Chapter 28: Seed Plants
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.