The Water Storers: Cactus Adaptations

In the Film

Most of the Baja California peninsula is desert. In this dry environment, plants have developed ways to use every available drop of water. Storing water, as cactus does, is an obvious solution. But even an adaptation such as slow growth serves to conserve energy in this parched land.

Concept

Desert plants store and conserve water.

Objective

To demonstrate absorption of water and other water conservation strategies

Content

Science, mathematics, language arts

Background

Desert plants have evolved various ways to conserve water. Cactus is well known for its ability to store water. A waxy coating on the stem and pads helps to reduce water loss. Further adaptations include spines, which are thought to help shade the plant by creating shadows, and plant orientation to sun exposure.

On the Web

Pages with photographs and information about cactus in the Field Guide: Barrel Cactus, Cardón, Cholla, Prickly-pear

Sponge (expanding type best, ordinary kitchen sponge will work), waxed paper, toothpicks, flashlight, 8 oz. plastic cups, water, scissors, scales (postage type will work), modeling clay

Part A   Plant Adaptations (whole class)

Discuss the need for plants to adapt to environmental conditions. Explore conditions in a desert environment such as hot, dry, wide range of temperatures, high rate of evaporation.

Part B   Sun Shade (small groups)

• Cut two sponges of equal size to resemble a cactus.
• Take one of the dry sponge "cacti." Place toothpicks in it to represent spines. Stand it up with a piece of clay. Shine a flashlight (sun) on the spines. Do you see shadows? How might shadows help to cool the plant?
• Turn the plant so that the flat, wide part of the pad faces away from the flashlight (sun). Estimate the surface area exposed (thin, narrow edge) to light (sun) versus the surface area not exposed to the sun. How might orientation to sunlight affect a cactus?

Part C   Storing Water (small groups)

• Remove the toothpicks and clay from the "cactus" used in part B.
• Weigh each "cactus." Record the weights.
• Put 2 ounces (60 milliliters) of water in each plastic cup and add a sponge "cactus" to each cup. Set aside for one hour. Observe the results.
• Carefully remove each sponge and weigh it. Record the weights. How much weight did each sponge gain? Compare the weights.
• Set each sponge aside to dry. Cover the top of one with a piece of waxed paper. Check daily for several days. Weigh and record the weights. Note the differences in the weights. Which one dried out faster? How would a waxy covering help a desert plant?

Local Connection
Do you live in a wet, dry, or moderate climate? Select some plants that are native to your area. What kind of adaptations do they exhibit that help them survive in your climate?

Key Words
adaptation, evolve

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