A Houseplant that Holds Its Breath

As winter approaches, many northerners turn to house plants for a touch of growing greenery - tough, reliable plants that can survive the low light and humidity of most homes during the cold season.

Among the plants often found on northern windowsills are the succulent jade plants, Crassula portulacea and its kin. Their thick, rounded green leaves don't look much like jade, but they are not much harder to care for than pet rocks (and sometimes seem to grow little faster), so a stony name seems suitable.

All members of the Crassula family came originally from the Southern Hemisphere, and many species live naturally in some of the hottest and driest portions of southern Africa. That may seem an unlikely source for plants that do well in Alaska, but the relative humidity of our indoor winter climates can sink lower than the 15 or 20 percent prevailing in summer on the Crassulas' native deserts. The mechanisms these plants have evolved to survive in their harsh habitats serve them well in our homes.

Some of these mechanisms even add to their decorative appeal. A jade plant's chunky, tree-like stem and plump leaves are well designed for retaining moisture. The leaves' attractive glossiness comes from a waxy cuticle that seals in the retained water. In nature, dew condensing on those waxy leaves would run off before it evaporated, dripping to the ground where the roots could absorb it.

The most unusual mechanism doesn't show. In adapting to life under conditions that would cause most plants to dry up and blow away, jade plants have tinkered with photosynthesis, the most basic process in green plants. And they've also learned to hold their breath--all day.

The wild green plants that surround Alaskans in summertime make a good comparison to the Crassulas' behavior. During the long summer days, our native plants absorb moisture and minerals from the soil, take in carbon dioxide through leaf pores, and use sunlight to make these raw materials into more plant tissues and carbohydrates. The plants give off oxygen as a waste product, and water vapor passes freely through the leaf pores.

At night, plants still need energy, but without light to power photosynthesis, they turn instead to respiration: they take in oxygen, use it to consume some of the carbohydrates built up during the day, and give off water and carbon dioxide as byproducts. (Actually, the plants respire all the time, not just at night, but it's comparatively unnoticeable by day because so much photosynthesis is going on.)

Crassulas play by different rules. Their leaf pores open only in darkness--they'd loose too much water if they opened during the hot days. Carbon dioxide flows in through the pores and, through a complex chemical reaction involving the consumption of carbohydrates, becomes attached to an acid in the plants. At sunrise, the leaf pores close tightly. The plant breaks the bonds holding the carbon dioxide to the acid, releasing the gas. Photosynthesis then proceeds as usual, but it's using last night's stored carbon dioxide. Each jade plant leaf operates like a sealed factory by day, taking in only water from its roots and sunshine from the sky.

It's hard not to imagine sunset in Africa being accompanied by a great green gasp, as all those plants breathe at once.

This trick of storing carbon dioxide by binding it overnight to an acid is found in nearly all succulent plants, but was first identified in the hardy jade plant family. Thus it is known as Crassulacean Acid Metabolism, or CAM. It's not as efficient as normal photosynthesis; CAM plants expend more energy in fixing and releasing carbon dioxide than do plants that simply absorb it from the air. That may be part of the reason that succulents often seem to grow more slowly than other plants.

Less efficient it may be, but CAM is also a very good solution for a photosynthetic organism trying to make a living at the edge of an African desert--or an Alaskan windowsill. And, if you're in the habit of saying good night to your house plants, be sure the lights are off before you speak to your jade plants. Once it's dark, they can really use the carbon dioxide and moisture your farewell leaves behind.