Movement in Plants continued

Phloem Phloem transport sugars, hormones and other organic molecules throughout the plant. This process is called translocation. While xylem transport is generally one direction, the transport of materials within the phloem can go in any direction. In addition, phloem is composed of living cells. How materials move within the phloem is yet to be determined. One of the most supported theories is the Pressure Flow Hypothesis, also called the Mass Flow Hypothesis.

In the leaf, glucose is produced by photosynthesis and is converted into sucrose for transportation. The sucrose is actively transported into the cells of the phloem. This lowers the concentration of water in the phloem. Water from the xylem moves into the phloem by osmosis and increases the pressure in the phloem area. As the pressure increases, the sucrose will move to areas of lower pressure.

Once the sucrose arrives at the lower pressure areas it is actively removed from the phloem and into the surrounding cells. The cells will convert the sucrose back to glucose or another organic compound. When the sucrose is removed, the pressure drops as water also moves out of the phloem by osmosis.

Tropisms A Venus fly trap plant is one of the few carnivorous plants that exist. Through highly sensitive hairs, the lobes of the Venus fly trap can rapidly close upon stimulation trapping the unsuspecting insect or spider. Once trapped inside, any movement by the insect triggers the lobes to close tighter and digestive enzymes are secreted by glands inside the lobes. This type of rapid movement is seen in very few plants.

muscipula/video-00.html muscipula/video-00.html However, all plants show movement in response to an external stimulus like the sun, gravity or touch. This type of motion is called tropism. If a plant moves towards the stimulus, then it is a positive tropism. If the plant moves away from the stimulus, then it is negative tropism. Some parts of a plant may show positive tropism and other parts of the same plant may show negative tropism.

Phototropism This tropism is a response to light. The tips and stems of plants demonstrate positive tropism as they grow towards the light. The roots of plants grow away from the light and are negatively phototropic. You will recall that the hormone auxin is released to stimulate growth in stem cells that are not exposed to sunlight.

Geotropism or Gravitropism This type of tropism is affected by gravity. Roots will display positive tropism and stems and shoots will demonstrate negative tropism. Charles Darwin was one of the first scientists to identify the growth responses in both roots and stems in relation to gravity.

Thigmotropism This growth movement is in response to physical contact with another object. Ivy clinging to the surface of walls is an example of positive tropism. An example of negative tropism includes roots growing away from objects in the soil.

Hydrotropism Growth of roots towards water is an example of a positive hydrotropism. Roots will gravitate towards a supply of water.

Nastic Response The movement shown by the Venus fly trap is an excellent example of nastic movement. The nastic movement of plants is not dependent on the direction of the stimulus. Rather the movements are random and are not permanent.

Homework Various tropism videos: otion/movements/tropism/tropisms.html otion/movements/tropism/tropisms.html topnav/materials/listbytype/Tropisms_of_Plan ts.html#.VWt2rtLBxjM topnav/materials/listbytype/Tropisms_of_Plan ts.html#.VWt2rtLBxjM