BLY 121: General Survey of Higher Plants

Higher Plants: Spermatophyta

• Higher plants are classified under the Division Spermatophyta, characterized by the production of seeds.
• Spermatophytes are the most highly developed and evolved of all plants.

Characteristics of Higher Plants

• Spermatophytes produce seeds, which are specialized structures for reproduction.
• They exhibit alternation of generation, characterized by the alternation of two generations (gametophyte and sporophyte) within the life cycle.
• Many members of Spermatophyta are trees and shrubs.
• They are not dependent on water for sexual reproduction, possessing pollen tubes that transport male gametes to female gametes.
• Spermatophytes are characterized by the phenomenon of pollination, a process in their sexual reproduction.

Gymnosperms

• Gymnosperms are a major subdivision within the Division Spermatophyta, less highly developed than the Angiosperms.
• They are characterized by producing seeds in the naked form, without being enclosed within pericarps.
• Examples of gymnosperms include cycads and conifers.
• Cycads are tropical palm-like trees that form symbiotic associations with nitrogen-fixing bacteria.
• Conifers are often large and can dominate plant life in some ecosystems, with stems that continue to expand in width and length throughout their life.
• Conifer leaves are needle or scale-like.

Life Cycle of Gymnosperms

• Gymnosperms are spore-bearing plants (sporophytes) with a sporophyte-dominant life cycle.
• Two spore types, microspores and megaspores, are produced in pollen cones or ovulate cones, respectively.
• A short-lived multicellular haploid gamete-bearing phase (gametophyte) develops inside the spore wall.
• Pollen grains (microgametophytes) mature from microspores and produce sperm cells.
• Megagametophyte tissue develops in the megaspore of each ovule and produces multiple egg cells.
• During pollination, pollen grains are physically transferred between plants, from pollen cone to ovule.
• Two main modes of fertilization are found in gymnosperms: motile sperm that swim directly to the egg inside the ovule, and sperm with no flagella that are conveyed to the egg along a pollen tube.

Angiosperms

• Angiosperms, or flowering plants, are one of the major groups of extant seed plants, with at least 260,000 living species classified in 453 families.
• Angiosperms occupy every habitat on Earth except extreme environments such as the highest mountaintops, the regions immediately surrounding the poles, and the deepest oceans.
• They are crucial for human existence, as the vast majority of the world's crops are angiosperms, as are most natural clothing fibers.

Characteristics of Angiosperms

• Angiosperms are clearly united by shared, derived features, including:
  • Ovules that are enclosed within a carpel (a structure made up of an ovary, which encloses the ovules, and the stigma, where pollen germination takes place).
  • Double fertilization, which leads to the formation of an endosperm (a nutritive tissue within the seed that feeds the developing plant embryo).
  • Stamens with two pairs of pollen sacs.
  • Features of gametophyte structure and development.
  • Phloem tissue composed of sieve tubes and companion cells.

Angiosperm Longevity

• Angiosperms vary widely in longevity, with some living hundreds of years (trees, for instance) while others die after only one season.
• Life cycles are measured in a circular fashion, from seed to seed.
• Annuals live for one growing season or one year, biennials live for two years or growing seasons, and perennials live for three or more growing seasons.

Annuals, Biennials, and Perennials

• Annuals sprout from a seed, creating vegetative growth, flowering, and producing seeds before dying.
• Biennials sprout from a seed, creating vegetative growth, but instead of flowering, they create food storage organs like bulbs or tubers to fuel future growth.
• Perennials may be woody or herbaceous, sprouting from seed, growing, and flowering, with the stems of herbaceous perennials dying over the winter and the roots surviving, while the woody perennials go into dormancy over the winter and renew their growth the following spring.