get water to the top of very tall plants, like sequoia trees. This is called the cohesion theory. Food, messenger RNA, plant hormones, and certain organic substances made by the plant cells are transported in the phloem of the plant. The main sugar made in plants is sucrose. This, along with amino acids and small organic molecules enter the cells and pass through plasmodesmata that connect cells to adjacent cells. Once in the phloem, the molecules can be transported to any part of the cell. The process of girdling a tree (removing a band of bark around the tree) removes the phloem. The tree will live for a period of time but will ultimately die because the roots are starved. Transfer of food content through the phloem is dependent on the metabolism of the phloem cells, which is completely different from the xylem. Sugar will leave the sieve tubes near the leaves and stems (pumped out through active transport), with water following by osmosis. This increases the turgor (or pressure) in these areas, which causes the food and water to be pushed through the phloem under this high pressure. The sugar is used by the plant for nutrition and any that is left over becomes starch. Starch is insoluble in water and doesn’t affect the concentration of sugar in the phloem.
TRANSPIRATION This is the evaporation of water from primarily the leaves. Leaves have stomata that are open for CO2 and O2 to pass as part of photosynthesis. The surrounding air doesn’t have a hundred percent humidity, making it drying for the leaves and resulting in substantial evaluation. This transpired water needs to be replaced by the transport of water from the roots to the leaves through the xylem. This provides the engine for pulling water up from the roots, bringing minerals along with it. The process also cools the leaves, similar to the way that the evaporation of water from human skin will cool the body. There are several factors that impact the amount of transpiration that goes on, including the following: •
The amount of light—there is greater transpiration during light hours. This is because light will warm the leaf and will stimulate the opening of the stomata. 181
