Plant Diversity II: Plant Diversity II:  The Evolution of Seed PlantsThe Evolution of Seed Plants

Overview of seed plant evolution

There were three (3) major reproductive adaptations in the evolution of seed plants.

1) Reduction of gametophyte (Gametophytes of seed plants are microscopic, so they can exist within the sporophyte).

2) Seeds became an important means of dispersal.

3) Pollen (Sperm cells) eliminated the liquid-water requirement for fertilization.

1) Reduction of the gametophyte continued with the evolution of seed plants.

In seed plants, the very small female (1N) gametophyte develops from spores retained in the sporangia of the (2N) sporophyte. Why??

Delicate female gametophytes are protected from environmental stress.

Embryos that are produced are also protected from stress.                       

Three variations on gametophyte/sporophyte relationships

2) Seeds became an important means of dispersal.                        Seeds are resistant to environmental stress, and are multi-cellular complex structures that consist of a sporophyte embryo, a food supply, and a protective coat. The embryo develops from the fertilized egg (derived from a spore) retained within the sporangium.

The sperm (derived from a spore) is carried (produced by) a pollen grain released from a sporangium. 

All seed plants are heterosporous.

Female spores give rise to female gametes; male spores give rise to male gametes.                                    The megasporangia produce megaspores (female) and the microsporangia produce microspores (male). The megasporangium is enveloped by layers of tissue called integuments.  The whole structure (integuments + megasporangium) is called the ovule.

From ovule to seed.

3) Pollen (with sperm cells) eliminated the liquid-water requirement for fertilization.                        Pollen travels by air or on animals.  It eliminates the need for water to be present during fertilization.

Hypothetical phylogeny of the seed plants.

Note- two clades for seed plants.

Gymnosperms

Gymnosperms are vascular plants that bear naked seeds – seeds not enclosed in specialized chambers. 

Mesozoic era was the age of gymnosperms

Gymnosperms were the most common plants during the Mesozoic era (the age of dinosaurs). Four phyla of extant gymnosperms 

– Phylum Ginkgophyta: e.g., Ginkgo biloba.– Phylum Cycadophyta: e.g., cycads.– Phylum Gnetophyta: e.g., ephedra– Phylum Coniferophyta: e.g., pines, firs, spruces

Life cycle of a pine demonstrates the key reproductive adaptations of seed plants                          1.  The tree is the sporophyte. 2.  Female gametophyte develops within the sporangium. 3.  Pollen cone has microsporangium that develops into pollen (male gametophyte). 4.  After fertilization, the embryo develops and is surrounded by food reserves and a seed coat. 5.  Embryo grows to produce a new sporophyte.

The life cycle of a pine.

Winged seed of a White Pine (Pinus

strobus)

Angiosperms are flowering plants that form seeds inside a protective chamber called an ovary.

Examples: 

Monocots: Orchids, lilies, grasses, palms, bamboo

(Eu)dicots: Daisies, maples, snapdragon, pea, oaks

Angiosperms (Flowering Plants)

Systematists are identifying angiosperm clades.

Originally, only:                        Monocots are angiosperms that possess one embryonic seed leaf (cotyledon). Dicots are angiosperms that possess two embryonic seed leaves (cotyledons). However, angiosperms have recently been divided into new taxonomic groups.

A comparison of monocots and dicots

The flower is the defining reproductive adaptation of angiosperms                        Flowers are made up of four types of modified leaves sepals, petals, stamens, and carpels.                                    a.  Stamens are the male reproductive organs that produce microspores. b.  Carpels are female reproductive organs that produce megaspores. 

The structure of a flower.

A fruit is a mature ovary                                   Protect dormant seeds. Fruit aids in seed dispersal.                     - Wind dispersal                     - Attachment and transportation                     - Consumption – berries contain seeds to be

passed in feces 

Fruit adaptations that enhance seed dispersal

Life cycle of an angiosperm                        Mature plant is the sporophyte Some plants easily self-pollinate, but most have mechanisms to ensure cross-pollination.   Male gametophytes reach female gametophytes by producing a pollen tube. 

Next slide: The life cycle of an angiosperm.

Angiosperms dominated the earth at the end of the Mesozoic era                        Radiation of angiosperms represents the transition from Mesozoic to Cenozoic Angiosperms and animals have affected one another’s evolution                        Coevolution is the mutual influence on the evolution of two different species interacting with each other and reciprocally influencing each other’s adaptations.                                    e.g., Pollinator-plant relationships

Plants and Human Welfare

Agriculture is almost totally dependent on angiosperms. Plant diversity is a non-renewable resource.                        Many medicines are obtained from plant materials.  

Deforestation is an international practice

Deforestation in the United

States

Fragmentation of a forest ecosystem

A sampling of medicines derived from plants