CHANGES OF STATE:
PLANTS

  • plants have a xylem and phloem
    • xylem takes the water up from the roots to the leaves
    • phloem takes the water from the leaves to the roots
  • most of the water taken up to the leaves throught the xylem is lost through a process called transpiration
  • Transpiration
    • the process through which water is lost through pores on the leaves called stomata
    • as water is lost through the leaves, it creates a negative pressure
    • it is this negative pressure that causes more water to move up the stem and take its place
    • this flow of water is called the transpiration stream
      • the transpiration stream can move up to 45 meters per hour
      • water must be highly viscous in order to travel at these speeds






















  • Natural Cooling System
    • as water evaporates from the leaves, it takes energy with it
    • this cools the leaves
    • this cooling system keeps the leaves from overheating
      • if the temperature is too high, the cells in the leaves would stop functioning
  • on a hot, dry, sunny day, a plant can loose 100% of its water in less than an hour
    • this water needs to be replaced at a steady rate throughout the day
  • if water is on the leaves in direct sunlight, the leaves will be scorched
    • the water acts as a magnifying lens and burns the leaf tissue
  • How Trees Survive Winter
    • in winter, when water freezes into ice, it's impossible for trees to obtain water
      • therefore, transpiration is impossible
    • there are two types of trees: deciduous and evergreen
    • deciduous
      • these trees loose their leaves
      • this process helps them retain there water supply
      • when they loose their leaves in autumn, they already start to grow buds containing new leaves
      • these buds must survive the winter, for they are need for photosynthesis
      • these buds as well as the bark are fairly water-tight
      • similar to seeds, these buds "know" when to open, for they can tell when winter has ended and spring has arrived
      • some tell time using the number of hours of sunlight in a day, while others rely on temperature
    • evergreens
      • these trees maintain their leaves on their branches
      • their leaves are usually covered in a thick layer of wax to reduce water loss
      • conifer leaves are also shaped to reduce water loss
      • evergreen sap is also special, for it becomes rich in sugars to keep it from freezing
    • trees gather lots of nutrients during the summer to supply them throughout the winter
  • Cohesion
    • particles of water have a slight electrical attraction to one another
    • this attraction in known as cohesion
    • when a water particle leaves the stomata, it is this attraction that causes the particle to pull another particle out of the stomata with it
  • Adhesion
    • by combining cohesion and adhesion, water can travel up the xylem in a continuous flow, allowing transpiration to occur
    • this is the attraction of the water particles to the sides of the xylem
    • this attraction allows it to travel up the xylem instead of flowing back down




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