What is a very tall cactus that can store water inside there stem?

Water, dissolved nutrients, and byproductsof photosynthesis are stored in the cactus stem. The water and the dissolved nutrients are sent up into the stem from the cactus plant's roots. They're held in the stem until they can be used in the photosynthetic interaction with sunlight. From that interaction are made the energizing carbohydrates, starches and sugars that the cactus needs for such activities as flowering, fruiting, growing, reproducing, and responding to stresses and stressors.