Biology by Karl Irvin Baguio (the ebook reader TXT) 📕

and vascular plants. Nonvascular plants do not have specialized tissues to transport fluids, while vascular plants do have specialized tissues. The bryophytes (the mosses and liverworts) are the only major group of nonvascular plants. There are three large groups of vascular plants: the seedless vascular plants (for example, ferns), the vascular plants with unprotected seeds (for example, pines), and the vascular plants with protected seeds (for example, flowering plants). While animals are classified in phyla, plants are classified in divisions.

 

The life cycle of plants has both a multicellular haploid and multicellular diploid phase. Because both phases of the life cycle are multicellular, this type of life cycle is an alternation of generations. In contrast, animal life cycles have a multicellular diploid phase and a unicellular haploid phase.

 

The alternating generations of plants are the sporophyte generation and the gametophyte generation. Individuals in the gametophyte generation (often called gametophytes) form gametes, or sex cells. Gametes are haploid cells (they contain one set of chromosomes). Haploid gametes fuse in fertilization. This fusion produces fertilized eggs, which are diploid cells (they have two sets of chromosomes). The plants that develop are diploid plants of the sporophyte generation. Individuals in the sporophyte generation (sporophytes) undergo meiosis to produce haploid spores.

 

Plants produce their gametes in specialized structures. In the nonvascular bryophytes and in the vascular plants, the egg cells are formed in structures called archegonia (the singular is archegonium). Sperm cells are produced in structures called antheridia (the singular is antheridium). In some specialized plants, these structures are reduced, and the sporophyte generation is dominant over the gametophyte generation in the life cycle.

 

Nonvascular Plants 

 

Nonvascular plants belong to the division Bryophyta, which includes mosses, liverworts, and hornworts. These plants have no vascular tissue, so the plants cannot retain water or deliver it to other parts of the plant body. The bryophytes do not possess true roots, stems, or leaves, although the plant body is differentiated into leaflike and stemlike parts. In some species, there are rootlike structures called rhizoids. With no vascular tissue, the bryophytes cannot retain water for long periods of time. Consequently, water must be absorbed directly from the surrounding air or another nearby source. This explains the presence of mosses in moist areas, such as swamps and bogs, and on the shaded sides of trees.

 

The life cycle of the moss is typical of the bryophytes. Flask-shaped archegonia, located among the top leaves of the female gametophytes, produce one egg cell each. Antheridia, located similarly on the male gametophyte, produce many sperm cells that swim in drops of rainwater or dew into the neck of the archegonium to fertilize the egg cell.

 

The zygote that results from the fertilization develops into a young sporophyte within the archegonium. The sporophyte grows out of the archegonium, taking its nourishment from the gametophyte, and differentiates into a slender stalk with a spore capsule near the tip. Haploid spores are produced by meiosis in this capsule, and when the tip of the capsule opens, the spores are freed. The spores settle in the soil and germinate into gametophytes, which represent the next stage in the alternation of generations.

 

The life cycles of all bryophytes are uniform, and although the gametophyte generation is the most familiar aspect of the life cycle, neither the sporophyte nor the gametophyte generation is dominant.

 

Vascular Plants 

 

Vascular plants encompass several divisions of plants and are collectively known as tracheophytes. Tracheophytes are characterized primarily by the presence of a vascular system composed of two types of specialized tissue: xylem and phloem. Xylem conducts water and minerals upward from the roots of a plant, while phloem transports sugars and other nutrients from the leaves to the other parts of the plant. Both xylem and phloem are distributed throughout the plant. The vascular tissue also serves as a means for mechanical support in the plant, so some tracheophytes (such as trees) can grow quite tall.

 

Seedless vascular plants (ferns)

 

In many plants, seeds are the structures from which the sporophyte generation emerges. Seeds protect the embryonic plant during its early stages and store food. Many plants do not form seeds in their life cycles, but they have flourished nevertheless. Among the seedless vascular plants are the ferns, classified in the division Pteridophyta.

 

A mature fern produces spores by meiosis. The spores are stored in cases called sori (the singular is sorus) on the underside of the fern leaf. The leaf is a frond, and the sori resemble dots on the underside of the frond. The fern plant is the sporophyte generation, and this generation dominates the life cycle.

 

After the spores are dispersed, they germinate into small heart-shaped haploid plants if they reach moist ground. Each plant is called a prothallus and is a gametophyte, meaning it produces gametes for reproduction. The antheridia and archegonia are found on the underside of the prothallus. Sperm produced in the antheridia swim through moisture on the plant to the archegonia and fertilize the egg cells. The resulting zygote grows within the protection of the archegonium and develops into a young sporophyte. The sporophyte eventually grows out of the archegonium into a slender stalk. The stalk develops into a sporophyte, which, in its adult form, is the familiar fern plant.

 

Other divisions of the seedless vascular plants are the Psilophyta (the whisk ferns), the Lycophyta (the club mosses), and the Sphenophyta (the horsetails).

 

Vascular plants with unprotected seeds (gymnosperms)

 

Vascular plants having unprotected, or naked, seeds are known as gymnosperms. Their seeds are not enclosed in female tissues and are therefore said to be naked. There are four divisions of gymnosperms: Cycadophyta (Cycads), Ginkgophyta (Ginkgo), Gnetophyta (Gnetae), and Coniferophyta (Conifer).

 

Coniferophyta is the largest and most familiar division of the gymnosperms. These plants are cone-bearers and are therefore called conifers. Their seeds are borne on the surface of the female cone scales. Members of this division include trees such as cedars, firs, spruces, pines, and giant redwoods. The leaves are generally needle-shaped and contain vascular tissue.

 

The full-grown conifer (for example, a pine tree) is the sporophyte generation of the plant. The sporophyte produces male and female cones on the same tree. These cones produce spores that undergo meiosis and produce the male and female gametophytes. Male gametophytes are the pollen grains, each consisting of four cells. The male gametophyte produces sperm cells in the pollen grains. The female gametophyte produces two or three egg cells that develop within protective structures called ovules.

 

In the spring, the male cone releases pollen, which is blown about by the wind. Some pollen gets trapped on the female cone, where it germinates and forms a pollen tube that makes its way into the ovule. A sperm cell then fertilizes the egg. The zygote that is produced develops into an embryo within the ovule. In time, the embryo matures into a seed. Eventually, the seed falls from the cone and germinates, and the germinating embryo becomes a new pine tree.

 

The function of dispersal in gymnosperms is assumed by the seeds. Growth of the embryo depends on food supplied from the parent sporophyte, where food-rich tissue surrounds the embryo. The gametophyte generation is little more than a reproductive mechanism in the gymnosperms. Both male and female gametophytes are tiny and entirely dependent on the parent sporophyte.

 

Vascular plants with protected seeds (angiosperms)

Angiosperms are the most developed and most complex vascular plants. They are the flowering plants, of which more than 250,000 species have been identified. Almost all vegetables, flowers, fruits, cereals, grains, grasses, and hardwood trees are angiosperms, the dominant life form on Earth today.

 

Angiosperm means “seed vessel,” a reference to the female tissues that enclose the seed. The tissue is endosperm. During embryonic development, the endosperm serves as a source of nourishment. In many angiosperms, the endosperm develops into the fruit of the plant. Thus, the protected seed is often found within a fruit. The two most distinguishing features of angiosperms are the flower and the fruit.

 

The flower of an angiosperm (see Figure 18-1) consists of a ring of modified leaves called sepals that enclose and protect the growing flower bud. In some species, the sepals are small and green, while in others they become colored and resemble the petals, the next ring. Flower petals are colorful and are useful in attracting pollinating animals, especially insects. Within the petals are the organs of reproduction: the male stamen and, at the center, the female pistil.

 

Figure 18-1   The flower of a plant and its structural features. 

 

The stamen of a flower consists of a thin, stemlike filament and an anther, where haploid pollen grains are produced. Each pollen grain is a male gametophyte containing sperm cells. The pistil consists of a sticky stigma; a style, which is a narrow stalk connecting the stigma to the top of the ovary; and the ovary, which is where the ovules are enclosed. In each ovule, a single mother cell divides to produce four cells, one of which will develop into a female gametophyte. The female gametophyte is a sac in which there are eight cells, one of which is the egg cell.

 

In the angiosperm life cycle, a pollen grain lands on the stigma and produces a pollen tube that grows from the pollen grain down through the style, penetrating the ovary to reach the ovule. One of the sperm cells in the pollen tube fertilizes the egg cell in the ovary to produce a diploid zygote. The zygote becomes the new plant embryo. Another sperm cell fuses with two other cells in the female gametophyte to produce a triploid (three sets of chromosomes) endosperm nucleus, which develops into the endosperm that will feed the growing embryo. The ovule becomes the seed in which the embryo develops, and the ovary ripens into a fruit.

During plant development, the cells become more specialized. In angiosperms, the embryo remains dormant for a while. When the seed germinates, the embryo grows further. Germination may depend on the availability of oxygen, a suitable temperature, or adequate light.

 

Germination requires the availability of stored food within the seed. In plants called monocots (monocotyledon plants), most of the food is stored in the endosperm in one seed leaf called a cotyledon (see Figure 18-2). In dicots (dicotyledon plants), most of the food is stored in endosperm in two cotyledons. Among the monocots are grasses, orchids, irises, and lilies. Dicots have two seed leaves (cotyledons) that provide nutrients for the plants as they grow for the seed. Monocots have a single cotyledon. Monocots usually have parallel leaf veins and flower parts occurring in threes. Dicots, in contrast, usually have netlike leaf veins and flower parts in fours or fives. Most of the flowering plants are dicots.

 

Figure 18-2   Growth of monocotyledon (monocot) and dicotyledon (dicot) plants. Other parts of the young plant are illustrated.

 

 

Chapter 19: Vascular Plants: Structure and Function

Vascular Plants

 

The plant world is conveniently separated into two major groups: nonvascular plants and vascular plants. The nonvascular plants include the bryophytes, while the vascular plants include the ferns, gymnosperms, and angiosperms (see Chapter 19). The nonvascular plants have no internal transport system. The vascular plants do have such a system, and they are more structurally and functionally complex.

 

Highly specialized tissues occur in the vascular plants. (A tissue is a group of cells working together to carry out a specialized function.) The tissues are organized into specialized organs called