How would not cutting the stem of a plant under water affect the rate of transpiration when using a potometer?

Answer 1

Cutting the stem in the air instead of under water allows air to be introduced into the xylem. Transpiration will still happen, but it might not draw up water to be measured in the potometer because a bubble of air will be in the column of water that is supposed to be continuous up the xylem.

This is the same reason why we are supposed to cut the stems of flowers for an arrangement under the water. Otherwise the flowers may not be able to draw up water adequately to last very long in the flower arrangement.