Title: READ: Plants
Course: Biology (B)

READ: Plants

Introduction

This lush green landscape is thickly carpeted with trees and a myriad of other plants. Much of Earth’s land is dominated by plants. Yet compared to our active existence as animals, plants are—literally—rooted to the ground. Their sedentary lives may seem less interesting than the active lives of animals, but plants are very busy doing extremely important work. All plants are chemical factories. Each year, they transform huge amounts of carbon (from carbon dioxide) into food for themselves and virtually all other land organisms.

Plants are complex organisms that carry out complex tasks. But unlike animals, they don’t have nerves, bones, or muscles to do their work. How do plants do it? Read on to find out.

Introduction: Like animals, plants have organs that are specialized to carry out complex functions. An organ is a structure composed of more than one type of tissue. A tissue, in turn, is a group of cells of the same kind that do the same job. In this lesson, you will read about the tissues that do the important work of plants. The cells that make up plant tissues are described first.

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Objectives

Lesson Objectives

- Describe plant cell structures, and list types of plant cells.
- Compare and contrast different types of plant tissues.
- Explain how plants grow.
- 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.

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Vocabulary

Vocabulary

- bark
- deciduous plant
- evergreen plant
- fibrous root
- mesophyll
- root hair
- root system
- stomata (singular, stoma)
- taproot
- cuticle
- dermal tissue
- ground tissue
- meristem

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Plant Cells

Plant Cells

Plant cells resemble other eukaryotic cells in many ways. For example, they are enclosed by a plasma membrane and have a nucleus and other membrane-bound organelles. A typical plant cell is represented by the diagram in Figure below.

Plant cells have all the same structures as animal cells, plus some additional structures. Can you identify the unique plant structures in the diagram?

Plant Cell Structures

Structures found in plant cells but not animal cells include a large central vacuole, cell wall, and plastids such as chloroplasts.

- The large central vacuole is surrounded by its own membrane and contains water and dissolved substances. Its primary role is to maintain pressure against the inside of the cell wall, giving the cell shape and helping to support the plant.
- The cell wall is located outside the cell membrane. It consists mainly of cellulose and may also contain lignin, which makes it more rigid. The cell wall shapes, supports, and protects the cell. It prevents the cell from absorbing too much water and bursting. It also keeps large, damaging molecules out of the cell.
- Plastids are membrane-bound organelles with their own DNA. Examples are chloroplasts and chromoplasts. Chloroplasts contain the green pigment chlorophyll and carry out photosynthesis. Chromoplasts make and store other pigments. They give flower petals their bright colors.

Types of Plant Cells

There are three basic types of cells in most plants. The three types are described in Table below. The different types of plant cells have different structures and functions.

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Types of Plant Cells
Type of Cell | Structure | Functions | Example
 | Parenchymal | cube-shaped, loosely packed, thin-walled, relatively unspecialized, contain chloroplasts | photosynthesis, cellular respiration, storage | food storage tissues of potatoes
 | Collenchymal | elongated, irregularly thickened walls | support, wind resistance | strings running through a stalk of celery
 | Sclerenchymal | very thick cell walls containing lignin | support, strength | tough fibers in jute (used to make rope)

Plant Tissues

Plant Tissues

All three types of plant cells are found in most plant tissues. Three major types of plant tissues are dermal, ground, and vascular tissues.

Dermal Tissue

Dermal tissue covers the outside of a plant in a single layer of cells called the epidermis. You can think of the epidermis as the plant’s skin. It mediates most of the interactions between a plant and its environment. Epidermal cells secrete a waxy substance called cuticle, which coats, waterproofs, and protects the above-ground parts of plants. Cuticle helps prevent water loss, abrasions, infections, and damage from toxins.

Ground Tissue

Ground tissue makes up much of the interior of a plant and carries out basic metabolic functions. Ground tissue in stems provides support and may store food or water. Ground tissues in roots may also store food.

Vascular Tissue

Vascular tissue runs through the ground tissue inside a plant. It consists of xylem and phloem, which transport fluids. Xylem and phloem are packaged together in bundles, as shown in Figure below.

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Bundles of xylem and phloem run through the ground tissue inside this stalk of celery. What function do these tissues serve?

Growth of Plants

Growth of Plants

Most plants continue to grow throughout their lives. Like other multicellular organisms, plants grow through a combination of cell growth and cell division. Cell growth increases cell size, while cell division (mitosis) increases the number of cells. As plant cells grow, they also become specialized into different cell types through cellular differentiation. Once cells differentiate, they can no longer divide. How do plants grow or replace damaged cells after that?

The key to continued growth and repair of plant cells is meristem. Meristem is a type of plant tissue consisting of undifferentiated cells that can continue to divide and differentiate. Meristem at the tips of roots and stems allows them to grow in length. This is called primary growth. Meristem within and around vascular tissues allows growth in width. This is called secondary growth.

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Roots

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, their can’t absorb water and minerals as well. However, they may be even better at transporting fluids, anchoring the plant, and storing food.

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Stems

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, flower, 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 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.

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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

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 are many specialized cells in the leaf.

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 calledstomata (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.

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

Plant Life Cycles

Life Cycle of Seedless Vascular Plants

Unlike nonvascular plants, all vascular plants—including seedless vascular plants—have a dominant sporophyte generation. Seedless vascular plants include clubmosses and ferns. Figure below shows a typical fern life cycle.

In the life cycle of a fern, the sporophyte generation is dominant.

A mature sporophyte fern has the familiar leafy fronds. The undersides of the leaves are dotted with clusters of sporangia. Sporangia produce spores that develop into tiny, heart-shaped gametophytes. Gametophytes have antheridia and archegonia. Antheridia produce sperm with many cilia; archegonia produce eggs. Fertilization occurs when sperm swim to an egg inside an archegonium. The resulting zygote develops into an embryo that becomes a new sporophyte plant. Then the cycle repeats.

Life Cycle of Gymnosperms

Gymnosperms are vascular plants that produce seeds in cones. Examples include conifers such as pine and spruce trees. The gymnosperm life cycle has a very dominant sporophyte generation. Both gametophytes and the next generation’s new sporophytes develop on the sporophyte parent plant. Figure below is a diagram of a gymnosperm life cycle.

The gymnosperm life cycle follows the general plant life cycle, but with some new adaptations. Can you identify them?

Cones form on a mature sporophyte plant. Inside male cones, male spores develop into male gametophytes. Each male gametophyte consists of several cells enclosed within a grain of pollen. Inside female cones, female spores develop into female gametophytes. Each female gametophyte produces an egg inside an ovule.

Pollination occurs when pollen is transferred from a male to female cone. If sperm then travel from the pollen to an egg so fertilization can occur, a diploid zygote results. The zygote develops into an embryo within a seed, which forms from the ovule inside the female cone. If the seed germinates, it may grow into a mature sporophyte tree, which repeats the cycle.

Life Cycle of Angiosperms

Angiosperms, or flowering plants, are the most abundant and diverse plants on Earth. Angiosperms evolved several reproductive adaptations that have contributed to their success. Like all vascular plants, their life cycle is dominated by the sporophyte generation. A typical angiosperm life cycle is shown in Figure below.

Life cycle of an angiosperm

The flower in Figure above is obviously an innovation in the angiosperm life cycle. Flowers form on the dominant sporophyte plant. They consist of highly specialized male and female reproductive organs. Flowers produce spores that develop into gametophytes. Male gametophytes consist of just a few cells within a pollen grain and produce sperm. Female gametophytes produce eggs inside the ovaries of flowers. Flowers also attract animal pollinators.

If pollination and fertilization occur, a diploid zygote forms within an ovule in the ovary. The zygote develops into an embryo inside a seed, which forms from the ovule and also contains food to nourish the embryo. The ovary surrounding the seed may develop into a fruit. Fruits attract animals that may disperse the seeds they contain. If a seed germinates, it may grow into a mature sporophyte plant and repeat the cycle.

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