Plant Structure and Function

Types of Plants

Plants provide people with many essential materials for shelter, food, and clothing. Plants also provide beauty in our lives, from vast gardens to flower boxes in our windows. The variety of uses we have for plants belies the great variety of plants that exist. Plants can be simple or complex, small or gigantic, flowering or non-flowering.

Non-vascular Plants

The most primitive types of plants lack vascular tissue, the tube like structures through which water and other materials move inside a plant. Non-vascular plants are considered to be among the least complex living plants. They are usually small plants, which enables most materials to move through them easily. Non-vascular plants, such as liverworts and mosses, take in water through osmosis, the process that allows water to diffuse across a cell membrane. Because of this, their life cycles are dependent on damp places.

Liverworts and their relatives are small and grow on the surface of wet soil. Mosses are the small plants that form a green carpet on the floor of many forests. Mosses also grow on trees and rock surfaces. Liverworts and mosses, like all non-vascular plants, lack true roots, stems, and leaves.

Vascular Plants

Vascular plants, known as tracheophytes, are true land plants because they have evolved ways to survive independent of wet environments. They are able to move water from their surroundings through their bodies in vascular tissues. Vascular tissue allows plants to grow to a great height, like the giant redwoods of Northern California. Vascular plants include seedless plants like club mosses (which are not true mosses, despite their name), horsetails, and ferns, as well as seed plants-for example, violets, potatoes, and pine trees.

There are relatively few species of seedless vascular plants compared to vascular plants with seeds. The reproductive structures of seedless vascular plants are dust like spores. Club mosses are small evergreen plants with needle-like or scale-like leaves. The leaves of horsetails encircle the shoots. Horsetails may or may not be evergreens and can grow to be rather tall. Fem leaves look a great deal like green feathers. These seedless vascular plants reproduce in a two generation life cycle - one that produces spores, and one that does not.

Seed Plants

Seed plants are vascular plants that reproduce, as their name suggests, using seeds. They can be divided into two groups, gymnosperms and angiosperms, based on how their seeds are produced. Most of the plants people are familiar with, including trees, flowers, vegetables, and grains, are seed plants.

Gymnosperms

Gymnosperms are vascular plants that produce seeds that are not enclosed within a fruit. Gymnosperms, such as pines, firs, redwoods, and sequoias, do not produce flowers but form seeds in cones. The leaves of most gymnosperms are scale-like or needle-like. Many gymnosperms are called evergreens because some green leaves are always on their branches.

Angiosperms

Angiosperms are the most easily recognizable seed-producing plants. Angiosperms, or flowering plants, include familiar plants such as roses, corn, bamboo, orchids, daisies, and fruit trees. In angiosperms, seeds are enclosed in fruits. The fruit protects the seeds as they develop. Because most fruits are edible and the seeds not easily digested, animals often spread seeds far from the parent plants.

Although gymnosperms evolved long before flowering plants, today there are many more species of angiosperms than gymnosperms.

Parts of Plants

Plants have various parts that help them perform essential life functions. All vascular plants have roots, stems, and leaves, but only gymnosperms and angiosperms have seeds, and only angiosperms have flowers. Plants are made of cells, tissues, and organs that work together to perform specific functions that keep the plants alive. The four main organs of angiosperms are roots, stems, leaves, and flowers.

Roots

Roots come in various shapes and sizes. Roots give a plant the surface area it needs to absorb the water and minerals essential to its survival. The outermost cells of the roots absorb water and minerals. The more surface area, or outer part, the roots have, the more cells can absorb these essential substances. There are two main types of root systems: taproots and fibrous roots.

A taproot is a single thick structure that grows straight into the ground. Smaller branching roots grow out from its sides. A taproot securely anchors a plant in the soil and serves primarily as a storage organ for starch and sugar made by the plant. Taproots generally grow deep into the soil and can absorb water and nutrients there. Taproots are useful in areas where the water is deep in the soil.

Roots

Fibrous roots consist of a great many thin, branching roots that grow from a central point. They look like tufts of stringy hairs. Fibrous roots serve primarily to hold the plant in the ground and provide a large surface area for water and mineral absorption. The shallow branching nature of fibrous roots helps them absorb water quickly. Fibrous roots can help plants get water in sandy soil that does not hold water well.

Stems

Stems support the leaves, cones, fruits, flowers, and even seeds of plants. They hold a plant’s leaves up toward the sunlight. Stems contain two types of vascular tissue. Xylem is vascular tissue made up of tube-shaped cells that transport water and dissolved minerals through the roots to the rest of the plant. Lignin, a hard substance in xylem, helps give structure to the plant. Phloem is tube-shaped vascular tissue that transports organic molecules from the leaves throughout the plant.

There are two classifications of stems: herbaceous stems and woody stems. Herbaceous stems are green, soft, and flexible. Cells in herbaceous stems contain chloroplasts that use light to make food for the plant. Woody stems are hard, strong, and rigid. Trees, shrubs, and roses have woody stems.

Stems

Leaves

Leaves are plant organs whose main functions include capturing the energy of sunlight, making organic molecules, and exchanging gases with the environment. Most leaves are flat and have a relatively large surface area that receives sunlight.

Leaves have an outer layer of cells called the epidermis. The epidermis has various parts, two of which are the upper epidermis and the lower epidermis. The upper epidermis has a waxy, waterproof coating called the cuticle that prevents the plant from losing too much water. The lower epidermis has tiny pores, or openings, called stomata that allow molecules to move into and out of the plant. Between these layers is the tissue of the leaf where light energy is used to produce food for the plant in a process called photosynthesis.

The process by which most of the water passes out of leaves as water vapor through the stomata in the lower epidermis is called transpiration. Because water molecules attract other water molecules, water that exits the stomata by transpiration actually “pulls” water up from the roots.

Leaves

Flowers

Flowers are the main reproductive organs of flowering plants. Most flowers have four main parts: petals, sepals, stamens, and a pistil. Each flower part plays an important role in a plant’s ability to produce more of its kind.

Flowers

Pollination is the transfer of pollen grains from stamen to pistil. With few exceptions, pollination must take place before seeds can form. Pollen grains can be carried by wind, water, gravity, insects, or other animals. Once a pollen grain lands on the female part of the plant, a pollen tube develops. The sperm cells move from the pollen down the tube and into the ovary. Fertilization can then take place between a sperm and an egg. The resulting fertilized egg develops into a seed, the offspring of a plant. The ovary surrounding the seed swells and becomes a fruit in many species of flowering plants.

Seeds

Seeds vary widely in appearance, but they all have the same basic structure: a protective seed coat, an embryo, and stored food. A seed develops from a fertilized egg and contains an embryo.

Seeds that fall close to the parent plant often have to compete for available nutrients and other resources, such as light and water. Plants have evolved many ways to ensure that seeds are transported from where they are formed. This process is called seed dispersal.

Coconuts seeds are encased in tough husks made of strong fibers with air spaces between them. The seeds can float in water and are dispersed from one place to another on ocean currents. Dandelion seeds have small fluffy threads attached to them. The threads help the wind carry the seeds aloft.

The process by which the embryo in a seed begins to develop into a new plant is called germination. Germination begins when growing conditions are favorable. Water, oxygen, and favorable temperatures are common requirements of germination. Seeds often undergo a period of inactivity called dormancy until the conditions are right for germination.

Life Cycle of an Angiosperm

A life cycle is a continuous process and does not have a true beginning or end. One place to begin describing the process is at germination. The seed germinates, and a seedling begins to grow roots, stems, and leaves. An adult angiosperm grows flowers, which then go through the process of pollination. Once a flower has been pollinated, a seed begins to form with the fruit of the plant. Wind, water, or animals may carry the seed to a new location, and the life cycle continues.

Pollen