21: Seed Plants
- Page ID
- 124022
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Fossil from the Devonian period reveal fernlike plants that were heterosporous; that is, produced two kinds of spores: microspores and megaspores. The megaspores were not released from the parent sporophyte. Fertilization took place within the tissue of the parent sporophyte thus freed from dependence on surface water. However, the necessity for the microspores to be carried from one plant to another in order to reach the megagametophyte robbed them of their value as agents of dispersal. This function was taken over by seeds - dormant, protected, embryo sporophytes.
- 21.1: Introduction to Seed Plants
- 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, which grew wider and taller with secondary growth. Angiosperms then improved upon seed dispersal and pollination strategies with the evolution of fruits and flowers.
- 21.2: Gymnosperms
- 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.
- 21.3: Cycads and Ginkos
- Cycads and ginkgos are more ancestral lineages of gymnosperms. Ginkgos are represented by a single surviving species: Ginkgo biloba. These trees have fan-shaped, deciduous leaves. Paired ovules with fleshy coats are produced on separate trees than microstrobili. 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.
- 21.4: Conifers and Gnetophytes
- 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! Conifers are monoecious: seeds and pollen are produced in separate cones on the same plant. Most extant gymnosperms are conifers; this group includes some of Earth's oldest, largest, and longest lived organisms.
- 21.5: Angiosperms
- Angiosperms are plants that produce flowers and fruits. Within the ovule, double fertilization results in the formation of both the zygote and endosperm. New specialized cells are present in the vascular tissue. Meanwhile, the gametophytes are further reduced and archegonia are lost altogether. Nearly 90% of all plants belong to this group.
- 21.6: Flowers
- Flowers are specialized reproductive structures produced by angiosperms. These structures are composed of highly modified leaves in distinct whorls. The sterile whorls, the calyx and corolla, comprise the perianth. Pollen is produced by the androecium and ovules are produced in the gynoecium. Floral formulas are used to describe the composition and morphology of these whorls. An inflorescence involves the production of multiple florets in place of a flower.
- 21.7: Angiosperm Life Cycle
- Angiosperms have a complex life cycle. The microgametophyte is reduced to 2 cells, while the megagametophyte is now 7 cells and 8 nuclei. Ovules develop within the ovary or ovaries of the gynoecium. Production of gametophytes and fertilization happens within the flower. The spermatia each fertilize a cell within the ovule (double fertilization), one of which will grow into the embryo. Seeds are protected by the ovary wall, which becomes the fruit, a structure specialized for seed dispersal.
- 21.8: Chapter Summary
- A brief summary of the concepts covered in chapter 7.