The Gymnosperms Naked Seeds

The extant gymnosperms are a clade of seed plants that do not form flowers. Although there are probably fewer than 750 species of living gymnosperms, these plants are second only to the angiosperms in their dominance of the terrestrial environment.

There are four clades of living gymnosperms today. The cycads (phylum Cy-cadophyta) are palmlike plants of the Tropics and Subtropics, growing as tall as 20 meters (Figure 30.4a). Of the present-day gymnosperms, the cycads are probably closest to the earliest seed plants. Ginkgos (phylum Ginkgophyta), which were common during the Meso-zoic era, are represented today by a single genus and species, Ginkgo biloba, the maidenhair tree (Figure 30.4b). There are both male (microsporangiate) and female (megasporangiate) maidenhair trees. The difference is determined by X and Y sex chromosomes, as in humans; few other plants have sex chromosomes. The phylum Gnetophyta consists of three very different genera that share certain characteristics with

Example GnetophytaSex Maidenhair Tree

(d) Sequoiadendron giganteum

(b) Ginkgo biloba

(d) Sequoiadendron giganteum

(b) Ginkgo biloba

Gnetophyte

(c) Welwitschia mirabilis

30.4 Diversity among the Gymnosperms (a) Many cycads, such as this palmlike tree, have growth forms that resemble both ferns and palms. (b) The characteristic fleshy seed coat and broad leaves of the maidenhair tree. (c) A gnetophyte growing in the Namib Desert of Africa.Two huge, straplike leaves grow throughout the life of the plant, breaking and splitting as they grow. (d) Conifers, like this giant sequoia growing in Sequoia National Park, California, dominate many modern forests.

(c) Welwitschia mirabilis

30.4 Diversity among the Gymnosperms (a) Many cycads, such as this palmlike tree, have growth forms that resemble both ferns and palms. (b) The characteristic fleshy seed coat and broad leaves of the maidenhair tree. (c) A gnetophyte growing in the Namib Desert of Africa.Two huge, straplike leaves grow throughout the life of the plant, breaking and splitting as they grow. (d) Conifers, like this giant sequoia growing in Sequoia National Park, California, dominate many modern forests.

the angiosperms. One of the gnetophytes is Welwitschia (Figure 30.4c), a long-lived desert plant with just two straplike leaves that sprawl on the sand and can grow as long as 3 meters. By far the most abundant of the gymnosperms are the conifers (phylum Pinophyta), cone-bearing plants such as pines and redwoods (Figure 30.4d).

All living gymnosperms except the Gnetophyta have only tracheids as water-conducting and support cells in their xylem; they lack the more specialized vessels and fibers found alongside tracheids in the angiosperms. Although this difference may make the gymnosperm water transport and support system seem less efficient than that of the angiosperms, it serves some of the largest trees known. The coast redwoods of California are the tallest gymnosperms; the largest are well over 100 m tall. Secondary xylem— wood—produced by gymnosperms is the principal resource of the timber industry.

During the Permian period, the conifers and cycads flourished. Gymnosperm forests changed over time as the gym-nosperm groups evolved. Gymnosperms dominated the

Mesozoic era, during which the continents drifted apart and dinosaurs strode the Earth. They were the principal trees in all forests until less than 100 million years ago, and they still dominate many present-day forests. Let's look at the most abundant gymnosperms, the conifers, in more detail.

Conifers have cones but no motile cells

The great Douglas fir and cedar forests of the northwestern United States and the massive boreal forests of pine, fir, and spruce found in northern regions of Eurasia and North America, as well as on the upper slopes of mountain ranges everywhere, rank among the great vegetation formations of the world. All these trees belong to one phylum of gymnosperms, Pinophyta—the conifers, or cone-bearers. A cone is a short axis (a modified stem) bearing a tight cluster of scales, which are reduced branches specialized for reproduction (Figure 30.5a). A strobilus is a conelike cluster of scales that are modified leaves inserted on an axis (Figure 30.5b) . Megaspores are produced in seed cones, and microspores

(a) Pinus resinosa

Seed cones

(a) Pinus resinosa

Seed cones

Strobilus Betina Pinus

30.5 Cones and Strobili (a) The scales of seed cones are modified branches. (b) The spore-bearing structures in pollen strobili are modified leaves.

are produced in pollen strobili. Seed cones are much larger than pollen strobili.

We will use the life cycle of a pine to illustrate reproduction in gymnosperms (Figure 30.6). The production of male gametophytes in the form of pollen grains frees the plant completely from its dependence on liquid water for fertilization. Instead of water, wind assists conifer pollen grains in their first stage of travel from the strobilus to the female gametophyte inside the seed cone (see Figure 30.3). The pollen tube provides the sperm with the means for the last stage of travel by elongating and digesting its way through maternal sporophytic tissue. When it reaches the female ga-metophyte, it releases two sperm, one of which degenerates after the other unites with an egg.

The megasporangium, in which the female gametophyte will form, is enclosed in a layer of sporophytic tissue—the in-tegument—that will eventually develop into the seed coat. The integument, the megasporangium inside it, and the tissue attaching it to the maternal sporophyte constitute the ovule. The pollen grain enters through a small opening in the integument at the tip of the ovule, the micropyle.

Gymnosperms derive their name (which means "naked-seeded") from the fact that their ovules and seeds are not protected by ovary or fruit tissue. Most conifer ovules (which, upon fertilization, develop into seeds) are borne exposed on the upper surfaces of the modified branches that form the scales of the cone. Each cone scale lies in the angle between a modified leaf and the axis. The only protection of the ovules comes from the scales, which are tightly pressed against each other within the cone. As we have seen, some pines, such as the lodgepole pine, have such tightly closed seed cones that only fire suffices to split them open and release the seeds.

About half of the conifer species have soft, fleshy fruitlike tissues associated with their seeds; examples are the fleshy cones or "berries" of juniper and yew. Animals may eat these tissues and then disperse the seeds in their feces, often carrying them considerable distances from the parent plant. These tissues, however, are not true fruits, which are characteristic of the plant phylum that is dominant today: the angiosperms.

30.5 Cones and Strobili (a) The scales of seed cones are modified branches. (b) The spore-bearing structures in pollen strobili are modified leaves.

The same plant has both pollen-producing strobili and egg-producing cones.

The same plant has both pollen-producing strobili and egg-producing cones.

Scale of

Section seed cone through i scale

30.6 The Life Cycle of a Pine Tree In conifers and other gymnosperms, the gametophytes are microscopically small and nutritionally dependent on the sporophyte generation.

Scale of

Section seed cone through i scale

Ovule Megasporangium

Ovule Megasporangium

I Functional megaspore

Pine Megaspore And Microspore

Microspore mother cells

Meiosis

Pollen chamber

Microspore mother cells

Seed coat - Female gametophyte

Section Scale of pollen thr°u8h strobilus scale

DIPLOID (2n) Sporophyte generation

Microspores

HAPLOID (n)

Gametophyte generation

Pollen grain

Reduced archegonium

Seed coat - Female gametophyte

Section Scale of pollen thr°u8h strobilus scale

DIPLOID (2n) Sporophyte generation

HAPLOID (n)

Gametophyte generation

Pollen grain

Reduced archegonium

Pollen Generation

Seed cone

Female gametophyte

I Functional megaspore

Pollen chamber

Ultrastucture Female Gametohyte
Female gametophyte

Male gametophyte (germinating pollen grain)

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  • petra
    Is a Egg within a pine archegonium haploid or diploid?
    8 years ago

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