A group of students plan an experiment to test the effect of temperature on plant
growth. They plant radish seeds in 10
identical pots. Five pots are placed in a room
that is kept at 70 °F and given 10 hours of
light each day. Five pots are placed in
another room that is kept at 40 °F and given
8 hours of light each day. Both sets of plants
are watered every other day. After three
weeks, all the plants in the warmer room are
larger than the plants in the colder room.
The students concluded that radish plants
grow faster at 70 °F than at 40 °F, but their
teacher told them they had made an error in
their experiment. Which of the following is
the error that the students made in their
experimental set up?