Grades 3–5 · Organisms
How can a desert cactus store water and lose less?
A cactus's thick stem can hold water collected during a brief rain, helping it last through dry weather. Many desert cacti take up rain with shallow roots near the soil surface, then move that water into living tissues, including the fleshy stem.
The stem is a store, but it is not a hollow bottle. Water is held within tissue. In a ribbed cactus such as a saguaro, folds allow the stem to expand as its water store increases and shrink as water is used. Storage connects a brief rain with later life during a dry period.
Keeping that store matters too. A waxy outer surface slows water loss, and spines can provide shade and reduce the drying effect of wind. Many desert cacti also open their gas-exchange pores mainly at night, when cooler, more humid conditions reduce water loss. They can store the collected carbon dioxide for use in making sugars with daylight energy.
The Stem cutaway represents water held in Fleshy stem tissue. Skin + spines help protect that store, while roots on the Whole plant provide a route for collecting rainwater.
In the diagramKeeping water in a desert cactusWhole plantStem cutawaySkin +spinesFleshy stem: Stores waterA simplified example; not every cactus is alike.
Collecting water, storing it and exchanging gases with less water loss work together. Their details differ among cactus species. These inherited adaptations were shaped across generations; an individual cactus does not invent them when thirsty. A useful water store depends on both supply and protection.
About this explanation
Starting knowledge: Know that plants need water and can have stems, leaves and roots; distinguish holding water from losing water.
Sources
- National Park Service: Saguaro National Park — Cacti and Desert Succulents
- National Park Service: Joshua Tree National Park — Cacti / Desert Succulents
- Next Generation Science Standards — 3-LS4-3: Organisms in habitats