#16 "Stem - Water Transort"

Color Code For This Biogram

• Colorless: root hair (b) and cortex (c)
• Blue: arrows, water (a) and xylem (f)
• Tan: endospermis (d)
• Yellow: pericycle (e)
• Orange: phloem (g)

   

  Transpirational "Pull"

  Water has a natural cohesiveness, surface tension, so will rise in a tube (capillary action), but not very high. How then does water rise from the soil to the leaves of tall trees? The air surrounding mesophyll cells in leaf tissue has a humidity of 100% making the air saturated with water. Air outside the plant usually has a lower humidity unless it is raining. The evaporation of moist air as it escapes the leaf tissue (through the stomata) provides a "pull" for water movement within the plant. Loss of water by evaporation from leaf surfaces is called transpiration. Water (a) moves upward in the plant from an area of higher water concentration in the soil, to an area of lower water concentration, the dry air outside the leaves.

  Pathway of Water Transport

  Water (a) in the soil is absorbed by the epidermal root hairs (b), then moves across the root through living cortical cells (c), is "filtered" through the semipermeable membranes of endodermal cells (d), passes through the pericycle (e), and enters the water-conducting cells of the xylem (f). Once in the xylem of the vascular system, water rises up the root to the stem and into the leaves. The maximum rate of water rise recorded in the roots of several oak species is 60 meters per hour. From the xylem, water enters the mesophyll tissue of the leaves and evaporates into the atmosphere through the stomata - when they are open. Much smaller amounts of water are lost from leaf cuticle and bark lenticels in the stems.

  Environmental Influences The rate of water transport in most plants is at a maximum during the daytime when air temperatures are high, relative humidity is low, and stomata are fully open. When the rate of water loss by transpiration exceeds the rate of water uptake from the soil, the stomata usually close. Many succulent plants that live in the desert open their stomata only at night in order to conserve water. Thus, water transport and transpiration in these plants occur primarily at night.

  Color Biogram #16, answer these questions and add both of them to your portfolio.

  1. Water tends to rise inside a tube. What is this called?
  2. How does water get out of leaf tissue?
  3. Where is water filtered before reaching the xylem?
  4. What type of tissue holds water in leaves?
  5. What three conditions exist when the rate of water transport is at its highest in plants?
  6. What happens in leaves when the rate of water loss by transpiration exceeds the rate of water uptake from the soil?