Positive Geotropism

• The plant tends to bend its stem toward the light in order to get full sunlight.
• Auxin builds up on the opposite side to make the bending.

2.81) Positive Phototropism

Positive phototropism = the growth towards the light

• Let's say we're given a tip of a stem. The direction in which the stem would grow toward depends on which side the light was shone on.
• For example, the light was shone from directly above the stem. The stem would then grow upward towards the light.
• If the light was shone only on the right side of the stem, then the stem would grow right towards the light.
• This happens because of a plant hormone called auxin. Auxin builds up on the side of stem with lesser light intensity, causing it to bend toward the opposite side (the side with greater light intensity). 

2.80) Geotropism

Geo= gravity; Tropism =growth; Geotropism =plant's growth and response to the gravity

• If a root/stem grows toward the gravity (downward), then it is a positive geotropism. If it grows against the gravity (upward), it is the negative geotropism. 
• For example, on a seed there is the embryotic shoot which grows upward (negative) and embryotic root which grows downward (positive).
• One experiment is that we rotate the seed by 90degree so that both embryotic shoot and root are parallel to the ground.
• The embryotic shoot would continue to grow upward, and in the same time the embryotic root would continue to grow downward.  This proved that for different structure the plant would produce different but specific response to the environment (in this case, the gravity).  

2.79) Plants and Stimulli

• Plant respond to stimuli.
• Stimuli --changes in the environment; Receptor --detect the stimuli and turn them into response; Response --reaction towards the change (often take in the form of growth)
• The response taken in the form of growth is called tropism. 
• There're 2 kinds of tropism we're going to discuss: Light -phototropism and gravity -geotropism.
• Receptor produce response by plant hormones (plant growth regulator). One example is auxin.

2.54) Transpiration

Recall that transpiration is the evaporation of water from the surface of a plant.

• In order for liquid water to evaporate into water vapour, we need the HEAT energy from sunlight which was absorbed by the chloroplasts and other cells in the leaves. (Sunlight travels as ultraviolet rays, pass through particles, and turned into long infrared rays which gives off heat.)
• Water from the roots moves up the stem and into the leaves.
• Water moves from xylem into the spongy mesophyll layer of the leaf. (It is here that the changing of water from liquid to gas takes place.)
• Sunlight absorbed gives the heat energy and turn liquid water into gas water vapour.
• Air in the leaf is full of water with 100% humidity. It is therefore denser than the outer environment. (Air outside has mostly 70% humidity.)
• The water vapour then diffuses out of the leaves from the stomata. (Diffusion is the movement of particles from area with greater concentration to area with lesser concentration.)

2.53) Uptake of Water

•  The roots have branching areas which increase the surface area for greater absorption of water, and along the branches there were root hairs for same purpose.
• Root hairs are the epidermal cell formed by the extension of cell wall.
•  Firstly, roots get minerals from the soil into the root hair by active transport. The intake of minerals makes it more concentrated inside than the outside.
• Water then is absorbed by osmosis into the root hairs. (Remember, is the movement of water from area with greater concentration to area with lesser concentration.)
• Water then moved out from the root cells and into the xylem.