PHYLUM ANTHOPHYTA

I. Anthophyta - commonly called the angiosperms or flowering plants

A. About 235,00 species, representing at least 90% of all true plants

B. They arose very abruptly in the fossil record about 130 million years ago and they quickly became the dominant plants on earth

C. Angiosperm = "vessel seed". The ovules became enclosed in a carpel, a megasporophyll which rolled up to enclose them. The carpel is the "vessel"

D. Major characteristics:

1. Flower = a determinate shoot of the sporophyte of a higher plant that is modified for reproduction and consists of a shortened axis bearing modified leaves

2. They have xylem vessels. Recall that vessels have perforate end walls and they are stacked upon each other to form long tubes throughout the plant body

3. Double fertilization - one sperm nucleus fuses with the egg, another sperm nucleus fuses with the polar nuclei to form the stored food (endosperm) within the seed

II. Two classes of angiosperms:

A. Monocotyledones (monocots)

1. About 65,000 species

2. Includes the grasses, sedges, lilies, irises, orchids and palms

B. Eudicotyledones (dicots) 

1. About 165,000 species

2. Includes most other flowering plants, including all woody flowering plants

III. Floral parts - Flower = a determinate shoot of the sporophyte of a higher plant that is modified for reproduction and consists of a shortened axis bearing modified leaves. Even the ancient Greeks recognized that floral parts were modified leaves

A "typical" flower

Click here to see many examples of flowers

A. Peduncle - the stalk of an inflorescence or a solitary flower

1. Pedicel - the stalk of an individual flower in an inflorescence.

B. Receptacle - the shoot tip upon which the floral parts are attached

C. Sepal - outermost part of the flower, typically but not always green in color

1. Calyx - collective term for all sepals

D. Petal - the structures in between the sepals and stamens. Usually showy and brightly colored

1. Corolla - collective term for the petals

E. Perianth - collective term for both the calyx and corolla

F. Stamen - the male structure. It consists of:

1. Anther - pollen sacs (microsporangia)

2. Filament - the stalk of the stamen.

3. Androecium - collective term for all the male parts of a flower

G. Pistil  - the female part. It may may be made of 1 carpel (simple) or many fused carpels (compound). It consists of:

1. Stigma - the tip upon which the pollen lands

2. Style - a column which connects the stigma to the ovary

3. Ovary - the part which contains the ovules

a. Gynoecium - collective term for all the female parts of a flower

b. Placentation - attachment of ovules within ovary. 

H. Many floral parts may be fused together or they may be lacking:

1. Complete flower - all 4 floral parts (sepals, petals, stamens, pistils) are present

2. Incomplete flower - 1 or more floral parts are lacking

3. Perfect (hermaphroditic) flower - has both sexual parts (stamens and pistils)

4. Imperfect flower - lacks one or more sexual parts:

a. Staminate flower - has only stamens

b. Pistillate flower - has only pistils

c. Imperfect flowers may be arranged in two ways:

Monoecious - unisexual flowers on same plant, e.g. corn, cattail

Dioecious - unisexual flowers on different plants, e.g. willows

I. Insertion of parts - describes the relative positions of the floral parts to the ovary:

1. Hypogynous (superior ovary) - other parts are attached below the ovary

2. Epigynous (inferior ovary) - other parts are attached above the ovary

3. Perigynous (superior ovary) - sepals, petals, and stamens are fused to form a cup around ovary

IV. Inflorescence = the arrangement of one or more flowers on an axis. Types of inflorescences:

A. Spike - sessile (stalkless) flowers on an axis

B. Raceme - the flowers are attached to the axis via stalks (pedicels)

C. Panicle - basically a compound raceme, i.e. the pedicels are branched

D. Corymb - a flat topped inflorescence in which the pedicels arise from different points along the peduncle

E. Umbel - a flat topped inflorescence in which the pedicels arise from the same point on the peduncle. Common in the carrot family

F. Head - an inflorescence with a large receptacle containing many small flowers. Occur primarily in the sunflower family

G. Catkin - a spike like inflorescence of unisexual flowers subtended by bracts. E.g. willow, oak or birch

V. Angiosperm reproduction

A. Pollination = transfer of pollen (carrying male gametophyte) from the anther to the stigma

B. Fertilization (syngamy) = union of sperm and egg

C. Male development:

1. Microsporogenesis = the formation of microspores within the microsporangia (pollen sacs). Within the developing anther sporogenous cells become microsporocytes which divide meiotically to produce 4 single celled microspores. These eventually develop a hard, resistant coat

2. Microgametogenesis = the development of microspores into microgametophytes (pollen grains). Each microspore divides mitotically to become 2 celled, a tube cell and a generative cell. The pollen may or may not be released at this time

D. Female development:

1. Megasporogenesis = formation of megaspores within the megasporangium. Within the megasporangium a single megasporocyte develops and divides meiotically to form four haploid megaspores. Three disintegrate leaving one functional megaspore

2. Megagametogenesis = the development of the megaspore into a megagametophyte. The one functional megaspore divides mitotically three times to form eight nuclei. These eight nuclei will arrange themselves:

a. 3 remain at the micropylar end. The egg, flanked by 2 synergids

b. 3 remain at the chalazal (opposite) end. They are called antipodals

c. 2 two migrate to the center of the megasporangium and they often fuse into one cell (polar cell). They are called polar nuclei

3. This eight nucleate, seven celled embryo sac is the mature female gametophyte

E. Following pollination the pollen tube grows down through the style and the generative cell divides to form two sperm nuclei

F. The pollen tube enters the ovule through the micropyle and one sperm fuses with the egg to form the zygote and the other sperm fuses with the two nucleate polar cell to form a primary endosperm nucleus

1. This is called double fertilization. It ensures that energy is not wasted forming stored food unless fertilization has taken place

VI. Seed development:

A. Primary endosperm nucleus divides many times mitotically to form endosperm (stored food)

B. Zygote forms the embryo, the young sporophyte

C. Integument forms the seed coat

D. Ovary wall (pericarp) and sometimes other structures develop into the fruit

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