Chapter 8 - Plants

Section 1 - The Plant Kingdom


  Nearly all plants are autotrophs, organisms that produce their own food. All plants are eukaryotes that contain many cells. In addition, all plants cells are surrounded by cell walls. Plant cells are enclosed by a cell wall and contain chloroplasts and vacuoles.

  Most plants live on land. For plants to survive on land, they must have ways to obtain water and other nutrients from their surroundings, retain water, transport materials within their bodies, support their bodies, and reproduce. Most plants have a waxy, waterproof layer covering their leaves called a cuticle. The cuticle helps keep water inside a plant cell rather than let it evaporate into the air. Some plants have vascular tissue, a system of tubelike structures inside the plant through which food, minerals, and water move. All plants undergo sexual reproduction that involves fertilization, the joining of a sperm cell with an egg cell. The fertilized egg is called a zygote.

  Scientists informally group plants into two major groups- nonvascular and vascular plants. Nonvascular plants do not have a welldeveloped system of tubes for transporting water and other materials. They grow low to the ground. Vascular plants have well-developed vascular tissue to transport and move materials quickly and efficiently throughout the plant's body. Vascular tissue also provides strength, stability, and support to a plant. Thus, vascular plants are able to grow quite tall.

  Biologists infer that ancient green algae were the ancestors of today's land plants because land plants and green algae contain the same forms of chlorophyll.

  Plants have complex life cycles that include two different stages, the sporophyte stage and the gametophyte stage. In the sporophyte stage, the plant produces spores, tiny cells that can grow into new organisms. A spore develops into the plant's other stage, called the gametophyte stage. In the gametophyte stage, the plant produces two kinds of sex cells: sperm cells and egg cells.




Section 2 - Plants Without Seeds


  There are three major groups of nonvascular plants: mosses, liverworts, and hornworts. These low-growing plants live in moist areas where they can absorb water and other nutrients directly from their environment. The watery surroundings also enable sperm cells to swim to egg cells during reproduction.

  The familiar green, fuzzy part of the moss is the gametophyte generation of the plant. Structures that look like tiny leaves grow off a small, stemlike structure. Thin, rootlike structures called rhizoids anchor the moss and absorb water and nutrients. The sporophyte generation grows out of the gametophyte. The sporophyte includes a long, slender stalk with a capsule at the end. The capsule contains spores.

  Liverworts are often found growing as a thick crust on moist rocks or soil along the sides of a stream. Hornworts grow hornlike structures that are the sporophytes. Hornworts usually live in moist soil, often mixed in with grass plants.

  Ferns, club mosses, and horsetails have true vascular tissue and they do not produce seeds. Instead of seeds, these plants reproduce by releasing spores. Seedless vascular plants have true stems, roots, and leaves.

  Vascular plants can grow quite tall because their vascular tissue provides an effective way of transporting materials throughout the plant. Vascular tissue also gives the plants strength and stability.   Seedless vascular plants need to grow in moist surroundings because they produce spores. These spores grow into gametophytes. When the gametophytes produce egg cells and sperm cells, there must be enough water available for the sperm to swim toward the eggs.

  Most ferns have underground stems. Leaves grow upward from the top side of the stems, while roots grow downward from the bottom of the stems. The roots anchor the fern to the ground and absorb water and nutrients from the soil. The leaves of ferns are called fronds. The upper surface of each frond is coated with a cuticle that helps the plant retain water. The familiar fern, with its visible fronds, is the sporophyte stage of the plant. Spores develop in tiny spore cases on the underside of the mature fronds. The spores develop into tiny gametophytes that grow low to the ground.

  Two other groups of seedless vascular plants are the horsetails and club mosses. There are relatively few species of horsetails and club mosses alive today.




Section 3 - The Characteristics of Seed Plants


  Seed plants share two important characteristics. They have vascular tissue, and they use pollen and seeds to reproduce. They all have body plans that include leaves, stems, and roots.

  Water, food, and minerals are transported throughout plants in vascular tissue. There are two types of vascular tissue. Phloem is the vascular tissue through which food moves. When food is made in the plant's leaves, it enters the phloem and travels to other parts of the plant. Water and minerals travel in the vascular tissue called xylem.The plant's roots absorb water and minerals from the soil. These materials enter the root's xylem and move upward into the stems and leaves.

  Seed plants can live in a variety of environments. They produce pollen, tiny structures that contain the cells that later become sperm cells. Pollen delivers sperm cells directly near the eggs, therefore seed plants do not need water for fertilization to occur. Seeds are structures that contain a young plant inside a protective covering. Inside a seed is a partially developed plant. If a seed lands in an area where conditions are favorable, it sprouts out of the seed and begins to grow. A seed has three main parts-an embryo, stored food, and a seed coat.

  The young plant that develops from the zygote, or fertilized egg, is called the embryo and has the beginnings of roots, stems, and leaves. The embryo also has one or two seed leaves, or cotyledons.In some seeds, the cotyledons store food. The outer covering of a seed is called the seed coat. A seed may remain inactive for awhile. Germination occurs when the embryo begins to grow again and pushes out of the seed.

  Roots anchor a plant in the ground, absorb water and minerals from the soil, and sometimes store food. The tip of the root is covered by a root cap. The root cap protects the root from injury from rocks as the root grows through the soil.

  The stem carries substances between the plant's roots and leaves. The stem also provides support for the plant and holds up the leaves so they are exposed to the sun. Stems can be herbaceous or woody. In woody stems, a layer of cells called the cambium divide to produce new phloem and xylem.

  Leaves capture the sun's energy and carry out the food-making process of photosynthesis.The surfaces of the leaf have stomata that open and close to control when gases enter and leave the leaf. The process by which water evaporates from the stomata in a plant's leaves is called transpiration.




Section 4 - Gymnosperms and Angiosperms


  A gymnosperm is a seed plant that produces naked seeds-seeds that are not enclosed by a protective fruit. Every gymnosperm produces naked seeds. In addition, many gymnosperms have needle-like or scalelike leaves and deep-growing root systems. Gymnosperms are classified into cycads, conifers, ginkgoes, and gnetophytes.

  Most gymnosperms have reproductive structures called cones. Male cones produce pollen. Female cones contain at least one ovule. An ovule is a structure that contains an egg cell. After being fertilized, the ovule develops into a seed. To reproduce, first, pollen falls from a male cone onto a female cone. In time, a sperm cell and an egg cell join together in an ovule on the female cone. The transfer of pollen from a male reproductive structure to a female reproductive structure is called pollination.

  Angiosperms are a group of seed plants. All angiosperms, or flowering plants, share two important characteristics. First, they produce flowers. Second, in contrast to gymnosperms, which produce uncovered seeds, angiosperms produce seeds that are enclosed in fruits.

  Flowers come in all sorts of shapes, sizes, and colors. But, despite their differences, all flowers have the same function-reproduction. Aflower is the reproductive structure of an angiosperm. A flower bud is enclosed by leaflike structures called sepals that protect the developing flower. Most flowers have petals-colorful, leaflike structures. Within the petals are the flower's reproductive parts. Thin stalks topped by small knobs inside the flower are stamens, the male reproductive parts. A stamen consists of an anther and a filament. Pollen is produced in the anther. The female part, or pistil, consists of a stigma, style, and ovary. An ovary is a flower structure that protects seeds as they develop. An ovary contains one or more ovules. For angiosperms to reproduce, first, pollen falls on a flower's stigma. In time, the sperm cell and egg cell join together in the flower's ovule. The zygote develops into the embryo part of the seed.As the seed develops, the ovary eventually becomes a fruit-a ripened ovary and other structures that enclose one or more seeds.

  Angiosperms are divided into two major groups: monocots and dicots. Monocots are angiosperms that have only one seed leaf. Dicots produce seeds with two seed leaves.

  Seed plants have many uses. For example, paper, lumber, turpentine, and other products come from gymnosperms. Angiosperms provide food and are used to make clothing, rubber, and furntiure.




Section 5 - Plant Responses and Growth


  A plant's growth response toward or away from a stimulus is called a tropism. Touch, light, and gravity are three important stimuli to which plants show growth responses, or tropisms.

  A hormone produced by a plant is a chemical that affects how the plant grows and develops. In addition to tropisms, plant hormones also control germination, the formation of flowers, stems, and leaves, the shedding of leaves, and the development and ripening of fruit. One important plant hormone is named auxin. Auxin speeds up the rate at which a plant's cells grow.

  Plant responses to seasonal changes include photoperiodism and dormancy. Photoperiodism is a plant's response to seasonal changes in length of night and day. Short-day plants flower when nights are longer than a critical night length. Long-day plants flower when nights are shorter than a critical night length. The critical night length is the number of hours of darkness that determines whether or not a plant will flower.

  Short-day plants bloom in the fall or winter, when nights are growing longer. Long-day plants flower in the spring or summer, when nights are getting shorter. The flowering cycle of day-neutral plants is not sensitive to periods of light and dark.

  As winter draws near, many plants prepare to go into a state of dormancy. Dormancy is a period when an organism's growth or activity stops. Dormancy helps plants survive freezing temperatures and the lack of liquid water.

  Angiosperms are classified as annuals, biennials, or perennials based on the length of their life cycles. Flowering plants that flower and die in the same year are called annuals. Annuals include marigolds and petunias. Wheat and cucumbers are also annuals. Biennials complete their life cycle in two years. Parsley and celery are biennials. Flowering plants that live for more than two years are called perennials. Maple trees and peonies are examples of perennials.



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