The Seed Plants

The most recent group to appear in the evolution of the tracheophytes is the seed plants. The earliest fossil evidence of seed plants is found in Devonian rocks. The earliest seed plants combined characteristics of rhyniophytes and heterosporous ferns, but they had tracheids of the type found in modern seed plants. They also differed from the plants around them by having extensively thickened woody

A Forest Ablaze Fires like this one in a northern Arizona forest can pose dangers to human life and property. But they play an essential role in the life cycles of many fire-adapted seed plants.

A Forest Ablaze Fires like this one in a northern Arizona forest can pose dangers to human life and property. But they play an essential role in the life cycles of many fire-adapted seed plants.

Ferns Adapted Forests

stems, which resulted from the proliferation of xylem. This type of growth in the diameter of stems and roots is called secondary growth. By the Carboniferous period, new lines of seed plants had evolved, including various seed ferns, which possessed fernlike foliage but had seeds attached to their leaves.

Two clades of early seed plants are known only as fossils. These clades are basal to the surviving seed plants, which fall into two groups, the gym-nosperms (such as pines and cycads) and the an-giosperms (flowering plants). There are four living phyla of gymnosperms and one of angiosperms (Figure 30.1). The phylogenetic relationships among these five clades have not yet been resolved. All living gymnosperms and many angiosperms show secondary growth. The life cycles of all seed plants share major features, as we are about to see.

Four Living Phyla Gymnosperms
30.1 The Phyla of Living Seed Plants There are four phyla of gymnosperms and one of angiosperms.Their exact evolutionary relationship is still uncertain.

Seed plants are heterosporous and have tiny gametophytes

In seed plants, the gametophyte generation is reduced even further than it is in the ferns (Figure 30.2). The haploid ga-metophyte develops partly or entirely while attached to and nutritionally dependent on the diploid sporophyte.

Among the seed plants, only the earliest types of gym-nosperms (and their few survivors) had swimming sperm. All other seed plants have evolved other means of bringing eggs and sperm together. The culmination of this striking

Sporophyte (2n)

evolutionary trend was independence from the liquid water that earlier plants needed for sexual reproduction.

Seed plants are heterosporous (see Figure 29.14b). They form separate megasporangia and microsporangia on structures that are grouped on short axes, such as the cones and strobili of conifers and the flowers of angiosperms.

As in other plants, the spores of seed plants are produced by meiosis within the sporangia, but in seed plants, the megas-pores are not shed. Instead, they develop into female gameto-

30.2 The Relationship between Sporophyte and Gametophyte Has Evolved In the course of plant evolution, the gametophyte has been reduced and the sporophyte has become more prominent.

Sporophyte (2n)

Plant Evolution

Gametophyte (n)

Female gametophyte (n)

Male gametophytes (n)

Anther Ovary

Gametophyte (n)

30.2 The Relationship between Sporophyte and Gametophyte Has Evolved In the course of plant evolution, the gametophyte has been reduced and the sporophyte has become more prominent.

Female gametophyte (n)

Male gametophytes (n)

Conifer Sporophyte And Gametophyte

phytes within the megasporangia. These megagametophytes are dependent on the sporophyte for food and water.

In most seed plant species, only one of the meiotic products in a megasporangium survives. The surviving haploid nucleus divides mitotically, and the resulting cells divide again to produce a multicellular female gametophyte. This megagametophyte is retained within the megasporangium, where it matures. The megagametophyte, in turn, houses the early development of the next sporophyte generation following fertilization of the egg. The megasporangium is surrounded by sterile sporophytic structures that form a protective integument.

Within the microsporangium, the meiotic products are microspores, which divide mitotically within the spore wall one or a few times to form a male gametophyte called a pollen grain. Pollen grains are released from the microsporangium to be distributed by wind, an insect, a bird, or a plant breeder (Figure 30.3). A pollen grain that reaches the appropriate surface of a sporophyte of the same species develops further. It produces a slender pollen tube that elongates and digests its way through the sporophytic tissue toward the female gametophyte.

When the tip of the pollen tube reaches the female game-tophyte, sperm are released from the tube, and fertilization occurs. The resulting diploid zygote divides repeatedly, forming a young sporophyte that develops to an embryonic stage at which growth is temporarily suspended (often referred to as a dormant stage). The end product at this stage is a multi-cellular seed.

The seed is a complex package

A seed may contain tissues from three generations. The seed coat develops from tissues of the diploid sporophyte parent (the integument). Within the megasporangium is the haploid female gametophytic tissue from the next generation, which contains a supply of nutrients for the developing embryo. (This tissue is fairly extensive in most gymnosperm seeds. In angiosperm seeds its place is taken by a tissue called endosperm, which we will describe below.) In the center of the seed is the third generation, the embryo of the new diploid sporophyte.

The seed of a gymnosperm or an angiosperm is a well-protected resting stage. The seeds of some species may remain viable (capable of growth and development) for many years, germinating when conditions are favorable for the growth of the sporophyte. In contrast, the embryos of non-seed plants develop directly into sporophytes, which either survive or die, depending on environmental conditions; there is no dormant stage in the life cycle.

During the dormant stage, the seed coat protects the embryo from excessive drying and may also protect it against potential predators that would otherwise eat the embryo and its nutrient reserves. Many seeds have structural adaptations that promote their dispersal by wind or, more often, by animals. When the young sporophyte resumes growth, it draws on the food reserves in the seed. The possession of seeds is a major reason for the enormous evolutionary success of the seed plants, which are the dominant life forms of Earth's modern terrestrial flora in most areas.

Larix Decidua Pollen Cones

The wind carries pollen from a pollen strobilus...

to a seed cone.

Larix decidua

30.3 Pollen Grains Pollen grains are the male gametophytes of seed plants. Conifers have strobili, which produce and release pollen. Their pollen is dispersed by the wind to cones, which contain female gametophytes.

The wind carries pollen from a pollen strobilus...

to a seed cone.

Larix decidua

30.3 Pollen Grains Pollen grains are the male gametophytes of seed plants. Conifers have strobili, which produce and release pollen. Their pollen is dispersed by the wind to cones, which contain female gametophytes.

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Responses

  • pamela
    Why gametophyte is reduce in plant as they evolved?
    8 years ago
  • veronica
    Why did the gametophyte become reduced in the seed plant?
    8 years ago
  • cirillo
    When did heterospory evolve on clade?
    8 years ago
  • Vanna
    Do all angiosperms and gymnosperms exhibit heterospory?
    8 years ago
  • rudigar
    When did heterospory evolve plants?
    7 years ago

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