The Evolution of Seed Plants

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Flowers - Specialized for Sexual Reproduction

The flower is an angiosperm structure specialized for sexual reproduction. It is a specialized shoot with up to four types of modified leaves:

Sepals - enclose the flower

Petals - brightly colored and attract pollinators

Stamens - produce pollen on their terminal anthers

Carpels - consist of an ovary containing ovules at the base and a style holding up a stigma, where pollen is received.

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Structure of an Idealized Flower

Carpel

Ovule

Sepal

Petal

Stigma

Style

Ovary

Stamen

Anther

Filament

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Fruits

A fruit typically consists of a mature ovary but can also include other flower parts.

Fruits protect seeds and aid in seed dispersal.

Mature fruits can be either fleshy or dry.

Various fruit adaptations help disperse seeds by wind, water, or animals to new locations.

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Fruits

Hazelnut

Ruby grapefruit

Tomato

Nectarine

Milkweed

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Fruit Adaptations for Seed Dispersal

Barbs

Seeds within berries

Wings

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The Angiosperm Life Cycle

The flower of the sporophyte is composed of both male and female structures.

Male gametophytes are contained within pollen grains produced by the microsporangia of anthers.

The female gametophyte = embryo sac, develops within an ovule contained within an ovary at the base of a stigma.

Most flowers have mechanisms to ensure cross-pollination between flowers from different plants of the same species.

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A pollen grain that has landed on a stigma germinates and the pollen tube of the male gametophyte grows down to the ovary.

Sperm enter the ovule through a pore opening called the micropyle.

Double fertilization occurs when the pollen tube discharges two sperm into the female gametophyte within an ovule.

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One sperm fertilizes the egg forming a zygote.

The other sperm combines with two nuclei and initiates development of food-storing endosperm.

The endosperm nourishes the developing embryo.

Within a seed, the embryo consists of a root and two seed leaves called cotyledons.

Double Fertilization: Produces

Zygote 2n and endosperm (food) 3n

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