How are the roots of a cactus plant an adaptation?

Cactus roots are adapted to take in water and store it below ground. Moisture may be taken from a wider below ground area by shallow, fibrous roots. Some of the water is sent upwards for joint food making with the sun through the process of photosynthesis. Some of the water is stored for use during extended droughts. For example, moisture may be stored below ground in swollen or tuberous roots.