2.6.4: Chapter Summary

Seeds represent one of the most important innovations in plant evolution: a protected, nutrient-supplied embryo with the ability to await appropriate conditions for germination. Seeds and pollen allowed plants to limit their reliance on water for completion of their life cycle. The first plants to evolve seeds were the gymnosperms. Angiosperms then improved upon seed dispersal and pollination strategies with the evolution of fruits and flowers.

In gymnosperms, protective seeds filled with nutritive tissue (including the megagametophtye) replace spores as the dispersal mechanism. Antheridia are lost in the microgametophyte, which is reduced to four cells and is dispersed as a whole (pollen). The evolution of secondary growth allows for the lateral deposition of woody tissues. This latter development, along with xerophytic leaves, allows gymnosperms to tolerate a wide variety of new environmental stressors.

Cycads and ginkgos are more ancestral lineages of gymnosperms. Ginkgos are represented by a single surviving species: Ginkgo biloba. Cycads are primarily tropical and usually have large pinnately compound leaves. Like the Ginkgos, seeds and pollen are produced in strobili produced on separate plants. Gnetophytes are a group of gymnosperms that have convergently evolved several characteristics with angiosperms: fruit-like cones, vessel elements, and double fertilization. However, it is likely that they are a highly-derived group within the conifers, sister to the pine family! Most extant gymnosperms are conifers; this group includes some of Earth's oldest, largest, and longest lived organisms.

The majority of plant species are angiosperms. This is the most recent lineage of plants and, though its origins are murky, its members appear to form a monophyletic clade. Within the angiosperms, there are two major groups: monocots and dicots (the latter of which can be further divided into some early diverging angiosperm lineages and the eudicots). Angiosperms have the most reduced gametophytes of all plant lineages: the mature microgametophyte is composed of just 2 cells, while the megagametophyte has 7 cells (with 8 nuclei!). These gametophytes are housed within flowers, structures composed of highly modified leaves specialized for pollination.

Flowers are composed of a series of concentric whorls. The outermost whorl is the calyx, composed of sepals. Inside the calyx is the corolla, which is composed of petals. Together, the calyx and corolla comprise the perianth, which is usually the showy part of the flower in animal-pollinated species. The internal whorls are the fertile whorls. The androecium is composed of stamens, formed from a filament and anther (microsporangia are produced within the anthers). The gynoecium is composed of carpels, formed from an ovary, style, and stigma. The stigma is where pollen is received. The pollen must grow a pollen tube from the stigma to the ovary, traveling down the style to get there. Within the ovary, ovules are housed (megasporangia are produced within the ovules).

During pollination, the generative cell of the pollen produces two spermatia. One spermatium fertilizes the zygote, as in most plants, but the second spermatium fertilizes a dikaryotic cell called the central cell, forming a triploid endosperm. This event is called double fertilization. After fertilization, the ovule becomes a seed and the ovary begins to develop into a fruit.

Other adaptations within this group include specialized vascular cells: vessel elements in the xylem and sieve tube elements with companion cells in the xylem.

After completing this chapter, you should be able to...

• Explain how heterospory led to the evolution of seeds.
• Connect r- and k-strategies to propagules produced by plants (e.g. spores, gametes, pollen, and seeds).
• List a few ways gymnosperms and angiosperms differ in life history traits.
• Describe the shared derived characteristics of gymnosperms.
• Connect these adaptations to the stressors this group of plants would have faced.
• Use morphological characteristics and life history traits to distinguish between ginkgos and flowering plants.
• Explain why Ginkgo biloba is called a living fossil.
• Use morphological characteristics and life history traits to distinguish between cycads and ferns.
• Define the term dioecious and provide an example.
• Use morphological features and life history traits to distinguish conifers from other plants.
• Connect the adaptations of conifers to dry and/or cold environments.
• Identify structures and phases in the Pinus life cycle; know their ploidy.
• Use morphological features and life history traits to distinguish gnetophytes from other plants.
• Describe the traits gnetophytes share with angiosperms.
• Describe the shared derived characteristics of angiosperms.
• Connect these characteristics to the stressors these plants would have faced.
• Identify the components of a flower and to which whorl each belongs.
• Write and interpret floral formulas.
• Differentiate between flowers and inflorescences.
• Explain the difference between raceme-based and cymose inflorescences.
• Identify the components of a flower and to which whorl each belongs.
• Write and interpret floral formulas.
• Differentiate between flowers and inflorescences.
• Explain the difference between raceme-based and cymose inflorescences.

Attribution

Content by Maria Morrow, CC BY-NC