Science

22.1 What Is a Plant?

Lesson Objectives

• Describe what plants need to survive. 

• Describe how the first plants evolved.

• Explain the process of alternation of generations.

Lesson Summary

Characteristics of Plants

•  Plants are eukaryotes that have cell walls containing cellulose. Mostly autotrophs, plants use chlorophyll a and b to carry out photosynthesis. 01

• Without moving about, plants get what they need from the environment. Sunlight: gathered by leaves arranged in ways that maximize absorption. Gas exchange: brings in oxygen and carbon dioxide and releases excess oxygen. Water: absorbed mostly from the soil and transported internally. Minerals: absorbed along with water from the soil.

The History and Evolution of Plants Ancestors of today's land plants were water-dwellers similar to today's green algae. Over time, the demands of life on land favored the evolution of plants more resistant to the drying rays of the sun, more capable of conserving water, and more capable of reproducing without water. The first land plants were dependent on water and lacked leaves and roots.

Five major groups of plants are classified based on four important features: embryo formation;

specialized water-conducting tissues, seeds, flowers

Plant life cycle: Shift between two phases 

Sporophyte: the multicellular diploid phase, a spore-producing plant

Gametophyte: the multicellular haploid phase, a gamete-producing plant

22.2 Seedless Plants

Lesson Objectives

• Identify the characteristics of green algae.

• Describe the adaptations of bryophytes.

• Explain the importance of vascular tissue.

Lesson Summary

Green algae are eukaryotic (contains a nucleus), photosynthetic, and mostly aquatic. They are found in fresh and saltwater, and in some moist areas on land. Most do not contain the specialized tissues found in other plants. Some may not alternate between haploid and diploid stages with every generation. Green algae form colonies providing a hint about how multicellular plants evolved. Although most cells in a Volvox colony are identical, a few are specialized for reproduction.

Mosses and Other Bryophytes The bryophytes have specialized reproductive organs.  Bryophytes are small because they lack vascular tissue, which is specialized for conducting water. 

Vascular Plants These plants are also known as tracheophytes. Vascular plants have vascular tissues that make it possible to move fluids through their bodies against the force of gravity. Tracheids are hollow tubelike water-conducting cells with thick cell walls strengthened by lignin. Tracheids are found in xylem, a tissue that carries water upward from the roots to every part of a plant. Phloems are a vascular tissue that carries nutrients and carbohydrates produced by photosynthesis.

22.3 Seed Plants

Lesson Objectives

• Describe the reproductive adaptations of seed plants.

• Identify the reproductive structures of gymnosperms.

Lesson Summary

The Importance of Seeds: A seed is a plant embryo and a food supply, encased in a protective covering. The embryo is an early stage of the sporophyte.

• Ancestors of seed plants evolved with many adaptations that allow seed plants to reproduce without open water. These include a reproductive process that takes place in cones or flowers, the transfer of sperm by pollination, and the protection of embryos in seeds. These adaptations enabled plants to survive on dry land.

• The gametophytes of seed plants grow and mature within the sporophyte.

• Gymnosperms are seed plants that bear their seeds directly on the scales of cones.

• The angiosperms are seed plants that bear their seeds in flowers inside a layer of tissue that protects the seed.

In seed plants, the entire male gametophyte is contained in a tiny structure called a pollen grain.

• The sperm are produced inside pollen grains and do not have to swim.

• Pollen grains are carried to female reproductive structures by wind or animals.

• The  transfer of pollen from the male reproductive structure to the female reproductive structure is called pollination.

After fertilization, the zygote in the seed grows into a tiny plant-the sporophyte embryo. A tough seed coat surrounds and protects the embryo and keeps the contents of the seed from drying out.

The Life Cycle of a Gymnosperm: The word gymnosperm means "naked seed." Gymno- sperms include cycads, ginkgoes, and conifers such as pines and firs.

• Conifers produce two types of cones: pollen cones that produce the pollen grains and seed cones that produce female gametophytes.

• Near the base of each scale of the seed cones are two ovules, the structures in which the female gametophytes develop.

• Wind carries pollen from pollen cones to new female cones.

• In gymnosperms, the direct transfer of pollen to the female cone allows fertilization to take place without the need for open water.

22.4 Flowering Plants

Lesson Objectives

• Identify the reproductive structures of angiosperms.

• Identify some of the ways angiosperms can be categorized.

Lesson Summary

Flowers and Fruits Angiosperms reproduce sexually by means of flowers.

• Flowers contain ovaries, which surround and protect the seeds. Angiosperm means "enclosed seed."

•  Flowers are an evolutionary advantage because they attract animals that carry pollen with them as they leave flowers.

• After fertilization, ovaries within flowers develop into fruits that surround, protect, and help disperse the seeds.

• A fruit is a structure containing one or more matured ovaries.

Angiosperm Diversity: Scientific classification reflects evolutionary relationships. Farmers, gardeners, and other people who work with plants group angiosperms according to the number of their seed leaves, the strength and composition of their stems, and the number of growing seasons they live.

• Monocots and dicots, grouped according to the number of cotyledons they produce, differ in several other characteristics, including:

• The distribution of vascular tissue in stems, roots, and leaves

• The number of petals per flower

• Monocots have single cotyledon, parallel veins, floral parts often in multiples of 3, vascular bundles scattered throughout stem, and fibrous roots. Dicots have two cotyledons, branched, veins, floral parts often in multiples of 4 or 5, vascular bundles arranged in a ring, and taproots.

Plants are also grouped by the characteristics of their stems.

• Woody plants have stems that are made primarily of cells with thick cell walls that support the plant body.

• Herbaceous plants have smooth and nonwoody stems.

Plants are grouped according to life span as annuals, biennials, or perennials. Annuals live one year; biennials live two years; and perennials (usually have woody stems and some have herbaceous stems that die each winter and are replaced in the spring) can live for several years.

21.4 Fungi

Lesson Objectives

• Identify the defining characteristics of fungi.

• Describe how fungi affect homeostasis.

Lesson Summary

What Are Fungi? Fungi are eukaryotic heterotrophs that have cell walls. The cell walls of fungi contain chitin, a complex carbohydrate.

• Most fungi are composed of thin filaments called hyphae. The fruiting body of a fungus- such as the above-ground part of a mushroom-is a reproductive structure that you can see. It grows from many hyphae tangled underground in a thick mass called a mycelium.

• Most fungi reproduce both asexually and sexually. Asexual reproduction can occur when cells or hyphae break off and begin to grow on their own. Some fungi also reproduce asexually by means of spores.

• Most fungi can also reproduce sexually. Spores are produced in structures called sporangia. Many fungi have minus (-) and plus (+) types that can reproduce sexually by fusing their nuclei when they meet.

The Ecology of Fungi Fungi do not ingest their food as animals do. Instead, fungi digest food outside their bodies and then absorb it. Many fungi feed by absorbing nutrients from decaying matter. Some fungi are parasites.

• Fungi help maintain equilibrium in nearly every ecosystem by recycling nutrients by breaking down the bodies and wastes of other organisms.

• Parasitic fungi cause serious plant and animal diseases. 

• Some fungi form mutualistic relationships in which both partners benefit.

• Lichens are symbiotic associations between a fungus and a photosynthetic organism. The photosynthetic organism provides a source of energy. The fungus provides water and minerals.

• Mutualistic associations of plant roots and fungi are called mycorrhizae. The plant's roots are woven into a partnership with the web of fungal hyphae.

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This relationship enhances nutrient absorption for the plant, particularly phosphorus, while the fungus receives carbohydrates produced by the plant through photosynthesis.