Plant Structure and Function
 

 

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Key Concepts

bulletAngiosperms and gymnosperms are the predominant plants on planet Earth. All are vascular seed plants with a basic shoot system and root system pattern of organization.
bulletAll plant growth originates from meristems. Growth in length is called primary growth.
bulletPlants have three tissue systems: dermal tissues, ground tissues, and vascular tissues. As in animals, a tissue is a group of  cells with similar structure working together for a common function.
bulletThe plant tissues are arranged to form a specific pattern in shoots, leaves , and stems. This pattern is different in monocot and dicot angiosperm plants.
bulletSome plants have persistent above ground growth that gets larger in diameter from year to year. This growth in diameter is called secondary growth.
bulletExcept for carbon dioxide, plants obtain water and nutrients from the soil through their roots. 
bulletWater is transported through the vascular tissue to the shoots and leaves. Plants also have several mechanisms to conserve water when needed.
bulletCarbon dioxide enters the leaf through openings in leaves (and sometimes stems) called stomata.
bulletAs organic materials are produced within the plant, they must be transported to other parts of the plant. This transport may have to be in any direction from the source of the organis material.
bulletPlants have five primary hormones that regulate much of plant growth and development.
bulletGrowth may be adjusted in direction and speed by environmental influences through tropisms.
bulletFlowering in plants is also done in response to environmental cues.

Objectives

Supplement the content material with the following links:

bulletGeneral sites.
bullet Starr and Taggart Textbook site.  This site has numerous links and additional information. It is organized by chapter.
 

PLANT STRUCTURE AND FUNCTION

 Click on the topic from the list below to get to a specific section of this unit.

Plant Form
Plant Tissues
Roots
Stems
Leaves
Secondary Growth
Water and Nutrient Acquisition

Fluid Transport
Regulation of Plant Growth

Plant Form

bulletThere is a great deal of material available on the web to support the chapters on plant structure and function. The following guide to this unit will direct you to a few of the better sites that contain text material and images of the structures in plants.
bulletWe start by looking at the overall structure of a plant. All seed plants consist of a shoot system above ground and a root system below ground. Roots systems can be very extensive. They do not primarily function to anchor the plant in the soil. Instead, the root system functions to absorb water and nutrients for the plant. Because those nutrients are located only down in the soil, the plant must grow long extensions into the soil to access those nutrients. Once that is being done, the plant is necessarily (and secondarily) anchored in place. The shoot system serves to hold the leaves up in the air to access sunlight to drive photosynthesis. Stems are then needed to connect the two parts. Look at the diagram in your text in section chapter 29.1. 

Plant Tissues

bulletLike all multicellular organisms, plants are constructed of a variety of tissues. Depending on the source, you will see three basic tissue systems for plants plus meristems. Meristems are a tissue where cell division occurs. They are the only cells in a plant where growth (cell division) occurs. So, if there is growth, there must be a meristem. The following link discusses plant tissues: click on the tissue types as we go through this outline of the tissues in plants.
bulletIf the growth adds to the length of the plants (tips of shoots and roots), it is called primary growth and occurs at a type of meristem called an apical meristem (it is located at the tip or apex of the structure, hence apical). 
bulletIf the growth adds to the diameter or girth of the plant (as occurs in trees and shrubs), it is called secondary growth and occurs at a type of meristem called a lateral meristem. We will look at two specific types of lateral meristem when we discuss woody stems.
bulletThe other three groups are referred to as tissue systems. 
bulletDermal tissues form the outer layer of the plant and secrete the cuticle (the waxy covering that helps regulate water loss). They stomata are also considered to be part of the dermal tissue. The following link discusses plant tissues; click on the tissue types as we go through this outline of the tissues in plants. A second link to an on-line botany book also looks at these tissues.
bulletGround tissues perform a variety of functions and are divided into three cells types. Parenchyma cells are thin walled and do much of the metabolic activity of plants. They are where photosynthesis occurs, serve as storage cells for plant products, and secrete plant products. Collenchyma  cells are thicker walled cells that provide support. Sclerenchyma cells are long cells and have even thicker walls with lignin in them. These provide the primary support for plants and make up most to the fibers that we use to make rope (hemp, sisal), cloth (linen, cotton), and paper. Some sclerenchyma cells are shorter and thicker--they make up the gritty material in the texture of pears or the hard coating around nuts. The following link discusses plant tissues; click on the tissue types as we go through this outline of the tissues in plants. A second link to an on-line botany book also looks at these tissues.
bulletVascular tissues form long conduits for movement of water and other materials through the plant. There are two types of vascular tissues. Xylem conducts water and dissolved nutrients from the roots up the plant. This is a one-way movement. Xylem cells are dead at maturity and do not actively move the water. The mechanism of action will be described later. Phloem consists of living cells that move plant products from one part of the plant to another. materials may move in any direction in this tissue. The following link discusses plant tissues; click on the tissue types as we go through this outline of the tissues in plants. A second link to a Botany On-line Site also looks at these tissues.
bulletThese tissues are then organized to form the basic structures of the plant--roots, stems, and leaves. All of the flowering plants can be divided into two major groups based on the differing arrangement of these tissues to form the structures. These two groups are moncotyledonous plants (monocot) and dicotyledonous (dicot) plants. The name is derived from the number of embryonic leaves in the seed--one in monocots and two in dicots. The internal anatomy of the root, stem , and leaf also differ. See this site on the difference between dicots and monocots. As you go through each of the three structures below, make sure you note the difference in structure (internal and external) between the monocot and dicot version.
bulletROOTS: Look first at the root tip--the growing portion of each root. Since there is primary growth, there must be an apical meristem. As cells are produced, the tip of the root (covered by a protective root cap) is pushed through the soil. Most of the actual increase in length comes from the elongation of the newly produced cells as they take in water. Farther from the root tip, the cells begin to differentiate into the different types of tissue systems. One feature of roots that is not found in stems is that the vascular tissues are separated from the rest of the ground and dermal tissues inside a structure called the vascular cylinder. The cylinder is surrounded by an endodermis which has a waxy substance called the Casparian Strip between the cells. This forces all water with its dissolved nutrients to go through a cell membrane before it can enter the vasculear cylinder and be transported to the rest of the plant. Just inside the endodermis is another layer of cells called the pericycle. This layer gives rise to branch roots--new roots. Remember that the root must be attached to the vascular cylinder or will not be functional. So new root growth begins by the pericycle growing and pushing out through the endodermis and the outer coverings of the root. This set of web pages from McDaniel College gives an excellent overview of root function, types of roots, anatomy and the differences between monocot and dicot roots. Make sure you note the different structures in the diagrams and the differences between monocot and dicot root structure.
bulletSTEMS:  Like roots, the stem has an apical meristem to provide primary growth. The apical meristem is located in the stem apex--a structure with a very different appearance from the root tip. Here is an excellent image of the stem apex. Each bud also has the beginings of a new apical meristem, usually called an axillary meristem until the new bud begins to actually grow. The tissue arrangement within the stem differs between monocots and dicots. This set of web pages from McDaniel College gives an excellent overview of stem function, types of stems, anatomy and the differences between monocot and dicot stems. Make sure you note the different structures in the diagrams and the differences between monocot and dicot stem structure.
bulletLEAVES:  Leaves are designed to do photosynthesis. Their broad surfaces maximize light absorption as does the internal arrangement of tissues. Stomata are usually more numerous on leaves as this is where carbon dioxide enters the plant. The guard cells around each stoma can help regulate water loss. As in the rest of the above ground plant, a distinct, non-cellular cuticle protects against water loss. The tissue arrangement within the leaf differs in monocots and dicots. This set of web pages from McDaniel College gives an excellent overview of leaf function, types of leaves, anatomy and the differences between monocot and dicot leaves. Make sure you note the different structures in the diagrams and the differences between monocot and dicot leaf structure.

Secondary Growth

bulletSecondary growth is growth in diameter of plants. It is normally found only in trees and shrubs--plants that have a permanent, above-ground structure. This above-ground structure increases in diameter each growing season. We refer to these more generally as woody plants. This new tissue is produced by two different lateral meristems. The vascular cambium produces new vascular tissue (xylem and phloem) each year. The new xylem and phloem is referred to as secondary xylem and phloem. The xylem accumulates in annual layers we call tree rings and the phloem is slowly crushed up against the outer layer of cork, leaving only this year's phloem still functioning. The cork layer must also increase in diameter to cover the growing stem. New cork cells are produced by a meristem called cork cambium. This set of web pages from McDaniel College gives an excellent presentation on woody stems. Another web site called the Ultimate Tree-ring pages has wonderful images of whole trees and tree rings. Scroll down the gallery page until you get to the tree rings section. The Douglas Fir Image is particularly good, but the rest of the annotated images will give you a good idea of what tree rings can be used for.

Water and Nutrient Acquisition

bulletPlants obtain all their nutrients except carbon dioxide from the soil. Most nutrients are dissolved in water that exists between the soil particles. The water can get through the outer portion of the root (the cortex) in two ways.  One way is through the cells of the cortex. This occurs because cells actively transport some nutrients into the cell and water follows by osmosis. The other method is that it can move between the cell walls and not actually enter a cell. Water moving between cells is stopped by the waxy Casperian strip and can only get into the vascular cylinder by entering the cells of the endodermis. This process ensures that cells of the plant have an opportunity to regulate what enters the vascular cylinder and is then transported to the rest of the plant. The plant roots also form associations with other soil inhabitants to enhance plant nutrition acquisition. This web site has a good section on plant nutrition and plant hormones.

Fluid Transport

bulletOnce fluids enter the vascular cylinder, the next issue is how to move water from the roots up to the leaves. 
bulletSince water doesn't flow up hill, some sort of work will have to be done to move it. The theory describing this process goes by several names; your text calls it the cohesion-tension theory. Other books call it the transpiration-cohesion-tension theory. In any case, the series of events in the process is the same. It begins with loss of water from the leaf. Water evaporates inside the leaf airspaces and then diffuses out of the open stomata along with oxygen from photosynthesis at the same time carbon dioxide enters. Plants can regulate water loss by changing the size of the stomata openings. The guard cells change shape to make this occur. This process involves changes in potassium and calcium ion levels in the guard cells (see animation on guard cell action by Terri Brown and how calcium works in this process). The loss of water from the leaf is called transpiration. The loss of water exerts tension on the hydrogen bonds holding the remaining water molecules together. Since water has the property of cohesion, it is literally pulled up the tiny xylem tubes. Make sure you do the tutorial on Water Relations from the Connecting Concepts web site of the University of Wisconsin. This series of three topics will help you to understand how and why water moves the way it does in plants. (See also McDaniel College site for more details of the tension-cohesion process.) The xylem cells do not actively contribute to the process as they are not alive. Basically, this is a solar-powered process.
bulletMovement of fluids containing plant products is done entirely differently. Sugars are used to fuel plant activities--production of ATP through cellular respiration as in animals. In addition, sugars may be stored in various locations--in flowers as nectar to attract pollinators, in fruits to attract animals to disperse seeds, and in the roots and stems as reserves for the winter and early spring growth. So, the phloem tissues needs to be able to move sucrose (and other plant products such as hormones) in different directions at different times. this requires a living, actively participating cell. The process here is referred to a the pressure flow theory. The flow is from a source (an area with an overabundance of a sucrose for example) to a sink (an area needing more sucrose). Basically, the companion cells at the source actively transport sucrose into the phloem tubes. The high concentration of sucrose causes water to enter the cells due to osmosis. This creates an elevated pressure area. at the sink, the reverse is occurring. Companion cells actively transport sucrose out of the phloem tubes leading to a loss of water and pressure. the fluid then simply flows from the high pressure area to the low pressure area. This overall process is sometimes also referred to as translocation. (see McDaniel College site for more details on this process.) A simplified animation by Terri Brown is also available.

Regulation of Plant Growth

bulletPlants regulate their growth and development in a number of ways. Three specific aspects of this issue will be considered in this section:  hormones, tropisms, and control of flowering (reproduction).
bulletHORMONES:  Plants have five major hormones that are relatively well understood. Each has numerous effects on the plant. Your text summarizes these effects. More extensive treatments of all five hormones is found at McDaniel College and at the Botany Online site. One of the effects of two hormones interacting is the phenomenon of apical dominance. In this situation, an apical meristem appears to inhibit growth of meristems imediately below it. The farther away a meristem is from the apical meristem, the less it is inhibited. This action leads to the typical plant shape--narrower at the top and increasingly broad toward the bottom (think of the "perfect" christmas tree.) We also manipulate hormones to deliberately control aspects of plant growth. Examples are solutions put on roots as new plants are planted and the use of ethylene to control ripening. A commercial site about ethylene shows some of the applications.
bulletTROPISMS:  Tropisms are growth responses of plants to environmental stimuli. There are three basic tropisms:  gravitropism (sometimes called geotropism, it is a response to gravity), phototropism (response to light), and thigmotropism (response to physical contact). The McDaniel College site covers these topics very well. The Botany Online site has good information on all three tropisms:  Gravitropism page, phototropism page, and thigmotropism page. There is also an excellent, interactive site on gravitropism from Penn State and and some excellent movies of plant motions.
bulletFLOWERING:  Flowering in plants is also a response to external cues, primarily day/night length. this response is called photoperiodism. Plants contain light sensitive pigments that change form depending on the wavelength of light to which they are exposed. Some plant flower only when days are getting shorter (nights longer); they flower in the Fall. Others flower only as days are getting longer (nights shorter); they flower in the Spring and summer. There are plants that do not respond to these cues at all; they are day-neutral and flower anytime. Dandelions are a good example of day-neutral plants. The McDaniel College site covers this topic at the end of the same page that covered tropisms. Lastly, there is an excerpt from the book The Rose's Kiss that has a well written discussion of flowering written for the layperson.

Assignments

1. Go to the Plant Assignment 1. Complete the questions in the assignment. BE COMPLETE. Submit the answers to these questions for grade.

2. Go to Plant Assignment 2. Complete the questions in the assignment. BE COMPLETE. Submit the answers to these questions for grade.

3. Go to Plant Assignment 3. Complete the questions in the assignment. BE COMPLETE. Submit the answers to these questions for grade.

REMEMBER:  Complete each assignment in each unit. Each will be  graded and averaged into your course grade. Once you take the exam on this unit, I will no longer accept assignments or corrections of assignments for this unit. At my discretion, 5% per day may be deducted from the score of any assignment not submitted by the due date.

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