An Introduction to Seed Plants

• The two groups of seed plants are the gymnosperms and the angiosperms.

  • Gymnosperms produce seeds that are totally exposed or borne on the scales of cones; an ovary wall does not surround the ovules of gymnosperms.

  • Angiosperms are flowering plants that produce their seeds within a fruit (a mature ovary).

Gymnosperms

• A pine tree is a mature sporophyte; pine gametophytes are extremely small and nutritionally dependent on the sporophyte generation.

  • Pine is heterosporous and produces microspores and megaspores in separate cones.

• Male cones produce microspores that develop into pollen grains (immature male gametophytes) that are carried by air currents to female cones.

  • Female cones produce megaspores.

  • One of each four megaspores produced by meiosis develops into a female gametophyte within an ovule (megasporangium).

• After pollination, the transfer of pollen to the female cones, a pollen tube grows through the megasporangium to the egg within the archegonium.

  • After fertilization, the zygote develops into an embryo encased inside a seed adapted for wind dispersal.

• Unlike bryophytes, gymnosperms are vascular plants.

  • Unlike bryophytes and ferns, gymnosperms produce seeds.

  • Gymnosperms also produce wind-borne pollen grains, a feature absent in ferns and other seedless vascular plants.

• Conifers (phylum Coniferophyta), the largest phylum of gymnosperms, are woody plants that bear needles (slender leaves that are usually evergreen) and produce seeds in cones.

  • Most conifers are monoecious and have male and female reproductive parts in separate cones on the same plant.

• Cycads (phylum Cycadophyta) are palmlike or fernlike in appearance.

  • They are dioecious—they have male and female reproductive structures on separate plants—but reproduce with pollen and seeds in conelike structures.

• Ginkgo biloba, the only surviving species in phylum Gink- gophyta, is a deciduous, dioecious tree.

  • The female ginkgo produces fleshy seeds directly on branches.

• Gnetophytes (phylum Gnetophyta) are an obscure clade of gymnosperms that has a few traits associated with angiosperms.

Flowering Plants

• Flowering plants, or angiosperms (phylum Anthophyta), constitute the phylum of vascular plants that produce flowers and seeds enclosed within a fruit.

  • They are the most diverse and most successful group of plants.

• The flower, which may contain sepals, petals, stamens, and carpels, functions in sexual reproduction.

  • Unlike those of gymnosperms, the ovules of flowering plants are enclosed within an ovary.

  • After fertilization, the ovules become seeds, and the ovary develops into a fruit.

• The sporophyte generation is dominant in flowering plants; gametophytes are extremely reduced in size and nutritionally dependent on the sporophyte generation.

  • Flowering plants are heterosporous and produce microspores and megaspores within the flower.

• Each microspore develops into a pollen grain (immature male gametophyte).

  • One of each four megaspores produced by meiosis develops into an embryo sac (female gametophyte).

  • Within the embryo sac, the egg cell and the central cell with two polar nuclei participate in fertilization.

• Double fertilization, which results in the formation of a diploid zygote and triploid endosperm, is characteristic of flowering plants.

• Most monocots (class Monocotyledones) have floral parts in threes, and their seeds each contain one cotyledon.

  • The nutritive tissue in their mature seeds is endosperm.

• Eudicots (class Eudicotyledones) usually have floral parts in fours or fives or multiples thereof, and their seeds each contain two cotyledons.

  • The nutritive organs in their mature seeds are usually the cotyledons, which have absorbed the nutrients in the endosperm.

• Flowering plants reproduce sexually by forming flowers.

  • After double fertilization, seeds form within fruits.

  • Flowering plants have efficient water-conducting vessel elements in their xylem and efficient carbohydrate-conducting sieve tube elements in their phloem.

  • Wind, water, insects, or other animals transfer pollen grains in various flowering plants.

The Evolution of Seed Plants

• Seed plants arose from seedless vascular plants.

  • Progymnosperms were seedless vascular plants that had megaphylls and “modern” woody tissue.

  • Progymnosperms probably gave rise to conifers as well as to seed ferns, which in turn likely gave rise to cycads and ginkgo.

• The evolution of the gnetophytes is unclear, although molecular data indicate that they are closely related to conifers.

• Flowering plants probably descended from ancient gymnosperms that had specialized features, such as leaves with broad, expanded blades and closed carpels.

  • Flowering plants likely arose only once; that is, there is only one line of evolution from the gymnosperms to the flowering plants.