Explain the role of auxin in phototropism

Phototropism is the growing or turning of an organism in response to a unidirectional light source
Auxins (e.g. IAA) are plant hormones that are produced by the tip of a shoot and mediate phototropism
Auxin makes cells enlarge or grow and, in the shoot, are eradicated by light
The accumulation of auxin on the shaded side of a plant causes this side only to lengthen, resulting in the shoot bending towards the light
Auxin causes cell elongation by activating proton pumps that expel H+ ions from the cytoplasm to the cell wall
The resultant decrease in pH within the cell wall causes cellulose fibres to loosen (by breaking the bonds that hold them together)
This makes the cell wall flexible and capable of stretching when water influx promotes cell turgor
Auxin can also alter gene expression to promote cell growth (via the upregulation of expansins)

Explain how water is carried by transpiration stream, including the structure of xylem vessels, transportational pull, cohesion, adhesion, and evaporation

Some of the light energy absorbed by leaves changes into heat, converting water in the spongy mesophyll into vapor
This vapor diffuses out of the stomata and is evaporated, creating a negative pressure gradient in the leaf
New water is drawn from the xylem (mass flow), which is replaced by water from the roots (enters from soil via osmosis)
The flow of water through the xylem from the roots to the leaf is called the transpiration stream
Water rises through xylem vessels because of two qualities:
Cohesion:  Water molecules are weakly attracted to each other via hydrogen bonds
Adhesion:  Water molecules form hydrogen bonds with the xylem cell wall
These properties create a suction effect (or transpiration pull) in the xylem
The xylem has a specialized structure to facilitate transpiration:
The inner lining is composed of dead cells that have fused to create a continuous tube
These cells lack a cell membrane, allowing water to enter the xylem freely
The outer layer is perforated (contains pores), allowing water to move out of the xylem into the leaves
The outer cell wall contains annular ligin rings which strengthens the xylem against the tension created by the transpiration stream

Compare growth, due to apical and lateral meristems in dicots

Apical Meristems
Occurs at the tips of roots and shoots
Responsible for primary growth
Develops into primary xylem and phloem
Produces new leaves and flowers

Lateral Meristems
Occurs at the cabium
Adds lateral growth to stem (Increase width)
Responsible for secondary growth
Produces secondary xylem and phloem
Produces bark on trees

Similarities in close they are composed of totipotent cells (Like stem cells)
and the are found in dicots