### Video Transcript

The graph provided shows the
relationship between the time of day and the rate of water flow through a plant. Between which hours was the rate of
transpiration the greatest? (A) 2 AM and 8 AM, (B) 2 AM and 6
PM, (C) 10 AM and 2 PM, (D) 12 PM and 6 PM, or (E) 8 PM and 10 PM.

First, let’s work out what the
graph is showing us. On the 𝑥-axis we have the time of
day, from midnight through to 10 PM. On the 𝑦-axis, we have the rate of
water flow through a plant. You may recall that a rate is how
frequently something happens in a given period of time. So, the rate of water flow is how
frequently water flows through the plant from the roots up to the leaves in a given
period of time.

The flow of water through a plant
is largely driven by transpiration pull. This is where water is lost from
the leaves through the stomata, so more water is pulled up the xylem vessels of the
plant to replace it. Therefore, the rate of water flow
is equivalent to the rate of transpiration.

This question asks us for the hours
between which the rate of transpiration was the greatest. We are therefore simply looking for
the highest point of the curve, which is here. Of the time intervals given in the
answer options, this peak falls between 12 PM and 6 PM.

So, the correct answer is (D). The rate of transpiration was the
greatest between 12 PM and 6 PM.