28.1 Why Do Biologists Study Green Algae and Land Plants?

• An ecosystem consists of all the organisms in a particular area, along with physical components of the environment such as the atmosphere, precipitation, surface water, sunlight, soil, and nutrient

• Green algae and land plants provide ecosystem services because they enhance the life-supporting attributes of the atmosphere, soil, surface water, and other physical components of an ecosystem.

• By actively selecting individuals with the largest and most nutritious seeds, leaves, or other plant parts year after year, our ancestors gradually changed the characteristics of certain wild species. This process is called artificial selection

28.2 How Do Biologists Study Green Algae and Land Plants?

• To understand how green plants originated and diversified, biologists analyze

  • Morphological traits

  • the fossil record

  • phylogenetic trees estimated from similarities and differences in DNA sequences from homologous genes and whole genomes.

• Based on morphology, the land plants were traditionally clustered into three broad categories:

  • Nonvascular plants lack vascular tissue-specialized groups of cells that conduct water and nutrients from one part of the plant body to another

  • Seedless vascular plants have vascular tissue but do not make seeds.

  • Seed plants have vascular tissue, and they produce seeds.

• A seed consists of an embryo and a store of nutritive tissue, surrounded by a tough protective layer.

• The first seed plants to appear were gymnosperms

• The flowering plants, or angiosperms followed with flowers and seeds contained in fruits, which can aid in seed dispersal.

• Cuticle is a watertight barrier that coats the aboveground parts of today’ s land plants and helps them resist drying.

• Sporopollenin is a waxy substance (similar to cuticle) that encases the spores and pollen of modern land plants and helps them resist drying.

• The fossilized spores have been found in association with spore-producing structures, called sporangia

28.3 What Themes Occur in the Diversification of Land Plants?

• A stoma consists of an opening surrounded by specialized guard cells

• Natural selection favored early land plants with three main adaptations that solved the drying problem by:

  • preventing water loss, which kept cells from drying out and dying

  • providing protection from harmful ultraviolet (UV) radiation

  • moving water from tissues with direct access to water to tissues without direct access.

• Lignin is a complex polymer built from six-carbon rings. It is extraordinarily strong for its weight and is particularly effective in resisting compressing forces such as gravity.

• Tracheids are long, thin, tapering cells that have the following components:

  • A thickened, lignin-containing secondary cell wall in addition to a cellulose-based primary cell wall

  • Pits in the sides and ends of the cell where the secondary cell wall is absent, where water can flow efficiently from one tracheid to the next

• Vessel elements are shorter and wider than tracheids, and their upper and lower ends have perforations where both the primary and secondary cell walls are missing.

• In the stems and branches of some vascular plant species, tracheids or a combination of tracheids and vessels form the extremely strong support material called wood

• Three innovations that occurred early in land plant evolution were instrumental for efficient reproduction in a dry environment:

  • spores were produced that resist drying

  • gametes were produced in complex, multicellular structures

  • the embryo was retained on the parent (mother) plant and was nourished by it.

• The fossilized structures of early plants contain specialized reproductive organs called gametangia

• The sperm-producing gametangium is called an antheridium

• The egg-producing gametangium is called an archegonium

• When alternation of generations occurs, individuals exist in either a multicellular haploid or diploid phase.

• The multicellular haploid stage is called the gametophyte; the multicellular diploid stage is called the sporophyte.

• Heterospory is the production of two distinct types of spores by different structures

• Homospory is the production of a single type of spore.

• Microsporangia (singular: microsporangium) are spore-producing structures that produce microspores.

• Microspores develop into male gametophytes, which produce sperm by mitosis.

• Megasporangia (singular: megasporangium) are spore-producing structures that produce megaspores.

• Megaspores develop into female gametophytes, which produce eggs by mitosis.

• Seeds develop from an ovule

• In heterosporous seed plants, the microspore germinates to form a tiny male gametophyte that is surrounded by a tough coat of sporopollenin, resulting in a pollen grain.

• A stamen includes a structure called an anther, where microsporangia develop.

• A carpel contains a protective structure called an ovary, where the ovules are found.

• The involvement of two sperm nuclei is called double fertilization.

• Once stamens and carpels evolved, they became enclosed by modified leaves called sepals and petals.

• Pollination is the transfer of pollen from one plant’s stamen to another plant’ s carpel.

• Under the directed-pollination hypothesis, natural selection has favored flower scents, shapes, and colors that attract particular types of pollinators

• Flowers are attractive because they provide pollinators with food in the form of protein-rich pollen or a sugar rich fluid known as nectar.

• A fruit is a structure at is derived from the ovary and then closes one or more seeds

• An adaptive radiation occurs when a single lineage produces a large number of descendant species that are adapted to a wide variety of habitats

• A cotyledon (“seed-leaf”) stores nutrients and supplies them to the developing embryonic plant

28.4 Key Lineages of Green Algae and Land Plants

• Individuals are anchored to soil, rocks, or ee bark by structures called rhizoids.

• Angiosperms can be annual (have a single growing season) or perennial (live for many years), with life spans ranging from a few weeks to several thousand years.