.

2. Microsporangia of flowering plants are called pollen sacs.

3. Each stamen usually bears 4 microsporangia.

4. The stamens of most flowering plants are differentiated to have a stalk-like basal part called a filament and a broader apical part bearing the pollen sacs called the anther.

5. Since pollen sacs are microsporangia, this is where the microsporocytes that will undergo meiosis to form microspores

--which lead to development of pollen.

The megasporophylls are called carpels and form region of flower called gynoecium.

1. The carpel is a closed structure that bears ovules in its internal space.

2. Flowering plants differ from other seed plants because of their possession of the carpel.

3. We can thus say that all flowering plants share the derived feature of the carpel.

--and this is one attribute that helps us to hypothesize that flowering plants form an evolutionary group.

4. Because the carpel is a closed structure, it prevents pollen from being deposited directly on the ovule at pollination.

5. The carpel has a specialized region on its surface called the stigma.

6. Pollen grains are deposited on the stigma at pollination.

7. Pollen tubes formed by the growth of the male gametophyte must thus grow from the stigma to the ovules.

8. The ovules are contained in a part of the carpel called the ovary.

9. In most flowering plants, carpels are differentiated such that they have elongate stalk-like region called the style that separates the stigma from the ovary.

Appendages that compose perianth, androecium, and gynoecium may be positioned helically along strobilus axis.

--This axis along which floral appendages are attached is called a receptacle.

- we observed this flower organization in Magnolia

Floral strobilus usually becomes more condensed (shorter) as serialization of appendages evolves.

More evolved flowers have simplified parts. This involves, for example several carpels being unified into one ovary. Petals can be unified into a cup form, and stamens can become stuck to the petals as in fuchsia.

Adnation occurs when separate appendages of different series become unified.

1. e.g., when set of sepals and petals become partly or wholly unified we call it adnate.

2. or when set of stamens and petals become partly or wholly unified is adnate.

Consider carpel and ovule structure:

Recall megasporophylls of cycads that had lamina with ovules along margins.

--Carpel much like such a megasporophyll folded with margin including ovules folded to inside.

Ovules of most flowering plants have 2 integuments.

--those ovules with only 1 integument are not primitive among flowering plants--appears to be reversal within this group

Inside nucellus is single megasporocyte.

1. megasporocyte undergoes meiosis to form 4 megaspores.

2. 3 megaspores degrade and 1 persists.

3. nucleus of megaspore undergoes set of divisions

4. division result in 8 nuclei inside megaspore cell.

5. cells eventually form around these 8 nuclei such that female gametophyte finally consists of 7 cells.

6. Female gametophyte consists of 3 cells at basal end (away form micropyle) called antipodal cells.

7. Also 3 cells at distal end (nearest micropyle) composing "egg apparatus".

8. cells of egg apparatus include 2 synergids and 1 egg.

9. Between antipodals and egg apparatus is largest cell called central cell which has 2 nuclei called polar nuclei.

Thus, although female gametophyte consists of 7 cells, it includes 8 nuclei.

The female gametophyte of flowering plants is much simpler than all other female gametophytes that we have discussed.

-- Simplest female gametophyte is among seed plants.

Microsporocytes formed in pollen sacs (microsporangia) of anther

-- meiosis of sporocytes results in tetrad of microspores.

Microspores usually separate from tetrad before development of male gametophyte begins inside of microspore (endosporous development)

Cells in male gametophyte include:

Male gametophyte of flowering plants consists of only 3 cells at its most complex.

--simplest male gametophyte we have encountered among embryophytes.

As mentioned before, area of pollen wall through which pollen tube emerges at germination is called aperture.

Great diversity among flowering plants in pollen grain apertures.

1. There may one or many apertures (colpi) on a pollen grain

2. pollen tube will emerge form one of the colpi.

Already noted that pollination is process of transferring pollen from microsporangium to ovule

--or in flowering plants this involves release of pollen from pollen sacs of stamens and transfer to stigma of carpel.

Let's consider manifestations of pollination.

1. cross-pollination: when carpel of one individual receives pollen from different individual of same pollination.

2. self-pollination: when carpel of individual receives pollen from same individual.

--since pollen and ovule of same individual have same genetic composition this form of pollination does not enhance genetic variation of population.

Important to recognition, however, transfer of pollen from one individual to another might be very difficult or even unlikely in certain circumstances.

--Thus, self-pollination may be important means of facilitating reproduction in certain circumstances.

Pollen tube grows from stigma, through style, and ovary tissue to ovule.

--Grows between cells and not through them.

Pollen tube enters micropyle and grows through nucellus.

Pollen tube enters embryo sac through one synergid.

--synergid is first cell that pollen tube actually enters.

Tube ruptures discharging two sperm and vegetative nucleus.

1. vegetative nucleus presumed to degrade.

2. one sperm fuses with egg and forms diploid zygote.

3. second sperm fuses with two polar nuclei of central cell.

a. this second fusion leads to 3N or triploid chromosomal complement of new nucleus.

b. triploid cells proliferate and form tissue known as endosperm.

Thus double fertilization in flowering plants results in:

1. sperm and egg forming zygote which leads to embryo

2. sperm and two polar nuclei forming triploid nucleus of initial endospermal cell.

Zygote develops into embryo and as embryo develops it uses endosperm as nutritive tissue.

Note difference between flowering plants and other seed plants.

1. Flowering plants have a very simple female gametophyte (7-celled) and have evolved a special nutritive tissue that begins to develop only after successful fertilization.

2. Nonflowering seed plants had a more extensive female gametophyte that served as nutritive tissue prior to evolution of endosperm.

Seed of flowering plant develops from ovule just as in other seed plants.

Seed has hard outer layer called seed coat.

--seed coat develops from integument in which cells gain thickened walls.

Seed of flowering plant includes tissues of parental sporophyte and offspring sporophyte as well as endosperm.

1. Tissues from parental sporophyte basically in integument (seed coat)

--Nucellus usually breaks down and degrades and not present in mature seed.

2. Offspring sporophyte present as embryo.

3. Endosperm usually surrounds the embryo.

No female gametophyte tissue left in seed of flowering plant unlike the seeds of other seed plants.