Scientists have meausured the circumfrence of trees at 2 A.M. and at 2 P.M. If they collect measurements when the ground has adequate moisture and the days are sunny and dry, they find that hte circumfrence (and therefore the diameter) of the tree trunk is smaller at 2 P.M. than at 2 A.M. From your knowledge of the mechanisms of water transport, suggest your reasons for this decrease in circumfrence.

During daylight when photosynthesis is at work, the trees draw up moisture through the sapwood (xylem) in the trunk. This would plump up this tissue.

When your homework has been checked, can you let us know if my guess was right?

My guess is because of the lack of photosynthesis at night. The water is not being 'pulled' up and out of the top of the tree through the leaves, so it settles back into the bole of the tree. The casparian strip that the water has to pass through going into the tree keeps the water from going back into the ground.