What part of the cactus takes in moisture?

The roots take in moisture for the cactus.

Specifically, water and dissolved nutrients are taken in underground by the cactus plant's roots. The water and dissolved nutrients are sent by capillary action up into the stem. In the stem, transformation into energizing foods takes place by way of the photosynthetic interaction with the sun.

Water and energizing carbohydrates, starches, and sugars are stored in the stem. They are released when needed for such cactus activities as flowering, fruiting, and reproducing. About 90% of the inside of the cactus is involved in the moving and storing of precious water.