What is a Plant Study Guide (page 2)
You can usually tell plants from other organisms because they (or parts of their bodies) are green. This is because plants contain a molecule called chlorophyll. Chlorophyll acts as a pigment to give plants their green color, but more importantly, it absorbs sunlight energy in the first step of the process known as photosynthesis. Plants also have chloroplasts and cell walls. This lesson introduces the world of plants.
What is Plant?
Many diverse organisms are classified within the Plant Kingdom, but they usually share certain characteristics that make them recognizable as plants. They are usually green (or part of their body is green), and they do not have the ability of locomotion, so they stay in one spot. They carry out the process known as photosynthesis, which turns carbon dioxide and water into sugars and oxygen gas. This process takes place in structures called chloroplasts. Plant cells have a hard cell wall made of the carbohydrate, cellulose.
Diverse Environments and Plants
Plants are found in nearly every place on Earth. They are located in the ocean and on mountaintops, they are in extremely warm and extremely cold places, and they even exist in very dry places like deserts. Plants are dependent upon light, so their access to a source of light is what limits where they can live. Water is also important to plant growth and development because much of plants' support comes from the water contained within its cells by the cell wall.
Long ago, two major groups of land plants evolved from algae, the bryophytes or nonvascular plants and the tracheophytes or vascular plants.
Bryophytes (Nonvascular Plants)
These plants lack roots, leaves, and stems, but they do have structures called rhizoids (root-like hairs) that absorb water and nutrients. However, the bryophytes have no vessels for conducting water throughout their bodies, so they rely on slow diffusion to distribute water and nutrients. This means that they cannot grow very large because the process of diffusion would be too inefficient to support large bodies. The most representative plants in this grouping are the liverworts and mosses.
Trachea refers to tube, and these vascular plants have tubes (vessels) that provide support and a means of transporting water and nutrients throughout their bodies. Vascular plants can thus grow very tall, up to hundreds of feet. This group is further broken down into two types, the seedless vascular plants and the seeded vascular plants.
Seedless vascular plants include club mosses, horsetails, and ferns. These plants must be in moist environments because they need water to reproduce. Millions of years ago, these types of plants dominated the Earth, and they grew to large sizes. Many of these types of plants are still in existence, but the seed plants have become dominant. The remains of the many seedless plants from millions of years ago have been transformed into oil and coal by tremendous heat and pressure.
Seed plants have become dominant today because they have developed pollen and seeds as adaptations. Pollen is a protective structure that ensures that the sperm cell used in pollination survives harsh conditions until it reaches the female part of a flower and can fertilize the egg found there. Seeds are an adaptation that allows these plants to undergo a period of inactivity in their life cycles. The seed contains and protects an immature plant in a state of dormancy. The small plant stays dormant until conditions are favorable, and then it germinates and forms a new plant.
Seeds are also very highly adapted to many ways of being dispersed. Some seeds are distributed by wind, some by water, and others by animals. This dispersal is a way plants can establish themselves in new areas because they cannot transport themselves. Seed plants are divided into two groups, flowering and nonflowering plants.
Flowers and Cones
Gymnosperms is the name given to seed plants that do not form flowers. These plants were present on Earth before the flowering seed plants. Representatives of this group include pines, spruce, and cypresses. Gymnosperms are adapted to cold, dry areas. They have very thin, small leaves covered with a waterproof layer that keeps them from drying out. They also retain green leaves all year long (these are the plants we call evergreens) so that they can continue making food all year long. They also produce a kind of biological antifreeze in their sap that keeps them from freezing. This substance is what produces the scent from a pine tree, for example. Gymnosperms also produce seeds in cones.
Angiosperms is the name given to seed plants that do form flowers. These plants now dominate the Earth (even more so than the gymnosperms) and are highly diverse with many different types of plants. The angiosperms have been successful because they have developed flowers, fruits, and broad leaves. Their broad leaves allow them to capture more sunlight and produce more of their own food than the narrow, thin leaves of the gymnosperms.
Their most attractive characteristic is their flowers. Flowers are structures that contain the male and female sexual parts where sperm and egg cells are produced. The structure of flowers is designed to attract animals that will assist in the pollination process. Thus, flowers are colorful and fragrant. They also often offer a "reward" of nectar or pollen that the animal, such as a bee, uses for food.
Additionally, fruits are clearly important to animals and humans. They are also important to flowering seed plants because fruits are the remnants of the flower and contain the fully developed seed. The fleshy, sweet-tasting fruit encourages animals to eat them and disperse the seeds they contain.
Many diverse organisms are classified within the Plant Kingdom, but they usually share certain characteristics that make them recognizable as plants. They are usually green (or part of their body is green), and they do not have the ability of locomotion, so they stay in one spot. They carry out the process known as photosynthesis, which turns carbon dioxide and water into sugars and oxygen gas. Long ago, two major groups of land plants evolved from algae, the bryophytes or nonvascular plants and the tracheophytes or vascular plants. Within vascular plants are two types, seedless and seeded. The seed plants also have two varieties, flowering and nonflowering.
Practice problems of this concept can be found at: What is a Plant Practice Questions
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