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 would the
tension in the xylem be highest? (A) 6 PM and 10 PM, (B) 12 PM and 6
PM, (C) 6 AM and 12 PM, (D) 4 AM and 10 AM, or (E) 2 AM and 8 AM.
First, let’s look at what the graph
is showing. 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 the plant. This just refers to how quickly
water travels up the stem of the plant in the xylem from the roots where it is
absorbed to the leaves where it evaporates during transpiration.
The loss of water from the leaves
of the plant that occurs during transpiration creates tension in the walls of the
xylem vessels. It’s this tension which is
responsible for pulling water up the xylem. We can therefore say that the
higher the rate of water flow through a plant, the higher the tension in the
xylem.
The question is asking us to state
the time interval between which the tension in the xylem is the highest. So, we need to find the point on
the graph at which the rate of water flow is highest, which is here. Of the time intervals given in the
answer options, this peak is found between 12 PM and 6 PM.
The correct answer is therefore
(B). The tension in the xylem would be
highest between 12 PM and 6 PM.
