Chapter 31: Seed Plants

Gnetophyta

Gnetophytes.Heterosporous. Sperm not motile; conducted to egg by a pollen tube. The only gymnosperms with vessels. Trees, shrubs, vines. Three very diverse genera (Ephedra, Gnetum, Welwitschia).

Water is not required to transport the male gametophyte to the female gametophyte

A pollen grain is a multicellular male gametophyte carrying the male gamete, a sperm cell.

Horizontal gene transfer occurred in land plants

Amborella trichopoda is the closest living relative to the earliest angiosperms, yet 20 out of 31 of its known mitochondrial protein genes hopped into the mitochondrial genome from other land plants through horizontal gene transfer (HGT).

Flowers house the gametophyte generation of angiosperms

Flowers are considered to be modified stems bearing modified leaves.

Gymnosperms: Plants with "Naked Seeds"

Gymnosperms encompass four of the five lineages of seed plants. The four groups of gymnosperms are the coniferophytes, cycadophytes, gnetophytes, and ginkgophytes. The fifth lineage of seed plants is the angiosperms. Although both gymnosperms and angiosperms produce seeds, only the angiosperms produce flowers and fruits (table 31.1). In gymnosperms, the ovule, which becomes a seed, rests exposed on a scale (a modified shoot or leaf) and is not completely enclosed by sporophyte tissues at the time of pollination. The name gymnosperm means "naked seed" because the seed develops on the surface of the scale.

Examples of Some Kinds of Fruits

Legumes and samaras are examples of dry fruits. Legumes open to release their seeds, but samaras do not. Drupes and true berries are simple fleshy fruits; they develop from a flower with a single pistil composed of one or more carpels. Aggregate and multiple fruits are compound fleshy fruits; they develop from flowers with more than one pistil or from more than one flower.

The Evolution of Seed Plants

Numerous evolutionary solutions to terrestrial challenges have resulted in over 400,000 species of seed plants dominating all terrestrial communities today.

Angiosperms: The Flowering Plants

Over 300,000 known species of flowering plants are called angiosperms because their ovules, unlike those of gymnosperms, are enclosed within diploid tissues at the time of pollination. The carpel, a modified leaf that encapsulates seeds, develops into the fruit, a unique angiosperm feature (figure 31.5). Although some gymnosperms, including the yew (Taxus Page 608species), have fleshlike tissue around their seeds, it is of a different origin and not a true fruit.

Specialized seed adaptations improve survival

Seeds of other plants germinate only when inhibitory chemicals leach from their seed coats, thus guaranteeing their germination when sufficient water is available. Still other seeds germinate only after they pass through the intestines of birds or mammals or are regurgitated by them, which both weakens the seed coats and ensures dispersal.

Only one species of the ginkgophytes remains extant

The fossil record indicates that members of the ginkgophytes (phylum Ginkgophyta) were once widely distributed, particularly in the northern hemisphere; today, only one living species, Ginkgo biloba, remains (figure 31.4c). This tree, which sheds its leaves in the fall, was first encountered by Europeans in cultivation in Japan and China; it apparently no longer exists in the wild. Like the sperm of cycads, those of Ginkgo have flagella. The ginkgo is dioecious—that is, the male and female reproductive structures are produced on separate trees. The fleshy outer coverings of the seeds of female ginkgo plants exude the foul smell of rancid butter, caused by the presence of butyric and isobutyric acids, which are also found in butter and vomit. As a result, male plants vegetatively propagated from shoots are preferred for cultivation. Because of its beauty and resistance to air pollution, Ginkgo is commonly planted along city streets.

Seeds Protect the Embryo

The seed is a vehicle for dispersing the embryo to distant sites. Being encased in the protective layers of a seed allows a plant embryo to survive in environments that might kill a mature plant. Seeds are an important adaptation in at least four ways: Seeds maintain dormancy under unfavorable conditions and postpone development until better conditions arise. Seeds afford maximum protection to the young plant at its most vulnerable stage of development. Seeds contain stored food that allows a young plant to grow and develop before photosynthetic activity begins. Perhaps most important, seeds are adapted for dispersal, facilitating the migration of plant genotypes into new habitats. A mature seed contains only about 5 to 20% water. Under these conditions, the seed and the young plant within it are very stable; its arrested growth is primarily due to the progressive and severe desiccation of the embryo and the associated reduction in metabolic activity. Germination cannot take place until water and oxygen reach the embryo. Seeds of some plants have been known to remain viable for hundreds and, in rare instances, thousands of years.

Fruits are Adapted for Dispersal

Three layers of ovary wall, also called the pericarp, can have distinct fates, which account for the diversity of fruit. Fruits contain three genotypes in one package. The fruit and seed coat are from the prior sporophyte generation. The embryo represents the next sporophyte generation (see figure 31.16). Finally, the endosperm is a transient, triploid product of fertilization.

Reproductive Structures

A pair of microsporangia form as sacs within each scale. Numerous microspore mother cells in the microsporangia undergo meiosis, each becoming four microspores. The microspores divide by mitosis to produce four-celled pollen grains, each with a pair of air sacs that give them added buoyancy when released into the air. A single cluster of male pine cones may produce more than a million pollen grains. Page 606 Female pine cones typically are produced on the upper branches of the same tree that produces male cones. Female cones are larger than male cones, and their scales become woody. Two ovules develop toward the base of each scale. Each ovule contains a megasporangium called the nucellus. The nucellus itself is completely surrounded by a thick layer of cells called the integument that has a small opening (the micropyle) toward one end. One of the layers of the integument later becomes the seed coat. A single megaspore mother cell within each megasporangium undergoes meiosis, becoming a row of four megaspores. Three of the megaspores break down, but the remaining one, over the better part of a year, slowly develops into a female gametophyte through mitotic divisions.

Learning Outcomes Review 31.3

Amborella, the closest living relative of the first angiosperms, provides clues to the origins of this very successful group. Although angiosperms are a clade, horizontal gene transfer has contributed genes from distantly related plant species. Angiosperms are characterized by ovules that at pollination are enclosed within an ovary at the base of a carpel, a structure unique to the phylum; the ovary develops into a fruit. Evolutionary innovations of angiosperms include flowers to attract pollinators, fruits to protect embryos and aid in their dispersal, rapid pollen tube growth, and double fertilization, which provides endosperm to help nourish the embryo.

Learning Outcomes Review 31.5

As a seed develops, the pericarp layers of the ovary wall develop into the fruit. A berry has a fleshy pericarp; a legume has a dry pericarp that opens to release seeds; the outer layers of a drupe pericarp are fleshy; and a samara is a dry structure with a wing. Animals often distribute the seeds of fleshy fruits and fruits with spines or hooks. Wind disperses lightweight seeds and samaras.

Coniferophyta

Conifers (including pines, spruces, firs, yews, redwoods, and others). Heterosporous seed plants. Sperm not motile; conducted to egg by a pollen tube. Leaves mostly needlelike or scalelike. Trees, shrubs. About 68 genera. Many produce seeds in cones

Cycadophyta

Cycads. Heterosporous. Sperm flagellated and motile but confined within a pollen tube that grows to the vicinity of the egg. Palmlike plants with pinnate leaves. Secondary growth slow compared with that of the conifers. Ten genera. Seeds in cones.

Fertilization and Seed Formation

Female cones usually take two or more seasons to mature. At first they may be reddish or purplish in color, but they soon turn green, and during the first spring, the scales spread apart. While the scales are open, pollen grains carried by the wind drift down between them, some catching in sticky fluid oozing out of the micropyle. The pollen grains within the sticky fluid are slowly drawn down through the micropyle to the top of the nucellus, and the scales close shortly thereafter.

Fire Induces Seed Release in Some Pines

Fire can destroy adult jack pines, but stimulate growth of the next generation. a. The cones of a jack pine are tightly sealed and cannot release the seeds protected by the scales. b. High temperatures lead to the release of the seeds.

Anthophyta

Flowering plants (angiosperms). Heterosporous. Sperm not motile; conducted to egg by a pollen tube. Seeds enclosed within a fruit. Leaves greatly varied in size and form. Herbs, vines, shrubs, trees. About 14,000 genera.

The Seed Protects the Embryo

From an evolutionary and ecological perspective, the seed represents an important advance. The embryo is protected by an extra layer or two of sporophyte tissue called the integument, creating the ovule (figure 31.1). Within the ovule, meiosis occurs in the megasporangium, producing a haploid megaspore. The megaspore divides by mitosis to produce a female gametophyte carrying the female gamete, an egg. The egg combines with the male gamete, a sperm, resulting in the zygote. The single-celled zygote divides by mitosis to produce the young sporophyte, an embryo. Seeds also contain a food supply for the developing embryo.

Ginkgophyta

Ginkgo. Heterosporous. Sperm flagellated and motile but conducted to the vicinity of the egg by a pollen tube. Deciduous tree with fan-shaped leaves that have evenly forking veins. Seeds resemble a small plum with fleshy, foul-smelling outer covering. One genus.

Learning Outcomes Review 31.2

Gymnosperms are mostly cone-bearing seed plants. In gymnosperms, the ovules are not completely enclosed by sporophyte tissue at pollination, and thus have "naked seeds." The four groups of gymnosperms are conifers, cycads, gnetophytes, and ginkgophytes.

Pine Morphology

Pines have tough, needle-like leaves produced mostly in clusters of two to five. Among the conifers, only pines have clustered leaves. The leaves, which have a thick cuticle and recessed stomata, represent an evolutionary adaptation for minimizing water loss. This strategy is important because many of the trees grow in areas where the topsoil is frozen for part of the year, making it difficult for the roots to obtain water. The leaves and other parts of the sporophyte have canals into which surrounding cells secrete resin. The resin deters insect and fungal attacks. The resin of certain pines is harvested commercially for its volatile liquid portion, called turpentine, and for the solid rosin, which is used on bowed stringed instruments. The wood of pines lacks some of the more rigid cell types found in other trees, and it is considered a "soft" rather than a "hard" wood. The thick bark of pines is an adaptation for surviving fires and subzero temperatures.

Pollination and the male gametophyte

Pollination is simply the mechanical transfer of pollen from its source (an anther) to a receptive area (the stigma of a flowering plant). Most pollination takes place between flowers of different plants and is brought about by wind, water, gravity, insects, and other animals. In some angiosperms, plants may pollinate themselves. This is called self-pollination. If the stigma is receptive, then the pollen grain's cytoplasm absorbs substances from the stigma and bulges through a pore in the pollen wall. The bulge develops into a pollen tube that responds to chemical and mechanical stimuli that guide it through the style and into the micropyle (opening) of an ovule. to the embryo sac. The pollen tube usually takes several hours to two days to reach the micropyle, but in a few instances, the journey may take up to a year. Pollen tube growth is more rapid in angiosperms than gymnosperms. Rapid pollen tube growth rate is an innovation that is believed to have preceded fruit development and been essential in the origins of angiosperms (figure 31.13)

Fruits

Survival of angiosperm embryos depends on fruit development as well as seed development. Fruits are most simply defined as mature ovaries (carpels). During seed formation, the ovary begins to develop into fruit (figure 31.16).

Fruit Development

The carpel (ovary) wall is composed of three layers: the exocarp, mesocarp, and endocarp. One, some, or all of these layers develops to contribute to the recognized fruit in different species. The seed matures within this developing fruit.

Fruits allow angiosperms to colonize large areas

The diversity of fruit types contributes to an array of dispersal methods. Fruits with fleshy coverings normally are dispersed by birds or other vertebrates (figure 31.18a). Like red flowers, red fruits signal an abundant food supply. By feeding on these fruits, birds and other animals may carry seeds from place to place and thus transfer plants from one suitable habitat to another. Such seeds require a hard seed coat to resist stomach acids and digestive enzymes. Water dispersal adaptations include air-filled chambers surrounded by impermeable membranes to prevent waterlogging. Coconuts and other plants that characteristically occur on or near beaches are regularly spread throughout a region by floating in water (figure 31.18d). This sort of dispersal is especially important in the colonization of distant island groups, such as the Hawaiian Islands. It has been calculated that the seeds of about 175 angiosperms, nearly one-third from North America, must have reached Hawaii to evolve into the roughly 970 species found there today. Some of these seeds blew through the air, others were transported on the feathers or in the guts of birds, and still others floated across the Pacific. Although the distances are rarely as great as the distance between Hawaii and the mainland, dispersal is just as important for mainland plant species that have discontinuous habitats, such as mountaintops, marshes, or rivers.

Learning Outcomes Review 31.1

The gymnosperms and angiosperms evolved from a common seed-producing ancestor. Whole-genome duplication likely contributed to the rise and dominance of seed plants. Seeds protect the embryo, aid in dispersal, and can allow for an extended pause in the life cycle. Seed plants produce male and female gametophytes; the male gametophyte is a pollen grain, which is carried to the female gametophyte by wind or other means. The male gametophyte carries a sperm cell, whereas the female gametophyte carries an egg cell.

Conifers are the largest gymnosperm phylum

The most familiar gymnosperms are conifers (phylum Coniferophyta), which include pines (figure 31.2), spruces, firs, cedars, hemlocks, yews, larches, cypresses, and others. Nearly 40% of the world's forests are composed of conifers.

Flower Morphology

The other flower parts typically are attached in circles called whorls. The outermost whorl is composed of sepals. Most flowers have three to five sepals, which are green and somewhat leaflike. The next whorl consists of petals that are often colored, attracting pollinators such as insects, birds, and some small mammals. The petals, which also commonly number three to five, may be separate, fused together, or missing altogether in wind-pollinated flowers. The third whorl consists of stamens and is collectively called the androecium. This whorl is where the male gametophytes, pollen, are produced. Each stamen consists of a pollen-bearing anther and a stalk called a filament, which may be missing in some flowers. At the center of the flower is the fourth whorl, the gynoecium, where the small female gametophytes are housed; the gynoecium consists of one or more carpels.

Diagram of an Angiosperm Flower

The ovary as it matures will become a fruit; as the ovule's outer layers (integuments) mature, they will become a seed coat.

Learning Outcomes Review 31.4

The seed coat originates from the integuments and encloses the embryo and stored nutrients. The four advantages conferred by seeds are dormancy, protection of the embryo, nourishment, and a method of dispersal. Fire, heavy rains, or passage through an animal's digestive tract may be required for germination in some species.

Cycads resemble palms, but are not flowering plants

The sporophytes of most of the 250 known species resemble palm trees (figure 31.4a) with trunks that can attain heights of 15 m or more. Unlike palm trees, which are flowering plants, cycads produce cones and have a life cycle similar to that of pines.

Sorting out angiosperm origins is complicated

There are many potential explanations for the takeover of the world's biomes by the angiosperms. An interesting new hypothesis is that the breakup of Pangea led to climate change and opportunities for the expansion of the angiosperms. The middle latitudes became more humid and temperate, and deserts between the tropic and temperate zones became smaller. The ability of angiosperms to move into these new environments was enhanced by their unique features, including flower production, the ability to attract insect pollinators, and the development of broad leaves with dense veins.

Gnetophytes Have Xylem Vessels

There are three genera and about 65 living species of gnetophytes (phylum Gnetophyta). They are the only gymnosperms with vessels in their xylem. Vessels are a particularly efficient conducting cell type that is a common feature in angiosperms.