Plant Leaves

3 Key excerpts on "Plant Leaves"

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  eBook - ePub

  The Handy Biology Answer Book

  • Patricia Barnes-Svarney, Thomas E. Svarney(Authors)
  • 2014(Publication Date)
  • Visible Ink Press

    (Publisher)

  M any houseplants, annual plants, and fruit trees exhibit a phenomenon in which the terminal bud produces hormones inhibiting the growth of axillary buds. This allows the plant to grow taller, increasing its exposure to light. Under certain conditions, the axillary buds begin to grow, producing branches. This can occur when the terminal bud is pruned (“pinched back”) on certain plants and fruit trees, stimulating the axillary bud growth and producing bushy, full-looking plants.

  What are leaves?

  Leaves are the main photosynthetic organ for plants; they are also organized to maximize sugar production while making sure little water loss occurs. Thus, they are also important in gas exchange and water movement throughout the whole plant. Leaves—outgrowths of the shoot tips—are found in a variety of shapes, sizes, and arrangements. Most leaves have a blade (the flattened portion of the leaf), a petiole (the slender stalk of the leaf), and leaflike stipules (found on some leaves and located at the base of the petiole where it joins the stem). Cross-sections of a leaf also show a variety of features, including the cuticle (the outer covering to minimize water loss), veins (also called vascular bundles, which carry water and nutrients from the soil to the leaf and also carry sugar), stoma (plural stomata; water escapes through the stoma; they open and close), guard cells (modified epidermal cells that contain chloroplasts and control the opening of the stoma), and palisade and spongy mesophyll cells (for photosynthesis).

  A cutaway of a typical leaf shows its interior structure.

  What is a cuticle in a plant leaf?

  The cuticle contains a waxy substance, called cutin, that covers the parts of the plant exposed to the air: the stem and leaves. It is relatively impermeable and provides a barrier to water loss, thus protecting the plant from desiccation.

  What is the purpose of the stomata?

  Stomata (singular “stoma” from the Greek term stoma , meaning “mouth”) are specialized pores in the leaves and sometimes in the green portions of the stems, as well as flowers and fruits. Carbon dioxide (CO2 ) enters the plant through the stomata, while water vapor escapes through the same pores. The guard cells that border the stomata expand and contract to control the passage of water, carbon dioxide (CO2 ), and oxygen (O2

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  eBook - ePub

  Biophysical Ecology

  • David M. Gates(Author)
  • 2012(Publication Date)
  • Dover Publications

    (Publisher)

  Chapter 3

  Application to Plants

  Introduction

  One must understand, in detail, the functioning of a leaf in order to understand many ecological phenomena concerning plants. Once the mechanisms by which a leaf carries out its vital functions are recognized, one can put together a complete analysis or model relating the properties of the environment to the vital functions of a leaf. Since photosynthesis and respiration are of primary importance to a plant, we shall first look at how these processes take place.

  Photosynthesis requires light, carbon dioxide, and a suitable temperature. The temperature of a leaf is the result of energy exchange between the leaf and the environment. The energy exchange involves radiation, and a component of the incoming radiation is the light which drives the photochemical reactions. Exchange of energy is affected by the diffusion of water vapor from the leaf mesophyll out through the stomates to the free air beyond the leaf’s adhering boundary layer. Carbon dioxide is supplied to the chloroplasts within the leaf mesophyll by diffusion inward through the stomates, cell walls, and cytoplasm. Once carbon dioxide arrives at the chloroplasts, it must undergo a chemical reaction in the process of photosynthesis compatible with its rate of diffusion to the chloroplasts. These chemical reactions are regulated by light, temperature, and the concentration of carbon dioxide, as well as by many other factors.

  Temperature is important to a leaf not only because of its influence on the rates of photosynthesis, respiration, and transpiration, but also because of its effect on the cytological state of plant cells. Protoplasmic streaming and the stability of plant proteins are temperature dependent. Plants must have the ability to withstand the high summer temperatures that may occur during the growing season and to withstand limited amounts of cold at the same time. In one form or another, either as seeds, roots, bulbs, or as a whole, plants must survive winter temperatures that may be very low. Depending upon where the plants are growing, whether in boreal, temperate, desert, or tropical habitats, extreme temperatures may be high or low, or both. Of course, the matter of high or low temperatures is relative. What is low for one plant may be high for another. Evergreens in temperate and boreal regions must be adapted to intense periods of cold, yet must also be able to become active and photosynthetically productive when daytime conditions of sunshine and air temperature warm the plant to temperatures above–10°C. It has been established that photosynthesis occurs at temperatures above 0°C, but there is considerable evidence that photosynthesis will proceed even at lower temperatures. Furthermore, in the spring of the year, most plants generate new leaves; these leaves must withstand brief periods of cold, respond quickly to the presence of warmth and sunshine, and may also have to survive becoming overheated. It is evident that the temperature of a leaf is vital to its survival, its well-being, and its ability to function physiologically. Basic to the process of photosynthesis in a whole leaf is energy exchange. Our first task is to show how energy exchange regulates leaf temperatures, which are in turn extremely important to the vital processes within the leaf.

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  eBook - ePub

  Biology For Dummies

  • Rene Fester Kratz(Author)
  • 2017(Publication Date)
  • For Dummies

    (Publisher)

  Part 5

  It’s Not Easy Being Green: Plant Structure and Function

  IN THIS PART … Get to know plants up close and personal. Explore how plants function. Passage contains an image Chapter 20

  Living the Life of a Plant

  IN THIS CHAPTER

  Looking at the structure of plants

  Acquiring what a plant needs to keep growing

  Exploring the differences between asexual and sexual reproduction

  A plant’s structure suits its lifestyle. After all, it has flat leaves for gathering sunlight, roots for drawing water up from the soil, and flowers and fruits for reproduction. Plants begin their lives from seeds or spores, grow to maturity, and then reproduce asexually or sexually to create new generations.

  In this chapter, I present the fundamental structures of plants, how they get the energy they need to grow, and their reproductive strategies.

  Presenting Plant Structure

  Like animals, plants are made of cells and tissues, and those tissues form organs, such as leaves and flowers, that are specialized for different functions. Plants have two basic organ systems: a root system (which exists underground) and a shoot system (found aboveground). The root system is responsible for anchoring the plant and also absorbing minerals and water from the soil. The shoot system ensures the plant gets enough sunlight to conduct photosynthesis; it also transports water upward from the roots and moves sugars throughout the plant.

  Within their organ systems, plants have up to three types of tissues. Biologists look at the types of tissues a plant has to help them classify plants into four different groups. They also look at the different structures of plant stems.

  Plant tissues

  All plants have tissues, but not all plants possess all three of the following types of tissues:

  • Dermal tissue: Consisting primarily of epidermal cells, dermal tissue covers the entire surface of a plant. Guard cells in a plant’s epidermis control the opening and closing of little holes called stomates that allow the plant to exchange gases with its environment (you can see a stomate in the leaf cross section in Figure 20-1

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