HortBotany

Lesson Plan #6 – instructor

Roots

  • Functions of a root system:
  • Absorption: the root hairs near the tips of actively growing roots absorb water and dissolved minerals. Root hairs aremodified epidermis cells. Root hairs increase the surface area for absorption.
  • Anchorage: the root system of a plant supports and anchors the plant in the soil.

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  • Plants, in general, have one of two types of root systems:
  • tap roots; more typical of the dicots. The tap root develops from the radicle, which is the first thing to emerge from a germinating seed. Eventually, smaller branch roots develop on the tap root.
  • fibrous root system; more typical of the monocots. In monocots the radicle is short-lived and the root system becomes highly branched.
  • Storage: some plants have root systems that have been modified for food storage. Plants store extra glucose produced during photosynthesis in modified roots as starches. It is an advantage, from the plants perspective, to store excess food underground and away from browsing animals.

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  • Two major types of root modifications for food storage:
  • Fleshy tap root; examples include carrots and beets.
  • Tuberous roots; examples include sweet potato and Dahlia.

  • Conduction: roots contain vascular tissue (xylem and phloem); water and dissolved minerals, absorbed by the roots, are transported through the xylem of the roots, into the stem xylem and eventually into the leaves. In a like manner, sugars are transported from the leaves down into stems and roots for storage.

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  • There are 3 main factors that affect the movement of water through plants. They are:
  • Osmosis
  • Transpiration
  • The properties of water molecules
  • Let’s begin by looking at osmosis. Definition of osmosis: The diffusion (movement) of water across a semi-permeable membrane.
  • Osmosis is occurring in the root hairs found at the tips of actively growing roots.
  • The semi-permeable membrane that water must cross as it moves from the soil into a root hair is the cell membrane of the root hair.
  • During osmosis water molecules attempt to equalize their concentration on both sides of the cell membrane of a root hair. The water typically found in the soil contains small quantities of dissolved “stuff” like salts (also known as minerals or nutrients). The protoplasm of a root hair, however, contains water but in that water is a large amount of dissolved “stuff” (sugars, various nutrients, salts, and other chemicals).

Compare the concentration of water, and the amount of “stuff dissolved in the water, between the soil and protoplasm of a root hair:

SOILPROTOPLASM

water concentration:highlow

amount of “stuff”

dissolved in the water:lowhigh

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  • So, what makes osmosis work? Why does water move from the soil into a root hair?
  • Water in the soil is in high concentration compared to the low concentration of water in the protoplasm of a root hair.
  • Water always moves from areas of high concentration to areas of low concentration.
  • Water moves from the soil (where it is in high concentration) into a root hair (where it is in lower concentration).

  • As water moves into a root hair it is stored in the central vacuole. As the vacuole expands it pushes the cell cytoplasm and membrane against the cell wall. When a cell is full of water it is said to be turgid.
  • Root Pressure is the force responsible for “pushing” water through roots and up stems.
  • Root pressure is driven by:
  • Osmosis
  • Turgor Pressure

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  • How does root pressure work?
  • The vacuoles in the cells of the epidermis and cortex fill with water.
  • As more water enters the root hairs by osmosis, it squeezes the water out of the cells and into the intercellular spaces between the cells.
  • Once in the intercellular spaces the water quickly moves across the cortex and into the xylem.
  • Root pressure can only move water a few feet up a stem.

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  • The second factor affecting water movement through plants is transpiration. Transpiration defined: The loss of water vapor through open leaf stomata.
  • Why are the leaf stomata open? During the day the plant is photosynthesizing, the stomata must be open to allow carbon dioxide into the leaf.
  • Root pressure pushes water up stems. Transpiration “pulls” or “sucks” water up stems in much the same way that you pull liquid up through a straw.
  • The rate of transpiration is affected by the following 3 things:
  • Humidity; during dry, less humid days the rate of transpiration increases.
  • Temperature; the rate of transpiration is greater on hot days than on cooler, cloudy days.
  • Wind; the rate of transpiration is greater on windy days than calmer days.
  • What happens to the plant when water is in short supply?
  • When more water is lost to transpiration than the roots are able to absorb the stomata quickly close to conserve water.

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  • How does transpiration work?
  • On sunny, warm days the sun warms up the leaf surfaces.
  • The water in the cells of a leaf begins to evaporate.
  • Water vapor collects in the spongy mesophyll tissue of the leaf.
  • The water vapor moves out of spongy mesophyll, through the open stomata, and into the drier air surrounding the leaf.
  • As water evaporates from the spongy mesophyll, new water molecules are delivered to the leaf through the xylem in leaf veins.
  • Transpiration is the most important of the 3 factors affecting water movement in plants.

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  • The third factor affecting the movement of water through plants is the properties of water molecules.
  • Water molecules (H2O) are electrically charged. Having a positive charge at one end and a negative charge at the other end of the molecule.
  • Because of this water molecules stick to one another like little magnets.
  • Water moves through the xylem as long chains of molecules sticking to each other.

H+H+

O-

  • To summarize:
  • Osmosis is responsible for getting water into a root hair.
  • Root pressure pushes water through the intercellular spaces of the cortex and into the xylem.
  • Transpiration pulls water up through the stem xylem and ultimately into the vein xylem of leaves.Water moves through xylem as long chains of water molecules sticking to one another like little magnets.

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