Caitlin's+Project+Chapter+3

__**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

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 * **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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