Plants Study Guide (High School) Essay

KEY CONCEPT-Plant life began in the water and became adapted to land. Plants are multicellular eukaryotes, most of which produce their own food through photosynthesis and have adapted to life on land. Plants share many characteristics with green algae. Both are photosynthetic eukaryotes with the same types of chlorophyll. Both also use starch as a storage product and have cell walls that contain cellulose. One ancient species of green algae is the common ancestor of all plants. If it were alive today, it would be classified as a charophycean. Natural selection likely favored individuals of the ancestral charophycean species that could withstand dry periods, until eventually the first true plant species evolved. Life on land presents different challenges from life in the water. These challenges have acted as selective pressures for plant life on Earth. †¢ Retaining moisture: A cuticle is a waxy, waterproof layer that helps hold in moisture. Tiny holes in the cuticle, called stomata, can open and close allowing air to move in and out. †¢ Transporting resources: A vascular system is a collection of specialized tissues that bring water and mineral nutrients up from plant roots and disperse sugars down from the leaves. †¢ Growing upright: Lignin is a material that hardens the cell walls of some tissues, providing structure for plants to grow upright and space for vascular tissue. †¢ Reproducing on land: A pollen grain is a two-celled structure produced by seed plants that contains a cell that will divide to form sperm. Pollen can be carried by wind or animals to female reproductive structures. A seed is a storage device that also protects and nourishes a plant embryo. Plants evolve with other organisms in their environment. †¢ A mutualism is an interaction between two species in which both species benefit. Important mutualisms involving plants include those between plant roots and certain fungi and bacteria, and those between plants and their animal pollinators. †¢ Plant-herbivore interactions have brought about a variety of adaptations in plants that discourage animals from eating them. These adaptations include spines, thorns, and defensive chemicals. Mosses and their relatives are seedless nonvascular plants. These plants must grow close to the ground where they can absorb water and nutrients directly. They also rely on free-standing water to allow their sperm to swim to and fertilize an egg. Therefore, these plants usually live in damp environments. Three phyla fit into the category of seedless nonvascular plants. These include the liverworts (phylum Hepatophyta), the hornworts (phylum Anthocerophyta), and mosses (phylum Bryophyta). Club mosses and ferns are seedless vascular plants. Like seedless nonvascular plants, they rely on free-standing water to allow their sperm to swim to and fertilize an egg. However, a vascular system allows these plants to grow higher above the ground and still transport materials between the roots and the leaves. Two phyla fit into the category of seedless vascular plants. These include club mosses (phylum Lycophyta) and whisk ferns, horsetails, and ferns (phylum Pterophyta). Seeds plants are able to reproduce without free-standing water. Pollen can be carried by the wind or by animals to female reproductive structures, where sperm will form from each pollen grain. Pollination occurs when pollen meets female reproductive structures of the same plant species. Seeds nourish and protect plant embryos and allow plants to disperse to new areas. Seed plants can be grouped according to whether their seeds are enclosed in fruit. †¢ A gymnosperm is a seed plant whose seeds are not enclosed in fruit. A woody cone is the reproductive structure of most gymnosperms. Three phyla fit into the category of gymnosperms. These include cycads (phylum Cycadophyta), ginkgos (phylum Ginkgophyta), and conifers (phylum Coniferophyta). †¢ An angiosperm is a seed plant whose seeds are enclosed in fruit. Angiosperms belong to a phylum of their own, commonly called flowering plants (phylum Anthophyta). A flower is the reproductive structure of flowering plants. A fruit is the mature ovary of a flower. 20.3 KEY CONCEPT The largest phylum in the plant kingdom is the flowering plants. Flowering plants have unique adaptations that allow them to dominate the landmasses of Earth today. †¢ Flowers allow for efficient pollination. Although some flowering plants are pollinated by wind, many are pollinated by animals such as birds or insects. Animals often pollinate flowers while searching for food, so they transfer pollen from flower to flower in a very targeted way. †¢ Fruit, the mature ovary of a flowering plant, plays an important role in seed dispersal. Fruits can take on many forms. Animals may eat fleshy fruits, dispersing the seeds after they have passed through their digestive tracts. Other fruits take the form of burrs that cling to wildlife or fibers that help to spread seeds by wind. Botanists classify flowering plants into two groups based on the number of cotyledons present in the seed. A cotyledon is an embryonic leaf inside a seed. †¢ Monocots have one cotyledon. Monocots generally have leaves with parallel veins, flower parts in multiples of three, and bundles of vascular tissue scattered throughout the stem. Corn, grasses, irises, and lilies are monocots. †¢ Dicots have two cotyledons. Dicots generally have leaves with netlike veins, flower parts in multiples of four or five, and bundles of vascular tissue arranged in rings. Deciduous trees and peanuts are dicots. Flowering plants can also be categorized by stem type and life span. These characteristics help describe mature flowering plants and are commonly used by botanists, gardeners, landscape designers, and horticulturists. †¢ Two stem types are woody and herbaceous. Wood is made up of dead vascular tissue cells that have lots of lignin and cellulose in their cells walls. Woody stems are thick and stiff. Herbaceous stems do not contain wood. †¢ Three basic plant lifespans are annual, biennial, and perennial. Annuals mature from seeds, produce flowers, and die all in one year. Biennials take two years to complete their life cycle. Perennials live for more than two years.