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16.2: Plant Organs: Roots, Stems, and Leaves

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Lesson Objectives

  • Outline the structure, function, and growth of roots.
  • Give an overview of stem diversity and how stems function and grow.
  • Describe leaf variation, and explain how leaves make food and change seasonally.

Vocabulary

  • bark
  • deciduous plant
  • evergreen plant
  • fibrous root
  • mesophyll
  • root hair
  • root system
  • stomata (singular, stoma)
  • taproot

Introduction

Plants have specialized organs that help them survive and reproduce in a great diversity of habitats. Major organs of most plants include roots, stems, and leaves.

Roots

Roots are important organs in all vascular plants. Most vascular plants have two types of roots: primary roots that grow downward and secondary roots that branch out to the side. Together, all the roots of a plant make up a root system.

Root Systems

There are two basic types of root systems in plants: taproot systems and fibrous root systems. Both are illustrated in Figure below.

  • Taproot systems feature a single, thick primary root, called the taproot, with smaller secondary roots growing out from the sides. The taproot may penetrate as many as 60 meters (almost 200 feet) below the ground surface. It can plumb very deep water sources and store a lot of food to help the plant survive drought and other environmental extremes. The taproot also anchors the plant very securely in the ground.
  • Fibrous root systems have many small branching roots, called fibrous roots, but no large primary root. The huge number of threadlike roots increases the surface area for absorption of water and minerals, but fibrous roots anchor the plant less securely.

Dandelions have taproot systems; grasses have fibrous root systems.

Root Structures and Functions

As shown in Figure below, the tip of a root is called the root cap. It consists of specialized cells that help regulate primary growth of the root at the tip. Above the root cap is primary meristem, where growth in length occurs.

A root is a complex organ consisting of several types of tissue. What is the function of each tissue type?

Above the meristem, the rest of the root is covered with a single layer of epidermal cells. These cells may have root hairs that increase the surface area for the absorption of water and minerals from the soil. Beneath the epidermis is ground tissue, which may be filled with stored starch. Bundles of vascular tissues form the center of the root. Waxy layers waterproof the vascular tissues so they don’t leak, making them more efficient at carrying fluids. Secondary meristem is located within and around the vascular tissues. This is where growth in thickness occurs.

The structure of roots helps them perform their primary functions. What do roots do? They have three major jobs: absorbing water and minerals, anchoring and supporting the plant, and storing food.

  • Absorbing water and minerals: Thin-walled epidermal cells and root hairs are well suited to absorb water and dissolved minerals from the soil. The roots of many plants also have a mycorrhizal relationship with fungi for greater absorption.
  • Anchoring and supporting the plant: Root systems help anchor plants to the ground, allowing plants to grow tall without toppling over. A tough covering may replace the epidermis in older roots, making them ropelike and even stronger. As shown in Figure below, some roots have unusual specializations for anchoring plants.
  • Storing food: In many plants, ground tissues in roots store food produced by the leaves during photosynthesis. The bloodroot shown in Figure below stores food in its roots over the winter.

Mangrove roots are like stilts, allowing mangrove trees to rise high above the water. The trunk and leaves are above water even at high tide. A bloodroot plant uses food stored over the winter to grow flowers in the early spring.

Root Growth

Roots have primary and secondary meristems for growth in length and width. As roots grow longer, they always grow down into the ground. Even if you turn a plant upside down, its roots will try to grow downward. How do roots “know” which way to grow? How can they tell down from up? Specialized cells in root caps are able to detect gravity. The cells direct meristem in the tips of roots to grow downward toward the center of Earth. This is generally adaptive for land plants. Can you explain why?

As roots grow thicker, they can’t absorb water and minerals as well. However, they may be even better at transporting fluids, anchoring the plant, and storing food (see Figure below).

Secondary growth of sweet potato roots provides more space to store food. Roots store sugar from photosynthesis as starch. What other starchy roots do people eat?

Stems

In vascular plants, stems are the organs that hold plants upright so they can get the sunlight and air they need. Stems also bear leaves, flowers, cones, and secondary stems. These structures grow at points called nodes (shown in Figure below). At each node, there is a bud of meristem tissue that can divide and specialize to form a particular structure.

The stem of a vascular plant has nodes where leaves and other structures may grow.

Another vital function of stems is transporting water and minerals from roots to leaves and carrying food from leaves to the rest of the plant. Without this connection between roots and leaves, plants could not survive high above ground in the air. In many plants, stems also store food or water during cold or dry seasons.

Stem Diversity

Stems show variation because many stems are specialized. Figure below shows examples of stem specialization. With specialized stems, plants can exploit a diversity of niches in virtually all terrestrial ecosystems.

Stem specializations such as these let plants grow in many different habitats.

Stem Tissues and Functions

Like roots, the stems of vascular plants are made of dermal, vascular, and ground tissues.

  • A single-celled layer of epidermis protects and waterproofs the stem and controls gas exchange.
  • In trees, some of the epidermal tissue is replaced by bark. Bark is a combination of tissues that provides a tough, woody external covering on the stems of trees. The inner part of bark is alive and growing; the outer part is dead and provides strength, support, and protection.
  • Ground tissue forms the interior of the stem. The large central vacuoles of ground tissue cells fill with water to support the plant. The cells may also store food.
  • Bundles of vascular tissue run through the ground tissue of a stem and transport fluids. Plants may vary in how these bundles are arranged.

Stem Growth

The stems of all vascular plants get longer through primary growth. This occurs in primary meristem at the tips and nodes of the stems. Most stems also grow in thickness through secondary growth. This occurs in secondary meristem, which is located in and around the vascular tissues. Secondary growth forms secondary vascular tissues and bark. In many trees, the yearly growth of new vascular tissues results in an annual growth ring like the one in Figure below. When a tree is cut down, the rings in the trunk can be counted to estimate the tree’s age.

The number of rings in this cross-section of tree trunk show how many years the tree lived. What does each ring represent?

Leaves

Leaves are the keys not only to plant life but to all terrestrial life. The primary role of leaves is to collect sunlight and make food by photosynthesis. Despite the fundamental importance of the work they do, there is great diversity in the leaves of plants. However, given the diversity of habitats in which plants live, it’s not surprising that there is no single best way to collect solar energy for photosynthesis.

Leaf Variation

Leaves may vary in size, shape, and their arrangement on stems. Nonflowering vascular plants have three basic types of leaves: microphylls (“tiny leaves”), fronds, and needles. Figure below describes each type.

Leaf variation in nonflowering plants reflects their evolutionary origins. Can you explain how?

Flowering vascular plants also have diverse leaves. However, the leaves of all flowering plants have two basic parts in common: the blade and petiole (see Figure above). The blade of the leaf is the relatively wide, flat part of the leaf that gathers sunlight and undergoes photosynthesis. The petiole is the part that attaches the leaf to a stem of the plant. This occurs at a node.

Flowering plant leaves vary in how the leaves are arranged on the stem and how the blade is divided. This is illustrated in Figure below. Generally, the form and arrangement of leaves maximizes light exposure while conserving water, reducing wind resistance, or benefiting the plant in some other way in its particular habitat.

  • Leaves arranged in whorls encircle upright stems at intervals. They collect sunlight from all directions.
  • Leaves arranged in basal rosettes take advantage of warm temperatures near the ground.
  • Leaves arranged in alternate or opposing pairs collect light from above. They are typically found on plants with a single, upright stem.
  • The blades of simple leaves are not divided. This provides the maximum surface area for collecting sunlight.
  • The blades of compound leaves are divided into many smaller leaflets. This reduces wind resistance and water loss.

Leaf variation in flowering plants may include variations in the arrangement of leaves and the divisions of the blade.

Factories for Photosynthesis

You can think of a single leaf as a photosynthesis factory. A factory has specialized machines to produce a product. It’s also connected to a transportation system that supplies it with raw materials and carries away the finished product. In all these ways, a leaf resembles a factory. The cross section of a leaf in Figure below lets you look inside a leaf “factory.”

There’s more to a leaf than meets the eye. Can you identify the functions of each of the labeled structures in the diagram?

A leaf consists of several different kinds of specialized tissues that work together to make food by photosynthesis. The major tissues are mesophyll, veins, and epidermis.

  • Mesophyll makes up most of the leaf’s interior. This is where photosynthesis occurs. Mesophyll consists mainly of parenchymal cells with chloroplasts.
  • Veins are made primarily of xylem and phloem. They transport water and minerals to the cells of leaves and carry away dissolved sugar.
  • The epidermis of the leaf consists of a single layer of tightly-packed dermal cells. They secrete waxy cuticle to prevent evaporation of water from the leaf. The epidermis has tiny pores called stomata (singular, stoma) that control transpiration and gas exchange with the air. Figure below explains how stomata carry out this vital function.

For photosynthesis, stomata must control the transpiration of water vapor and the exchange of carbon dioxide and oxygen. Stomata are flanked by guard cells that swell or shrink by taking in or losing water through osmosis. When they do, they open or close the stomata.

Seasonal Changes in Leaves

Even if you don’t live in a place where leaves turn color in the fall, no doubt you’ve seen photos of their “fall colors” (see Figure below). The leaves of many plants turn from green to other, glorious colors during autumn each year. The change is triggered by shorter days and cooler temperatures. Leaves respond to these environmental stimuli by producing less chlorophyll. This allows other leaf pigments—such as oranges and yellows—to be seen.

A deciduous tree goes through dramatic seasonal changes each year. Can you identify the seasons in the photo?

After leaves turn color in the fall, they may all fall off the plant for the winter. Plants that shed their leaves seasonally each year are called deciduous plants. Shedding leaves is a strategy for reducing water loss during seasons of extreme dryness. On the downside, the plant must grow new leaves in the spring, and that takes a lot of energy and matter. Some plants may “bank” energy over the winter by storing food. That way, they are ready to grow new leaves as soon as spring arrives.

Evergreen plants have a different strategy for adapting to seasonal dryness. They don’t waste energy and matter growing new leaves each year. Instead, they keep their leaves and stay green year-round. However, to reduce water loss, they have needle-like leaves with very thick cuticle. On the downside, needle-like leaves reduce the surface area for collecting sunlight. This is one reason that needles may be especially rich in chlorophyll, as you can see from the dark green pine needles in Figure below. This is also an important adaptation for low levels of sunlight, allowing evergreens to live far from the equator.

Compare the color of the evergreen needles and the deciduous leaf. Why is the darker color of the needles adaptive?

Lesson Summary

  • Roots absorb water and minerals and transport them to stems. They also anchor and support a plant, and store food. A root system consists of primary and secondary roots. Each root is made of dermal, ground, and vascular tissues. Roots grow in length and width from primary and secondary meristem.
  • Stems hold plants upright, bear leaves and other structures, and transport fluids between roots and leaves. Like roots, stems contain dermal, ground, and vascular tissues. Trees have woody stems covered with bark.
  • The primary function of leaves is to collect sunlight and make food by photosynthesis. Specialized tissues in leaves work together to perform this function. In a deciduous plant, leaves seasonally turn color and fall off the plant. They are replaced with new leaves later in the year. An evergreen plant keeps its green leaves year-round. It may have needle-like leaves to reduce water loss.

Lesson Review Questions

Recall

1. What are root hairs? What is their role?

2. Identify three major functions of roots.

3. Describe two types of specialized stems. What is each type of stem specialized for?

4. What is bark? What purposes does it serve?

5. Name the two main parts of an angiosperm leaf. What is the function of each part?

6. Identify strategies used by deciduous and evergreen plants to adapt to seasonal dryness.

Apply Concepts

7. Apply lesson concepts to predict how the stem of a desert plant might be specialized for its environment.

8. Devise a model to demonstrate the concept that simple and compound leaves differ in the amount of light they absorb.

Think Critically

9. Contrast a taproot system with a fibrous root system.

10. Explain how roots “know” which way to grow.

11. Relate leaf variation to environmental variation.

12. Explain how a leaf is like a factory.

Points to Consider

In this lesson you read about the diversity of roots, stems, and leaves. The life cycles of plants are also diverse.

  • What do you already know about the life cycle of plants? What type of life cycle do plants have?
  • Predict how the life cycles of different plants might vary. For example, how might the life cycle of seed plants differ from the life cycle of seedless vascular plants?

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